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Chapter I
HETEROSTYLED DIMORPHIC PLANTS: PRIMULACEAE.
Primula veris or the cowslip.
Differences in structure between the two forms.
Their degrees of fertility when legitimately and illegitimately united.
P. elatior, vulgaris, Sinensis, auricula, etc.
Summary on the fertility of the heterostyled species of Primula.
Homostyled species of Primula.
Hottonia palustris.
Androsace vitalliana.
(FIGURE 1.1. Primula veris.
Left: Long-styled form.
Right: Short-styled form.)
It has long been known to botanists that the common cowslip (Primula veris,
Brit. Flora, var. officinalis, Lin.) exists under two forms, about equally
numerous, which obviously differ from each other in the length of their pistils
and stamens. (1/1. This fact, according to Von Mohl 'Botanische Zeitung' 1863
page 326, was first observed by Persoon in the year 1794.) This difference has
hitherto been looked at as a case of mere variability, but this view, as we
shall presently see, is far from the true one. Florists who cultivate the
Polyanthus and Auricula have long been aware of the two kinds of flowers, and
they call the plants which display the globular stigma at the mouth of the
corolla, "pin-headed" or "pin-eyed," and those which display the anthers,
"thrum-eyed." (1/2. In Johnson's Dictionary, "thrum" is said to be the ends of
weavers' threads; and I suppose that some weaver who cultivated the Polyanthus
invented this name, from being struck with some degree of resemblance between
the cluster of anthers in the mouth of the corolla and the ends of his threads.)
I will designate the two forms as the long-styled and short-styled.
The pistil in the long-styled form is almost exactly twice as long as that of
the short-styled. The stigma stands in the mouth of the corolla or projects just
above it, and is thus externally visible. It stands high above the anthers,
which are situated halfway down the tube and cannot be easily seen. In the
short-styled form the anthers are attached near the mouth of the tube, and
therefore stand above the stigma, which is seated in about the middle of the
tubular corolla. The corolla itself is of a different shape in the two forms;
the throat or expanded portion above the attachment of the anthers being much
longer in the long-styled than in the short-styled form. Village children notice
this difference, as they can best make necklaces by threading and slipping the
corollas of the long-styled flowers into one another. But there are much more
important differences. The stigma in the long-styled form is globular; in the
short-styled it is depressed on the summit, so that the longitudinal axis of the
former is sometimes nearly double that of the latter. Although somewhat variable
in shape, one difference is persistent, namely, in roughness: in some specimens
carefully compared, the papillae which render the stigma rough were in the long-
styled form from twice to thrice as long as in the short-styled. The anthers do
not differ in size in the two forms, which I mention because this is the case
with some heterostyled plants. The most remarkable difference is in the pollen-
grains. I measured with the micrometer many specimens, both dry and wet, taken
from plants growing in different situations, and always found a palpable
difference. The grains distended with water from the short-styled flowers were
about .038 millimetres (10 to 11/7000 of an inch) in diameter, whilst those from
the long-styled were about .0254 millimetres (7/7000 of an inch), which is in
the ratio of 100 to 67. The pollen-grains therefore from the longer stamens of
the short-styled form are plainly larger than those from the shorter stamens of
the long-styled. When examined dry, the smaller grains are seen under a low
power to be more transparent than the larger grains, and apparently in a greater
degree than can be accounted for by their less diameter. There is also a
difference in shape, the grains from the short-styled plants being nearly
spherical, those from the long-styled being oblong with the angles rounded; this
difference disappears when the grains are distended with water. The long-styled
plants generally tend to flower a little before the short-styled: for instance,
I had twelve plants of each form growing in separate pots and treated in every
respect alike; and at the time when only a single short-styled plant was in
flower, seven of the long-styled had expanded their flowers.
We shall, also, presently see that the short-styled plants produce more seed
than the long-styled. It is remarkable, according to Professor Oliver, that the
ovules in the unexpanded and unimpregnated flowers of the latter are
considerably larger than those of the short-styled flowers (1/3. 'Natural
History Review' July 1862 page 237.); and this I suppose is connected with the
long-styled flowers producing fewer seeds, so that the ovules have more space
and nourishment for rapid development.
To sum up the differences:--The long-styled plants have a much longer pistil,
with a globular and much rougher stigma, standing high above the anthers. The
stamens are short; the grains of pollen smaller and oblong in shape. The upper
half of the tube of the corolla is more expanded. The number of seeds produced
is smaller and the ovules larger. The plants tend to flower first.
The short-styled plants have a short pistil, half the length of the tube of the
corolla, with a smooth depressed stigma standing beneath the anthers. The
stamens are long; the grains of pollen are spherical and larger. The tube of the
corolla is of uniform diameter except close to the upper end. The number of
seeds produced is larger.
