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Chapter III
HETEROSTYLED DIMORPHIC PLANTS--continued.
Linum grandiflorum, long-styled form utterly sterile with own-form pollen.
Linum perenne, torsion of the pistils in the long-styled form alone.
Homostyled species of Linum.
Pulmonaria officinalis, singular difference in self-fertility between the
English and German long-styled plants.
Pulmonaria angustifolia shown to be a distinct species, long-styled form
completely self-sterile.
Polygonum fagopyrum.
Various other heterostyled genera.
Rubiaceae.
Mitchella repens, fertility of the flowers in pairs.
Houstonia.
Faramea, remarkable difference in the pollen-grains of the two forms; torsion of
the stamens in the short-styled form alone; development not as yet perfect.
The heterostyled structure in the several Rubiaceous genera not due to descent
in common.
(FIGURE 3.4. Linum grandiflorum.
Left: Long-styled form.
Right: Short-styled form.
s, s: stigmas.)
It has long been known that several species of Linum present two forms (3/1.
Treviranus has shown that this is the case in his review of my original paper
'Botanische Zeitung' 1863 page 189.), and having observed this fact in L. flavum
more than thirty years ago, I was led, after ascertaining the nature of
heterostylism in Primula, to examine the first species of Linum which I met
with, namely, the beautiful L. grandiflorum. This plant exists under two forms,
occurring in about equal numbers, which differ little in structure, but greatly
in function. The foliage, corolla, stamens, and pollen-grains (the latter
examined both distended with water and dry) are alike in the two forms (Figure
3.4). The difference is confined to the pistil; in the short-styled form the
styles and the stigmas are only about half the length of those in the long-
styled. A more important distinction is, that the five stigmas in the short-
styled form diverge greatly from one another, and pass out between the filaments
of the stamens, and thus lie within the tube of the corolla. In the long-styled
form the elongated stigmas stand nearly upright, and alternate with the anthers.
In this latter form the length of the stigmas varies considerably, their upper
extremities projecting even a little above the anthers, or reaching up only to
about their middle. Nevertheless, there is never the slightest difficulty in
distinguishing between the two forms; for, besides the difference in the
divergence of the stigmas, those of the short-styled form never reach even to
the bases of the anthers. In this form the papillae on the stigmatic surfaces
are shorter, darker-coloured, and more crowded together than in the long-styled
form; but these differences seem due merely to the shortening of the stigma, for
in the varieties of the long-styled form with shorter stigmas, the papillae are
more crowded and darker-coloured than in those with the longer stigmas.
Considering the slight and variable differences between the two forms of this
Linum, it is not surprising that hitherto they have been overlooked.
In 1861 I had eleven plants in my garden, eight of which were long-styled, and
three short-styled. Two very fine long-styled plants grew in a bed a hundred
yards off all the others, and separated from them by a screen of evergreens. I
marked twelve flowers, and placed on their stigmas a little pollen from the
short-styled plants. The pollen of the two forms is, as stated, identical in
appearance; the stigmas of the long-styled flowers were already thickly covered
with their own pollen--so thickly that I could not find one bare stigma, and it
was late in the season, namely, September 15th. Altogether, it seemed almost
childish to expect any result. Nevertheless from my experiments on Primula, I
had faith, and did not hesitate to make the trial, but certainly did not
anticipate the full result which was obtained. The germens of these twelve
flowers all swelled, and ultimately six fine capsules (the seed of which
germinated on the following year) and two poor capsules were produced; only four
capsules shanking off. These same two long-styled plants produced, in the course
of the summer, a vast number of flowers, the stigmas of which were covered with
their own pollen; but they all proved absolutely barren, and their germens did
not even swell.
The nine other plants, six long-styled and three short-styled, grew not very far
apart in my flower-garden. Four of these long-styled plants produced no seed-
capsules; the fifth produced two; and the remaining one grew so close to a
short-styled plant that their branches touched, and this produced twelve
capsules, but they were poor ones. The case was different with the short-styled
plants. The one which grew close to the long-styled plant produced ninety-four
imperfectly fertilised capsules containing a multitude of bad seeds, with a
moderate number of good ones. The two other short-styled plants growing together
were small, being partly smothered by other plants; they did not stand very
close to any long-styled plants, yet they yielded together nineteen capsules.
These facts seem to show that the short-styled plants are more fertile with
their own pollen than are the long-styled, and we shall immediately see that
this probably is the case. But I suspect that the difference in fertility
between the two forms was in this instance in part due to a distinct cause. I
repeatedly watched the flowers, and only once saw a humble-bee momentarily
alight on one, and then fly away. If bees had visited the several plants, there
cannot be a doubt that the four long-styled plants, which did not produce a
single capsule, would have borne an abundance. But several times I saw small
diptera sucking the flowers; and these insects, though not visiting the flowers
with anything like the regularity of bees, would carry a little pollen from one
form to the other, especially when growing near together; and the stigmas of the
short-styled plants, diverging within the tube of the corolla, would be more
likely than the upright stigmas of the long-styled plants, to receive a small
quantity of pollen if brought to them by small insects. Moreover from the
greater number of the long-styled than of the short-styled plants in the garden,
the latter would be more likely to receive pollen from the long-styled, than the
long-styled from the short-styled.
In 1862 I raised thirty-four plants of this Linum in a hot-bed; and these
consisted of seventeen long-styled and seventeen short-styled forms. Seed sown
later in the flower-garden yielded seventeen long-styled and twelve short-styled
forms. These facts justify the statement that the two forms are produced in
about equal numbers. The thirty-four plants of the first lot were kept under a
net which excluded all insects, except such minute ones as Thrips. I fertilised
fourteen long-styled flowers legitimately with pollen from the short-styled, and
got eleven fine seed-capsules, which contained on an average 8.6 seeds per
capsule, but only 5.6 appeared to be good. It may be well to state that ten
seeds is the maximum production for a capsule, and that our climate cannot be
very favourable to this North-African plant. On three occasions the stigmas of
nearly a hundred flowers were fertilised illegitimately with their own-form
pollen, taken from separate plants, so as to prevent any possible ill effects
from close inter-breeding. Many other flowers were also produced, which, as
before stated, must have received plenty of their own pollen; yet from all these
flowers, borne by the seventeen long-styled plants, only three capsules were
produced. One of these included no seed, and the other two together gave only
five good seeds. It is probable that this miserable product of two half-fertile
capsules from the seventeen plants, each of which must have produced at least
fifty or sixty flowers, resulted from their fertilisation with pollen from the
short-styled plants by the aid of Thrips; for I made a great mistake in keeping
the two forms under the same net, with their branches often interlocking; and it
is surprising that a greater number of flowers were not accidentally fertilised.
Twelve short-styled flowers were in this instance castrated, and afterwards
fertilised legitimately with pollen from the long-styled form; and they produced
seven fine capsules. These included on an average 7.6 seeds, but of apparently
good seed only 4.3 per capsule. At three separate times nearly a hundred flowers
were fertilised illegitimately with their own-form pollen, taken from separate
plants; and numerous other flowers were produced, many of which must have
received their own pollen. From all these flowers on the seventeen short-styled
plants only fifteen capsules were produced, of which only eleven contained any
good seed, on an average 4.2 per capsule. As remarked in the case of the long-
styled plants, some even of these capsules were perhaps the product of a little
pollen accidentally fallen from the adjoining flowers of the other form on to
the stigmas, or transported by Thrips. Nevertheless the short-styled plants seem
to be slightly more fertile with their own pollen than the long-styled, in the
proportion of fifteen capsules to three; nor can this difference be accounted
for by the short-styled stigmas being more liable to receive their own pollen
than the long-styled, for the reverse is the case. The greater self-fertility of
the short-styled flowers was likewise shown in 1861 by the plants in my flower-
garden, which were left to themselves, and were but sparingly visited by
insects.
On account of the probability of some of the flowers on the plants of both
forms, which were covered under the same net, having been legitimately
fertilised in an accidental manner, the relative fertility of the two legitimate
and two illegitimate unions cannot be compared with certainty; but judging from
the number of good seeds per capsule, the difference was at least in the ratio
of 100 to 7, and probably much greater.
Hildebrand tested my results, but only on a single short-styled plant, by
fertilising many flowers with their own-form pollen; and these did not produce
any seed. This confirms my suspicion that some of the few capsules produced by
the foregoing seventeen short-styled plants were the product of accidental
legitimate fertilisation. Other flowers on the same plant were fertilised by
Hildebrand with pollen from the long-styled form, and all produced fruit. (3/2.
'Botanische Zeitung' January 1, 1864 page 2.)
The absolute sterility (judging from the experiments of 1861) of the long-styled
plants with their own-form pollen led me to examine into its apparent cause; and
the results are so curious that they are worth giving in detail. The experiments
were tried on plants grown in pots and brought successively into the house.
FIRST.
Pollen from a short-styled plant was placed on the five stigmas of a long-styled
flower, and these, after thirty hours, were found deeply penetrated by a
multitude of pollen-tubes, far too numerous to be counted; the stigmas had also
become discoloured and twisted. I repeated this experiment on another flower,
and in eighteen hours the stigmas were penetrated by a multitude of long pollen-
tubes. This is what might have been expected, as the union is a legitimate one.
The converse experiment was likewise tried, and pollen from a long-styled flower
was placed on the stigmas of a short-styled flower, and in twenty-four hours the
stigmas were discoloured, twisted, and penetrated by numerous pollen-tubes; and
this, again, is what might have been expected, as the union was a legitimate
one.
SECONDLY.
Pollen from a long-styled flower was placed on all five stigmas of a long-styled
flower on a separate plant: after nineteen hours the stigmas were dissected, and
only a single pollen-grain had emitted a tube, and this was a very short one. To
make sure that the pollen was good, I took in this case, and in most of the
other cases, pollen either from the same anther or from the same flower, and
proved it to be good by placing it on the stigma of a short-styled plant, and
found numerous pollen-tubes emitted.
THIRDLY.
Repeated last experiment, and placed own-form pollen on all five stigmas of a
long-styled flower; after nineteen hours and a half, not one single grain had
emitted its tube.
FOURTHLY.
Repeated the experiment, with the same result after twenty-four hours.
FIFTHLY.
Repeated last experiment, and, after leaving pollen on for nineteen hours, put
on an additional quantity of own-form pollen on all five stigmas. After an
interval of three days, the stigmas were examined, and, instead of being
discoloured and twisted, they were straight and fresh-coloured. Only one grain
had emitted a quite short tube, which was drawn out of the stigmatic tissue
without being ruptured.
The following experiments are more striking:--
SIXTHLY.
I placed own-form pollen on three of the stigmas of a long-styled flower, and
pollen from a short-styled flower on the other two stigmas. After twenty-two
hours these two stigmas were discoloured, slightly twisted, and penetrated by
the tubes of numerous pollen-grains: the other three stigmas, covered with their
own-form pollen, were fresh, and all the pollen-grains were loose; but I did not
dissect the whole stigma.
SEVENTHLY.
Experiment repeated in the same manner, with the same result.
EIGHTHLY.
Experiment repeated, but the stigmas were carefully examined after an interval
of only five hours and a half. The two stigmas with pollen from a short-styled
flower were penetrated by innumerable tubes, which were as yet short, and the
stigmas themselves were not at all discoloured. The three stigmas covered with
their own-form pollen were not penetrated by a single pollen-tube.
NINTHLY.
Put pollen of a short-styled flower on a single long-styled stigma, and own-form
pollen on the other four stigmas; after twenty-four hours the one stigma was
somewhat discoloured and twisted, and penetrated by many long tubes: the other
four stigmas were quite straight and fresh; but on dissecting them I found that
three pollen-grains had protruded very short tubes into the tissue.
TENTHLY.
Repeated the experiment, with the same result after twenty-four hours, excepting
that only two own-form grains had penetrated the stigmatic tissue with their
tubes to a very short depth. The one stigma, which was deeply penetrated by a
multitude of tubes from the short-styled pollen, presented a conspicuous
difference in being much curled, half-shrivelled, and discoloured, in comparison
with the other four straight and bright pink stigmas.
I could add other experiments; but those now given amply suffice to show that
the pollen-grains of a short-styled flower placed on the stigma of a long-styled
flower emit a multitude of tubes after an interval of from five to six hours,
and penetrate the tissue ultimately to a great depth; and that after twenty-four
hours the stigmas thus penetrated change colour, become twisted, and appear
half-withered. On the other hand, pollen-grains from a long-styled flower placed
on its own stigmas, do not emit their tubes after an interval of a day, or even
three days; or at most only three or four grains out of a multitude emit their
tubes, and these apparently never penetrate the stigmatic tissue deeply, and the
stigmas themselves do not soon become discoloured and twisted.