I have examined a large number of flowers; and though the shape of the stigma
and the length of the pistil both vary, especially in the short-styled form, I
have never met with any transitional states between the two forms in plants
growing in a state of nature. There is never the slightest doubt under which
form a plant ought to be classed. The two kinds of flowers are never found on
the same individual plant. I marked many cowslips and primroses, and on the
following year all retained the same character, as did some in my garden which
flowered out of their proper season in the autumn. Mr. W. Wooler, of Darlington,
however, informs us that he has seen early blossoms on the Polyanthus, which
were not long-styled, but became so later in the season. (1/4. I have proved by
numerous experiments, hereafter to be given, that the Polyanthus is a variety of
Primula veris.) Possibly in this case the pistils may not have been fully
developed during the early spring. An excellent proof of the permanence of the
two forms may be seen in nursery-gardens, where choice varieties of the
Polyanthus are propagated by division; and I found whole beds of several
varieties, each consisting exclusively of the one or the other form. The two
forms exist in the wild state in about equal numbers: I collected 522 umbels
from plants growing in several stations, taking a single umbel from each plant;
and 241 were long-styled, and 281 short-styled. No difference in tint or size
could be perceived in the two great masses of flowers.
We shall presently see that most of the species of Primula exist under two
analogous forms; and it may be asked what is the meaning of the above-described
important differences in their structure? The question seems well worthy of
careful investigation, and I will give my observations on the cowslip in detail.
The first idea which naturally occurred to me was, that this species was tending
towards a dioecious condition; that the long-styled plants, with their longer
pistils, rougher stigmas, and smaller pollen-grains, were more feminine in
nature, and would produce more seed;--that the short-styled plants, with their
shorter pistils, longer stamens and larger pollen-grains, were more masculine in
nature. Accordingly, in 1860, I marked a few cowslips of both forms growing in
my garden, and others growing in an open field, and others in a shady wood, and
gathered and weighed the seed. In all the lots the short-styled plants yielded,
contrary to my expectation, most seed. Taking the lots together, the following
is the result:--
TABLE 1.1.
Column 1: Plant.
Column 2: Number of Plants.
Column 3: Number of Umbels Produced.
Column 4: Number of Capsules Produced.
Column 5: Weight of Seed In Grains.
Short-styled cowslips : 9 : 33 : 199 : 83.
Long-styled cowslips : 13 : 51 : 261 : 91.
If we compare the weight from an equal number of plants, and from an equal
number of umbels, and from an equal number of capsules of the two forms, we get
the following results:--
TABLE 1.2.
Column 1: Plant.
Column 2: Number of Plants.
Column 3: Weight of Seed in grains.
...
Column 4: Number of Umbels.
Column 5: Weight of Seed.
...
Column 6: Number of Capsules.
Column 7: Weight of Seed in grains.
Short-styled cowslips : 10 : 92 :: 100 : 251 :: 100 : 41.
Long-styled cowslips : 10 : 70 :: 100 : 178 :: 100 : 34.
So that, by all these standards of comparison, the short-styled form is the more
fertile; if we take the number of umbels (which is the fairest standard, for
large and small plants are thus equalised), the short-styled plants produce more
seed than the long-styled, in the proportion of nearly four to three.
In 1861 the trial was made in a fuller and fairer manner. A number of wild
plants had been transplanted during the previous autumn into a large bed in my
garden, and all were treated alike; the result was:--
TABLE 1.3.
Column 1: Plant.
Column 2: Number of Plants.
Column 3: Number of Umbels.
Column 4: Weight of Seed in grains.
Short-styled cowslips : 47 : 173 : 745.
Long-styled cowslips : 58 : 208 : 692.
These figures give us the following proportions:--
TABLE 1.4.
Column 1: Plant.
Column 2: Number of Plants.
Column 3: Weight of Seed in grains.
...
Column 4: Number of Umbels.
Column 5: Weight of Seed in grains.
Short-styled cowslips : 100 : 1585 :: 100 : 430.
Long-styled cowslips : 100 : 1093 :: 100 : 332.
The season was much more favourable this year than the last; the plants also now
grew in good soil, instead of in a shady wood or struggling with other plants in
the open field; consequently the actual produce of seed was considerably larger.
Nevertheless we have the same relative result; for the short-styled plants
produced more seed than the long-styled in nearly the proportion of three to
two; but if we take the fairest standard of comparison, namely, the product of
seeds from an equal number of umbels, the excess is, as in the former case,
nearly as four to three.
Looking to these trials made during two successive years on a large number of
plants, we may safely conclude that the short-styled form is more productive
than the long-styled form, and the same result holds good with some other
species of Primula. Consequently my anticipation that the plants with longer
pistils, rougher stigmas, shorter stamens and smaller pollen-grains, would prove
to be more feminine in nature, is exactly the reverse of the truth.
In 1860 a few umbels on some plants of both the long-styled and short-styled
form, which had been covered by a net, did not produce any seed, though other
umbels on the same plants, artificially fertilised, produced an abundance of
seed; and this fact shows that the mere covering in itself was not injurious.
Accordingly, in 1861, several plants were similarly covered just before they
expanded their flowers; these turned out as follows:--
TABLE 1.5.
Column 1: Plant.
Column 2: Number of Plants.
Column 3: Number of Umbels produced.
Column 4: Product of Seed.
Short-styled : 6 : 24 : 1.3 grain weight of seed, or about 50 in number.