This seems to me a remarkable physiological fact. The pollen-grains of the two
forms are undistinguishable under the microscope; the stigmas differ only in
length, degree of divergence, and in the size, shade of colour, and
approximation of their papillae, these latter differences being variable and
apparently due merely to the degree of elongation of the stigma. Yet we plainly
see that the two kinds of pollen and the two stigmas are widely dissimilar in
their mutual reaction--the stigmas of each form being almost powerless on their
own pollen, but causing, through some mysterious influence, apparently by simple
contact (for I could detect no viscid secretion), the pollen-grains of the
opposite form to protrude their tubes. It may be said that the two pollens and
the two stigmas mutually recognise each other by some means. Taking fertility as
the criterion of distinctness, it is no exaggeration to say that the pollen of
the long-styled Linum grandiflorum (and conversely that of the other form) has
been brought to a degree of differentiation, with respect to its action on the
stigma of the same form, corresponding with that existing between the pollen and
stigma of species belonging to distinct genera.
Linum perenne.
This species is conspicuously heterostyled, as has been noticed by several
authors. The pistil in the long-styled form is nearly twice as long as that of
the short-styled. In the latter the stigmas are smaller and, diverging to a
greater degree, pass out low down between the filaments. I could detect no
difference in the two forms in the size of the stigmatic papillae. In the long-
styled form alone the stigmatic surfaces of the mature pistils twist round, so
as to face the circumference of the flower; but to this point I shall presently
return. Differently from what occurs in L. grandiflorum, the long-styled flowers
have stamens hardly more than half the length of those in the short-styled. The
size of the pollen-grains is rather variable; after some doubt, I have come to
the conclusion that there is no uniform difference between the grains in the two
forms. The long-stamens in the short-styled form project to some height above
the corolla, and their filaments are coloured blue apparently from exposure to
the light. The anthers of the longer stamens correspond in height with the lower
part of the stigmas of the long-styled flowers; and the anthers of the shorter
stamens of the latter correspond in the same manner in height with the stigmas
of the short-styled flowers.
I raised from seed twenty-six plants, of which twelve proved to be long-styled
and fourteen short-styled. They flowered well, but were not large plants. As I
did not expect them to flower so soon, I did not transplant them, and they
unfortunately grew with their branches closely interlocked. All the plants were
covered under the same net, excepting one of each form. Of the flowers on the
long-styled plants, twelve were illegitimately fertilised with their own-form
pollen, taken in every case from a separate plant; and not one set a seed-
capsule: twelve other flowers were legitimately fertilised with pollen from
short-styled flowers; and they set nine capsules, each including on an average 7
good seeds, ten being the maximum number ever produced. Of the flowers on the
short-styled plants, twelve were illegitimately fertilised with own-form pollen,
and they yielded one capsule, including only 3 good seeds; twelve other flowers
were legitimately fertilised with pollen from long-styled flowers, and these
produced nine capsules, but one was bad; the eight good capsules contained on an
average 8 good seeds each. Judging from the number of seeds per capsule, the
fertility of the two legitimate to that of the two illegitimate unions is as 100
to 20.
The numerous flowers on the eleven long-styled plants under the net, which were
not fertilised, produced only three capsules, including 8, 4, and 1 good seeds.
Whether these three capsules were the product of accidental legitimate
fertilisation, owing to the branches of the plants of the two forms
interlocking, I will not pretend to decide. The single long-styled plant which
was left uncovered, and grew close by the uncovered short-styled plant, produced
five good pods; but it was a poor and small plant.
The flowers borne on the thirteen short-styled plants under the net, which were
not fertilised, produced twelve capsules, containing on an average 5.6 seeds. As
some of these capsules were very fine, and as five were borne on one twig, I
suspect that some minute insect had accidentally got under the net and had
brought pollen from the other form to the flowers which produced this little
group of capsules. The one uncovered short-styled plant which grew close to the
uncovered long-styled plant yielded twelve capsules.
From these facts we have some reason to believe, as in the case of L.
grandiflorum, that the short-styled plants are in a slight degree more fertile
with their own pollen than are the long-styled plants. Anyhow we have the
clearest evidence, that the stigmas of each form require for full fertility that
pollen from the stamens of corresponding height belonging to the opposite form
should be brought to them.
Hildebrand, in the paper lately referred to, confirms my results. He placed a
short-styled plant in his house, and fertilised about 20 flowers with their own
pollen, and about 30 with pollen from another plant belonging to the same form,
and these 50 flowers did not set a single capsule. On the other hand he
fertilised about 30 flowers with pollen from the long-styled form, and these,
with the exception of two, yielded capsules, containing good seeds.
It is a singular fact, in contrast with what occurred in the case of L.
grandiflorum, that the pollen-grains of both forms of L. perenne, when placed on
their own-form stigmas, emitted their tubes, though this action did not lead to
the production of seeds. After an interval of eighteen hours, the tubes
penetrated the stigmatic tissue, but to what depth I did not ascertain. In this
case the impotence of the pollen-grains on their own stigmas must have been due
either to the tubes not reaching the ovules, or to their not acting properly
after reaching them.
The plants both of L. perenne and grandiflorum, grew, as already stated, with
their branches interlocked, and with scores of flowers of the two forms close
together; they were covered by a rather coarse net, through which the wind, when
high, passed; and such minute insects as Thrips could not, of course, be
excluded; yet we have seen that the utmost possible amount of accidental
fertilisation on seventeen long-styled plants in the one case, and on eleven
long-styled plants in the other, resulted in the production, in each case, of
three poor capsules; so that when the proper insects are excluded, the wind does
hardly anything in the way of carrying pollen from plant to plant. I allude to
this fact because botanists in speaking of the fertilisation of various flowers,
often refer to the wind or to insects as if the alternative were indifferent.
This view, according to my experience, is entirely erroneous. When the wind is
the agent in carrying pollen, either from one sex to the other, or from
hermaphrodite to hermaphrodite, we can recognise structure as manifestly adapted
to its action as to that of insects when these are the carriers. We see
adaptation to the wind in the incoherence of the pollen,--in the inordinate
quantity produced (as in the Coniferae, Spinage, etc.),--in the dangling anthers
well fitted to shake out the pollen,--in the absence or small size of the
perianth,--in the protrusion of the stigmas at the period of fertilisation,--in
the flowers being produced before they are hidden by the leaves,--and in the
stigmas being downy or plumose (as in the Gramineae, Docks, etc), so as to
secure the chance-blown grains. In plants which are fertilised by the wind, the
flowers do not secrete nectar, their pollen is too incoherent to be easily
collected by insects, they have not bright-coloured corollas to serve as guides,
and they are not, as far as I have seen, visited by insects. When insects are
the agents of fertilisation (and this is incomparably the more frequent case
with hermaphrodite plants), the wind plays no part, but we see an endless number
of adaptations to ensure the safe transport of the pollen by the living workers.
These adaptations are most easily recognised in irregular flowers; but they are
present in regular flowers, of which those of Linum offer a good instance, as I
will now endeavour to show.
I have already alluded to the rotation of each separate stigma in the long-
styled form of Linum perenne. In both forms of the other heterostyled species
and in the homostyled species of Linum which I have seen, the stigmatic surfaces
face the centre of the flower, with the furrowed backs of the stigmas, to which
the styles are attached, facing outwards. This is the case with the stigmas of
the long-styled flowers of L. perenne whilst in bud. But by the time the flowers
have expanded, the five stigmas twist round so as to face the circumference,
owing to the torsion of that part of the style which lies beneath the stigma. I
should state that the five stigmas do not always turn round completely, two or
three sometimes facing only obliquely outwards. My observations were made during
October; and it is not improbable that earlier in the season the torsion would
have been more complete; for after two or three cold and wet days the movement
was very imperfectly performed. The flowers should be examined shortly after
their expansion, as their duration is brief; as soon as they begin to wither,
the styles become spirally twisted all together, the original position of the
parts being thus lost.
He who will compare the structure of the whole flower in both forms of L.
perenne and grandiflorum, and, as I may add, of L. flavum, will not doubt about
the meaning of this torsion of the styles in the one form alone of L. perenne,
as well as the meaning of the divergence of the stigmas in the short-styled form
of all three species. It is absolutely necessary as we know, that insects should
carry pollen from the flowers of the one form reciprocally to those of the
other. Insects are attracted by five drops of nectar, secreted exteriorly at the
base of the stamens, so that to reach these drops they must insert their
proboscides outside the ring of broad filaments, between them and the petals. In
the short-styled form of the above three species, the stigmas face the axis of
the flower; and had the styles retained their original upright and central
position, not only would the stigmas have presented their backs to the insects
which sucked the flowers, but their front and fertile surfaces would have been
separated from the entering insects by the ring of broad filaments, and would
never have received any pollen. As it is, the styles diverge and pass out
between the filaments. After this movement the short stigmas lie within the tube
of the corolla; and their papillous surfaces being now turned upwards are
necessarily brushed by every entering insect, and thus receive the required
pollen.
In the long-styled form of L. grandiflorum, the almost parallel or slightly
diverging anthers and stigmas project a little above the tube of the somewhat
concave flower; and they stand directly over the open space leading to the drops
of nectar. Consequently when insects visit the flowers of either form (for the
stamens in this species occupy the same position in both forms), they will get
their foreheads or proboscides well dusted with the coherent pollen. As soon as
they visit the flowers of the long-styled form they will necessarily leave
pollen on the proper surface of the elongated stigmas; and when they visit the
short-styled flowers, they will leave pollen on the upturned stigmatic surfaces.
Thus the stigmas of both forms will receive indifferently the pollen of both
forms; but we know that the pollen alone of the opposite form causes
fertilisation.
(Figure 3.5. Long-styled form of L. perenne var. Austriacum in its early
condition before the stigmas have rotated. The petals and calyx have been
removed on the near side. (3/3. I neglected to get drawings made from fresh
flowers of the two forms. But Mr. Fitch has made the above sketch of a long-
styled flower from dried specimens and from published engravings. His well-known
skill ensures accuracy in the proportional size of the parts.)
In the case of L. perenne, affairs are arranged more perfectly; for the stamens
in the two forms stand at different heights, so that pollen from the anthers of
the longer stamens will adhere to one part of an insect's body, and will
afterwards be brushed off by the rough stigmas of the longer pistils; whilst
pollen from the anthers of the shorter stamens will adhere to a different part
of the insect's body, and will afterwards be brushed off by the stigmas of the
shorter pistils; and this is what is required for the legitimate fertilisation
of both forms. The corolla of L. perenne is more expanded than that of L.
grandiflorum, and the stigmas of the long-styled form do not diverge greatly
from one another; nor do the stamens of either form. Hence insects, especially
rather small ones, will not insert their proboscides between the stigmas of the
long-styled form, nor between the anthers of either form (Figure 3.5), but will
strike against them, at nearly right angles, with the backs of their head or
thorax. Now, in the long-styled flowers, if each stigma did not rotate on its
axis, insects in visiting them would strike their heads against the backs of the
stigmas; as it is, they strike against that surface which is covered with
papillae, with their heads already charged with pollen from the stamens of
corresponding height borne by the flowers of the other form, and legitimate
fertilisation is thus ensured.
Thus we can understand the meaning of the torsion of the styles in the long-
styled flowers alone, as well as their divergence in the short-styled flowers.
One other point is worth notice. In botanical works many flowers are said to be
fertilised in the bud. This statement generally rests, as far as I can discover,
on the anthers opening in the bud; no evidence being adduced that the stigma is
at this period mature, or that it is not subsequently acted on by pollen brought
from other flowers. In the case of Cephalanthera grandiflora I have shown that
precocious and partial self-fertilisation, with subsequent full fertilisation,
is the regular course of events. (3/4. 'Fertilisation of Orchids' page 108; 2nd
edition 1877 page 84.) The belief that the flowers of many plants are fertilised
in the bud, that is, are perpetually self-fertilised, is a most effectual bar to
understanding their real structure. I am, however, far from wishing to assert
that some flowers, during certain seasons, are not fertilised in the bud; for I
have reason to believe that this is the case. A good observer, resting his
belief on the usual kind of evidence, states that in Linum Austriacum (which is
heterostyled, and is considered by Planchon as a variety of L. perenne) the
anthers open the evening before the expansion of the flowers, and that the
stigmas are then almost always fertilised. (3/5. H. Lecoq 'Etudes sur la Geogr.
Bot.' 1856 tome 5 page 325.) Now we know positively that, so far from Linum
perenne being fertilised by its own pollen in the bud, its own pollen is as
powerless on the stigma as so much inorganic dust.
Linum flavum.
The pistil of the long-styled form of this species is nearly twice as long as
that of the short-styled; the stigmas are longer and the papillae coarser. In
the short-styled form the stigmas diverge and pass out between the filaments, as
in the previous species. The stamens in the two forms differ in length; and,
what is singular, the anthers of the longer stamens are not so long as those of
the other form; so that in the short-styled form both the stigmas and the
anthers are shorter than in the long-styled form. The pollen-grains of the two
forms do not differ in size. As this species is propagated by cuttings,
generally all the plants in the same garden belong to the same form. I have
inquired, but have never heard of its seeding in this country. Certainly my own
plants never produced a single seed as long as I possessed only one of the two
forms. After considerable search I procured both forms, but from want of time
only a few experiments were made. Two plants of the two forms were planted some
way apart in my garden, and were not covered by nets. Three flowers on the long-
styled plant were legitimately fertilised with pollen from the short-styled
plant, and one of them set a fine capsule. No other capsules were produced by
this plant. Three flowers on the short-styled plant were legitimately fertilised
with pollen from the long-styled, and all three produced capsules, containing
respectively no less than 8, 9, and 10 seeds. Three other flowers on this plant,
which had not been artificially fertilised, produced capsules containing 5, 1,
and 5 seeds; and it is quite possible that pollen may have been brought to them
by insects from the long-styled plant growing in the same garden. Nevertheless,
as they did not yield half the number of seeds compared with the other flowers
on the same plant which had been artificially and legitimately fertilised, and
as the short-styled plants of the two previous species apparently evince some
slight capacity for fertilisation with their own-form pollen, these three
capsules may have been the product of self-fertilisation.
Besides the three species now described, the yellow-flowered L. corymbiferum is
certainly heterostyled, as is, according to Planchon, L. salsoloides. (3/6.
Hooker's 'London Journal of Botany' 1848 volume 7 page 174.) This botanist is
the only one who seems to have inferred that heterostylism might have some
important functional bearing. Dr. Alefeld, who has made a special study of the
genus, says that about half of the sixty-five species known to him are
heterostyled. (3/7. 'Botanische Zeitung' September 18, 1863 page 281.) This is
the case with L. trigynum, which differs so much from the other species that it
has been formed by him into a distinct genus. (3/8. It is not improbable that
the allied genus, Hugonia, is heterostyled, for one species is said by Planchon
(Hooker's 'London Journal of Botany' 1848 volume 7 page 525) to be provided with
"staminibus exsertis;" another with "stylis staminibus longioribus," and another
has "stamina 5, majora, stylos longe superantia.") According to the same author,
none of the species which inhabit America and the Cape of Good Hope are
heterostyled.
I have examined only three homostyled species, namely, L. usitatissimum,
angustifolium, and catharticum. I raised 111 plants of a variety of the first-
named species, and these, when protected under a net, all produced plenty of
seed. The flowers, according to H. Muller, are frequented by bees and moths.
(3/9. 'Die Befruchtung der Blumen' etc. page 168.) With respect to L.
catharticum, the same author shows that the flowers are so constructed that they
can freely fertilise themselves; but if visited by insects they might be cross-
fertilised. He has, however, only once seen the flowers thus visited during the
day; but it may be suspected that they are frequented during the night by small
moths for the sake of the five minute drops of nectar secreted. Lastly, L.
Lewisii is said by Planchon to bear on the same plant flowers with stamens and
pistils of the same height, and others with the pistils either longer or shorter
than the stamens. This case formerly appeared to me an extraordinary one; but I
am now inclined to believe that it is one merely of great variability. (3/10.
Planchon in Hooker's 'London Journal of Botany' 1848 volume 7 page 175. See on
this subject Asa Gray in 'American Journal of Science' volume 36 September 1863
page 284.)
PULMONARIA (BORAGINEAE).
Pulmonaria officinalis.
Hildebrand has published a full account of this heterostyled plant. (3/11.
'Botanische Zeitung' 1865 January 13 page 13.) The pistil of the long-styled
form is twice as long as that of the short-styled; and the stamens differ in a
corresponding, though converse, manner. There is no marked difference in the
shape or state of surface of the stigma in the two forms. The pollen-grains of
the short-styled form are to those of the long-styled as 9 to 7, or as 100 to
78, in length, and as 7 to 6 in breadth. They do not differ in the appearance of
their contents. The corolla of the one form differs in shape from that of the
other in nearly the same manner as in Primula; but besides this difference the
flowers of the short-styled are generally the larger of the two. Hildebrand
collected on the Siebengebirge, ten wild long-styled and ten short-styled
plants. The former bore 289 flowers, of which 186 (i.e. 64 per cent) had set
fruit, yielding 1.88 seed per fruit. The ten short-styled plants bore 373
flowers, of which 262 (i.e. 70 per cent) had set fruit, yielding 1.86 seed per
fruit. So that the short-styled plants produced many more flowers, and these set
a rather larger proportion of fruit, but the fruits themselves yielded a
slightly lower average number of seeds than did the long-styled plants. The
results of Hildebrand's experiments on the fertility of the two forms are given
in Table 3.19.
TABLE 3.19. Pulmonaria officinalis (from Hildebrand).
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Fruits produced.
Column 4: Average Number of Seeds per Fruit.
Long-styled by pollen of short-styled. Legitimate union :
14 : 10 : 1.30.
Long-styled 14 by own-pollen, and 16 by pollen of other plant of same form.
Illegitimate union :
30 : 0 : 0.
Short-styled by pollen of long-styled. Legitimate union:
16 : 14 : 1.57.
Short-styled 11 by own-pollen, 14 by pollen of other plant of same form.
Illegitimate union :
25 : 0 : 0.
In the summer of 1864, before I had heard of Hildebrand's experiments, I noticed
some long-styled plants of this species (named for me by Dr. Hooker) growing by
themselves in a garden in Surrey; and to my surprise about half the flowers had
set fruit, several of which contained 2, and one contained even 3 seeds. These
seeds were sown in my garden and eleven seedlings thus raised, all of which
proved long-styled, in accordance with the usual rule in such cases. Two years
afterwards the plants were left uncovered, no other plant of the same genus
growing in my garden, and the flowers were visited by many bees. They set an
abundance of seeds: for instance, I gathered from a single plant rather less
than half of the seeds which it had produced, and they numbered 47. Therefore
this illegitimately fertilised plant must have produced about 100 seeds; that
is, thrice as many as one of the wild long-styled plants collected on the
Siebengebirge by Hildebrand, and which, no doubt, had been legitimately
fertilised. In the following year one of my plants was covered by a net, and
even under these unfavourable conditions it produced spontaneously a few seeds.
It should be observed that as the flowers stand either almost horizontally or
hang considerably downwards, pollen from the short stamens would be likely to
fall on the stigma. We thus see that the English long-styled plants when
illegitimately fertilised were highly fertile, whilst the German plants
similarly treated by Hildebrand were completely sterile. How to account for this
wide discordance in our results I know not. Hildebrand cultivated his plants in
pots and kept them for a time in the house, whilst mine were grown out of doors;
and he thinks that this difference of treatment may have caused the difference
in our results. But this does not appear to me nearly a sufficient cause,
although his plants were slightly less productive than the wild ones growing on
the Siebengbirge. My plants exhibited no tendency to become equal-styled, so as
to lose their proper long-styled character, as not rarely happens under
cultivation with several heterostyled species of Primula; but it would appear
that they had been greatly affected in function, either by long-continued
cultivation or by some other cause. We shall see in a future chapter that
heterostyled plants illegitimately fertilised during several successive
generations sometimes become more self-fertile; and this may have been the case
with my stock of the present species of Pulmonaria; but in this case we must
assume that the long-styled plants were at first sufficiently fertile to yield
some seed, instead of being absolutely self-sterile like the German plants.
Pulmonaria angustifolia.
(FIGURE 3.6. Pulmonaria angustifolia.
Left: Long-styled form.
Right: Short-styled form.)
Seedlings of this plant, raised from plants growing wild in the Isle of Wight,
were named for me by Dr. Hooker. It is so closely allied to the last species,
differing chiefly in the shape and spotting of the leaves, that the two have
been considered by several eminent botanists--for instance, Bentham--as mere
varieties. But, as we shall presently see, good evidence can be assigned for
ranking them as distinct. Owing to the doubts on this head, I tried whether the
two would mutually fertilise one another. Twelve short-styled flowers of P.
angustifolia were legitimately fertilised with pollen from long-styled plants of
P. officinalis (which, as we have just seen, are moderately self-fertile), but
they did not produce a single fruit. Thirty-six long-styled flowers of P.
angustifolia were also illegitimately fertilised during two seasons with pollen
from the long-styled P. officinalis, but all these flowers dropped off
unimpregnated. Had the plants been mere varieties of the same species these
illegitimate crosses would probably have yielded some seeds, judging from my
success in illegitimately fertilising the long-styled flowers of P. officinalis;
and the twelve legitimate crosses, instead of yielding no fruit, would almost
certainly have yielded a considerable number, namely, about nine, judging from
the results given in Table 3.20. Therefore P. officinalis and angustifolia
appear to be good and distinct species, in conformity with other important
functional differences between them, immediately to be described.
TABLE 3.20. Pulmonaria angustifolia.
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Fruits produced.
Column 4: Average Number of Seeds per Fruit.
Long-styled by pollen of short-styled. Legitimate union :
18 : 9 : 2.11.
Long-styled by own-form pollen. Illegitimate union :
18 : 0 : 0.
Short-styled by pollen of long-styled. Legitimate union:
18 : 15 : 2.60.
Short-styled by own-form pollen. Illegitimate union :
12 : 7 : 1.86.
The long-styled and short-styled flowers of P. angustifolia differ from one
another in structure in nearly the same manner as those of P. officinalis. But
in Figure 3.6 a slight bulging of the corolla in the long-styled form, where the
anthers are seated, has been overlooked. My son William, who examined a large
number of wild plants in the Isle of Wight, observed that the corolla, though
variable in size, was generally larger in the long-styled flowers than in the
short-styled; and certainly the largest corollas of all were found on the long-
styled plants, and the smallest on the short-styled. Exactly the reverse occurs,
according to Hildebrand, with P. officinalis. Both the pistils and stamens of P.
angustifolia vary much in length; so that in the short-styled form the distance
between the stigma and the anthers varied from 119 to 65 divisions of the
micrometer, and in the long-styled from 115 to 112. From an average of seven
measurements of each form the distance between these organs in the long-styled
is to the same distance in the short-styled form as 100 to 69; so that the
stigma in the one form does not stand on a level with the anthers in the other.
The long-styled pistil is sometimes thrice as long as that of the short-styled;
but from an average of ten measurements of both, its length to that of the
short-styled was as 100 to 56. The stigma varies in being more or less, though
slightly, lobed. The anthers also vary much in length in both forms, but in a
greater degree in the long-styled than in the short-styled-form; many in the
former being from 80 to 63, and in the latter from 80 to 70 divisions of the
micrometer in length. From an average of seven measurements, the short-styled
anthers were to those from the long-styled as 100 to 91 in length. Lastly, the
pollen-grains from the long-styled flowers varied between 13 and 11.5 divisions
of the micrometer, and those from the short-styled between 15 and 13. The
average diameter of 25 grains from the latter, or short-styled form, was to that
of 20 grains from the long-styled as 100 to 91. We see, therefore, that the
pollen-grains from the smaller anthers of the shorter stamens in the long-styled
form are, as usual, of smaller size than those in the other form. But what is
remarkable, a larger proportion of the grains were small, shrivelled, and
worthless. This could be seen by merely comparing the contents of the anthers
from several distinct plants of each form. But in one instance my son found, by
counting, that out of 193 grains from a long-styled flower, 53 were bad, or 27
per cent; whilst out of 265 grains from a short-styled flower only 18 were bad,
or 7 per cent. From the condition of the pollen in the long-styled form, and
from the extreme variability of all the organs in both forms, we may perhaps
suspect that the plant is undergoing a change, and tending to become dioecious.
My son collected in the Isle of Wight on two occasions 202 plants, of which 125
were long-styled and 77 short-styled; so that the former were the more numerous.
On the other hand, out of 18 plants raised by me from seed, only 4 were long-
styled and 14 short-styled. The short-styled plants seemed to my son to produce
a greater number of flowers than the long-styled; and he came to this conclusion
before a similar statement had been published by Hildebrand with respect to P.
officinalis. My son gathered ten branches from ten different plants of both
forms, and found the number of flowers of the two forms to be as 100 to 89, 190
being short-styled and 169 long-styled. With P. officinalis the difference,
according to Hildebrand, is even greater, namely, as 100 flowers for the short-
styled to 77 for the long-styled plants. Table 3.20 shows the results of my
experiments.
We see in Table 3.20 that the fertility of the two legitimate unions to that of
the two illegitimate together is as 100 to 35, judged by the proportion of
flowers which produced fruit; and as 100 to 32, judged by the average number of
seeds per fruit. But the small number of fruit yielded by the 18 long-styled
flowers in the first line was probably accidental, and if so, the difference in
the proportion of legitimately and illegitimately fertilised flowers which yield
fruit is really greater than that represented by the ratio of 100 to 35. The 18
long-styled flowers illegitimately fertilised yielded no seeds,--not even a
vestige of one. Two long-styled plants which were placed under a net produced
138 flowers, besides those which were artificially fertilised, and none of these
set any fruit; nor did some plants of the same form which were protected during
the next summer. Two other long-styled plants were left uncovered (all the
short-styled plants having been previously covered up), and humble-bees, which
had their foreheads white with pollen, incessantly visited the flowers, so that
their stigmas must have received an abundance of pollen, yet these flowers did
not produce a single fruit. We may therefore conclude that the long-styled
plants are absolutely barren with their own-form pollen, though brought from a
distinct plant. In this respect they differ greatly from the long-styled English
plants of P. officinalis which were found by me to be moderately self-fertile;
but they agree in their behaviour with the German plants of P. officinalis
experimented on by Hildebrand.
Eighteen short-styled flowers legitimately fertilised yielded, as may be seen in
Table 3.20, 15 fruits, each having on an average 2.6 seeds. Four of these fruits
contained the highest possible number of seeds, namely 4, and four other fruits
contained each 3 seeds. The 12 illegitimately fertilised short-styled flowers
yielded 7 fruits, including on an average 1.86 seed; and one of these fruits
contained the maximum number of 4 seeds. This result is very surprising in
contrast with the absolute barrenness of the long-styled flowers when
illegitimately fertilised; and I was thus led to attend carefully to the degree
of self-fertility of the short-styled plants. A plant belonging to this form and
covered by a net bore 28 flowers besides those which had been artificially
fertilised, and of all these only two produced a fruit each including a single
seed. This high degree of self-sterility no doubt depended merely on the stigmas
not receiving any pollen, or not a sufficient quantity. For after carefully
covering all the long-styled plants in my garden, several short-styled plants
were left exposed to the visits of humble-bees, and their stigmas will thus have
received plenty of short-styled pollen; and now about half the flowers, thus
illegitimately fertilised, set fruit. I judge of this proportion partly from
estimation and partly from having examined three large branches, which had borne
31 flowers, and these produced 16 fruits. Of the fruits produced 233 were
collected (many being left ungathered), and these included on an average 1.82
seed. No less than 16 out of the 233 fruits included the highest possible number
of seeds, namely 4, and 31 included 3 seeds. So we see how highly fertile these
short-styled plants were when illegitimately fertilised with their own-form
pollen by the aid of bees.
The great difference in the fertility of the long and short-styled flowers, when
both are illegitimately fertilised, is a unique case, as far as I have observed
with heterostyled plants. The long-styled flowers when thus fertilised are
utterly barren, whilst about half of the short-styled ones produce capsules, and
these include a little above two-thirds of the number of seeds yielded by them
when legitimately fertilised. The sterility of the illegitimately fertilised
long-styled flowers is probably increased by the deteriorated condition of their
pollen; nevertheless this pollen was highly efficient when applied to the
stigmas of the short-styled flowers. With several species of Primula the short-
styled flowers are much more sterile than the long-styled, when both are
illegitimately fertilised; and it is a tempting view, as formerly remarked, that
this greater sterility of the short-styled flowers is a special adaptation to
check self-fertilisation, as their stigmas are eminently liable to receive their
own pollen. This view is even still more tempting in the case of the long-styled
form of Linum grandiflorum. On the other hand, with Pulmonaria angustifolia, it
is evident, from the corolla projecting obliquely upwards, that pollen is much
more likely to fall on, or to be carried by insects down to the stigma of the
short-styled than of the long-styled flowers; yet the short-styled instead of
being more sterile, as a protection against self-fertilisation, are far more
fertile than the long-styled, when both are illegitimately fertilised.
Pulmonaria azurea, according to Hildebrand, is not heterostyled. (3/12. 'Die
Geschlechter-Vertheilung bei den Pflanzen' 1867 page 37.)
[From an examination of dried flowers of Amsinckia spectabilis, sent me by
Professor Asa Gray, I formerly thought that this plant, a member of the
Boragineae, was heterostyled. The pistil varies to an extraordinary degree in
length, being in some specimens twice as long as in others, and the point of
insertion of the stamens likewise varies. But on raising many plants from seed,
I soon became convinced that the whole case was one of mere variability. The
first-formed flowers are apt to have stamens somewhat arrested in development,
with very little pollen in their anthers; and in such flowers the stigma
projects above the anthers, whilst generally it stands below and sometimes on a
level with them. I could detect no difference in the size of the pollen-grain or
in the structure of the stigma in the plants which differed most in the above
respects; and all of them, when protected from the access of insects, yielded
plenty of seeds. Again, from statements made by Vaucher, and from a hasty
inspection, I thought at first that the allied Anchusa arvensis and Echium
vulgare were heterostyled, but soon saw my error. From information given me, I
examined dried flowers of another member of the Boragineae, Arnebia
hispidissima, collected from several sites, and though the corolla, together
with the included organs, differed much in length, there was no sign of
heterostylism.]
Polygonum fagopyrum (Polygonaceae).
(FIGURE 3.7. Polygonum fagopyrum. (From H. Muller.)
Upper figure, the long-styled form; lower figure, the short-styled.
Some of the anthers have dehisced, others have not.)
Hildebrand has shown that this plant, the common Buck-wheat, is heterostyled.
(3/13. 'Die Geschlechter-Vertheilung' etc. 1867 page 34.) In the long-styled
form (Figure 3.7), the three stigmas project considerably above the eight short
stamens, and stand on a level with the anthers of the eight long stamens in the
short-styled form; and so it is conversely with the stigmas and stamens of this
latter form. I could perceive no difference in the structure of the stigmas in
the two forms. The pollen-grains of the short-styled form are to those of the
long-styled as 100 to 82 in diameter. This plant is therefore without doubt
heterostyled.
I experimented only in an imperfect manner on the relative fertility of the two
forms. Short-styled flowers were dragged several times over two heads of flowers
on long-styled plants, protected under a net, which were thus legitimately,
though not fully, fertilised. They produced 22 seeds, or 11 per flower-head.
Three flower-heads on long-styled plants received pollen in the same manner from
other long-styled plants, and were thus illegitimately fertilised. They produced
14 seeds, or only 4.66 per flower-head.
Two flower-heads on short-styled plants received pollen in like manner from
long-styled flowers, and were thus legitimately fertilised. They produced 8
seeds, or 4 per flower-head.
Four heads on short-styled plants similarly received pollen from other short-
styled plants, and were thus illegitimately fertilised. They produced 9 seeds,
or 2.25 per flower-head.
The results from fertilising the flower-heads in the above imperfect manner
cannot be fully trusted; but I may state that the four legitimately fertilised
flower-heads yielded on an average 7.50 seeds per head; whereas the seven
illegitimately fertilised heads yielded less than half the number, or on an
average only 3.28 seeds. The legitimately crossed seeds from the long-styled
flowers were finer than those from the illegitimately fertilised flowers on the
same plants, in the ratio of 100 to 82, as shown by the weights of an equal
number.
About a dozen plants, including both forms, were protected under nets, and early
in the season they produced spontaneously hardly any seeds, though at this
period the artificially fertilised flowers produced an abundance; but it is a
remarkable fact that later in the season, during September, both forms became
highly self-fertile. They did not, however, produce so many seeds as some
neighbouring uncovered plants which were visited by insects. Therefore the
flowers of neither form when left to fertilise themselves late in the season
without the aid of insects, are nearly so sterile as most other heterostyled
plants. A large number of insects, namely 41 kinds as observed by H. Muller,
visit the flowers for the sake of the eight drops of nectar. (3/14. 'Die
Befruchtung' etc. page 175 and 'Nature' January 1, 1874 page 166.) He infers
from the structure of the flowers that insects would be apt to fertilise them
both illegitimately as well as legitimately; but he is mistaken in supposing
that the long-styled flowers cannot spontaneously fertilise themselves.
Differently to what occurs in the other genera hitherto noticed, Polygonum,
though a very large genus, contains, as far as is at present known, only a
single heterostyled species, namely the present one. H. Muller in his
interesting description of several other species shows that P. bistorta is so
strongly proterandrous (the anthers generally falling off before the stigmas are
mature) that the flowers must be cross-fertilised by the many insects which
visit them. Other species bear much less conspicuous flowers which secrete
little or no nectar, and consequently are rarely visited by insects; these are
adapted for self-fertilisation, though still capable of cross-fertilisation.
According to Delpino, the Polygonaceae are generally fertilised by the wind,
instead of by insects as in the present genus.
[Leucosmia Burnettiana (Thymeliae).
As Professor Asa Gray has expressed his belief that this species and L.
acuminata, as well as some species in the allied genus Drymispermum, are
dimorphic or heterostyled (3/15. 'American Journal of Science' 1865 page 101 and
Seemann's 'Journal of Botany' volume 3 1865 page 305.), I procured from Kew,
through the kindness of Dr. Hooker, two dried flowers of the former species, an
inhabitant of the Friendly Islands in the Pacific. The pistil of the long-styled
form is to that of the short-styled as 100 to 86 in length; the stigma projects
just above the throat of the corolla, and is surrounded by five anthers, the
tips of which reach up almost to its base; and lower down, within the tubular
corolla, five other and rather smaller anthers are seated. In the short-styled
form, the stigma stands some way down the tube of the corolla, nearly on a level
with the lower anthers of the other form: it differs remarkably from the stigma
of the long-styled form, in being more papillose, and in being longer in the
ratio of 100 to 60. The anthers of the upper stamens in the short-styled form
are supported on free filaments, and project above the throat of the corolla,
whilst the anthers of the lower stamens are seated in the throat on a level with
the upper stamens of the other form. The diameters of a considerable number of
grains from both sets of anthers in both forms were measured, but they did not
differ in any trustworthy degree. The mean diameter of twenty-two grains from
the short-styled flower was to that of twenty-four grains from the long-styled,
as 100 to 99. The anthers of the upper stamens in the short-styled form appeared
to be poorly developed, and contained a considerable number of shrivelled grains
which were omitted in striking the above average. Notwithstanding the fact of
the pollen-grains from the two forms not differing in diameter in any
appreciable degree, there can hardly be a doubt from the great difference in the
two forms in the length of the pistil, and especially of the stigma, together
with its more papillose condition in the short-styled form, that the present
species is truly heterostyled. This case resembles that of Linum grandiflorum,
in which the sole difference between the two forms consists in the length of the
pistils and stigmas. From the great length of the tubular corolla of Leucosmia,
it is clear that the flowers are cross-fertilised by large Lepidoptera or by
honey-sucking birds, and the position of the stamens in two whorls one beneath
the other, which is a character that I have not seen in any other heterostyled
dimorphic plant, probably serves to smear the inserted organ thoroughly with
pollen.
Menyanthes trifoliata (Gentianeae).
This plant inhabits marshes: my son William gathered 247 flowers from so many
distinct plants, and of these 110 were long-styled, and 137 short-styled. The
pistil of the long-styled form is in length to that of the short-styled in the
ratio of about 3 to 2. The stigma of the former, as my son observed, is
decidedly larger than that of the short-styled; but in both forms it varies much
in size. The stamens of the short-styled are almost double the length of those
of the long-styled; so that their anthers stand rather above the level of the
stigma of the long-styled form. The anthers also vary much in size, but seem
often to be of larger size in the short-styled flowers. My son made with the
camera many drawings of the pollen-grains, and those from the short-styled
flowers were in diameter in nearly the ratio of 100 to 84 to those from the
long-styled flowers. I know nothing about the capacity for fertilisation in the
two forms; but short-styled plants, living by themselves in the gardens at Kew,
have produced an abundance of capsules, yet the seeds have never germinated; and
this looks as if the short-styled form was sterile with its own pollen.
Limnanthemum Indicum (Gentianeae).
This plant is mentioned by Mr. Thwaites in his Enumeration of the Plants of
Ceylon as presenting two forms; and he was so kind as to send me specimens
preserved in spirits. The pistil of the long-styled form is nearly thrice as
long (i.e. as 14 to 5) as that of the short-styled, and is very much thinner in
the ratio of about 3 to 5. The foliaceous stigma is more expanded, and twice as
large as that of the short-styled form. In the latter the stamens are about
twice as long as those of the long-styled, and their anthers are larger in the
ratio of 100 to 70. The pollen-grains, after having been long kept in spirits,
were of the same shape and size in both forms. The ovules, according to Mr.
Thwaites, are equally numerous (namely from 70 to 80) in the two forms.
Villarsia [sp.?] (Gentianeae).
Fritz Muller sent me from South Brazil dried flowers of this aquatic plant,
which is closely allied to Limnanthemum. In the long-styled
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