Long-styled : 18 : 74 : Not one seed.
Judging from the exposed plants which grew all round in the same bed, and had
been treated in the same manner, excepting that they had been exposed to the
visits of insects, the above six short-styled plants ought to have produced 92
grains' weight of seed instead of only 1.3; and the eighteen long-styled plants,
which produced not one seed, ought to have produced above 200 grains' weight.
The production of a few seeds by the short-styled plants was probably due to the
action of Thrips or of some other minute insect. It is scarcely necessary to
give any additional evidence, but I may add that ten pots of Polyanthuses and
cowslips of both forms, protected from insects in my greenhouse, did not set one
pod, though artificially fertilised flowers in other pots produced an abundance.
We thus see that the visits of insects are absolutely necessary for the
fertilisation of Primula veris. If the corolla of the long-styled form had
dropped off, instead of remaining attached in a withered state to the ovarium,
the anthers attached to the lower part of the tube with some pollen still
adhering to them would have been dragged over the stigma, and the flowers would
have been partially self-fertilised, as is the case with Primula Sinensis
through this means. It is a rather curious fact that so trifling a difference as
the falling-off of the withered corolla, should make a very great difference in
the number of seeds produced by a plant if its flowers are not visited by
insects.
The flowers of the cowslip and of the other species of the genus secrete plenty
of nectar; and I have often seen humble bees, especially B. hortorum and
muscorum, sucking the former in a proper manner, though they sometimes bite
holes through the corolla. (1/5. H. Muller has also seen Anthophora pilipes and
a Bombylius sucking the flowers. 'Nature' December 10, 1874 page 111.) No doubt
moths likewise visit the flowers, as one of my sons caught Cucullia verbasci in
the act. The pollen readily adheres to any thin object which is inserted into a
flower. The anthers in the one form stand nearly, but not exactly, on a level
with the stigma of the other; for the distance between the anthers and stigma in
the short-styled form is greater than that in the long-styled, in the ratio of
100 to 90. This difference is the result of the anthers in the long-styled form
standing rather higher in the tube than does the stigma in the short-styled, and
this favours their pollen being deposited on it. It follows from the position of
the organs that if the proboscis of a dead humble-bee, or a thick bristle or
rough needle, be pushed down the corolla, first of one form and then of the
other, as an insect would do in visiting the two forms growing mingled together,
pollen from the long-stamened form adheres round the base of the object, and is
left with certainty on the stigma of the long-styled form; whilst pollen from
the short stamens of the long-styled form adheres a little way above the
extremity of the object, and some is generally left on the stigma of the other
form. In accordance with this observation I found that the two kinds of pollen,
which could easily be recognised under the microscope, adhered in this manner to
the proboscides of the two species of humble-bees and of the moth, which were
caught visiting the flowers; but some small grains were mingled with the larger
grains round the base of the proboscis, and conversely some large grains with
the small grains near the extremity of the proboscis. Thus pollen will be
regularly carried from the one form to the other, and they will reciprocally
fertilise one another. Nevertheless an insect in withdrawing its proboscis from
the corolla of the long-styled form cannot fail occasionally to leave pollen
from the same flower on the stigma; and in this case there might be self-
fertilisation. But this will be much more likely to occur with the short-styled
form; for when I inserted a bristle or other such object into the corolla of
this form, and had, therefore, to pass it down between the anthers seated round
the mouth of the corolla, some pollen was almost invariably carried down and
left on the stigma. Minute insects, such as Thrips, which sometimes haunt the
flowers, would likewise be apt to cause the self-fertilisation of both forms.
The several foregoing facts led me to try the effects of the two kinds of pollen
on the stigmas of the two forms. Four essentially different unions are possible;
namely, the fertilisation of the stigma of the long-styled form by its own-form
pollen, and by that of the short-styled; and the stigma of the short-styled form
by its own-form pollen, and by that of the long-styled. The fertilisation of
either form with pollen from the other form may be conveniently called a
LEGITIMATE UNION, from reasons hereafter to be made clear; and that of either
form with its own-form pollen an ILLEGITIMATE UNION. I formerly applied the term
"heteromorphic" to the legitimate unions, and "homomorphic" to the illegitimate
unions; but after discovering the existence of trimorphic plants, in which many
more unions are possible, these two terms ceased to be applicable. The
illegitimate unions of both forms might have been tried in three ways; for a
flower of either form may be fertilised with pollen from the same flower, or
with that from a another flower on the same plant, or with that from a distinct
plant of the same form. But to make my experiments perfectly fair, and to avoid
any evil result from self-fertilisation or too close interbreeding, I have
invariably employed pollen from a distinct plant of the same form for the
illegitimate unions of all the species; and therefore it may be observed that I
have used the term "own-form pollen" in speaking of such unions. The several
plants in all my experiments were treated in exactly the same manner, and were
carefully protected by fine nets from the access of insects, excepting Thrips,
which it is impossible to exclude. I performed all the manipulations myself, and
weighed the seeds in a chemical balance; but during many subsequent trials I
followed the more accurate plan of counting the seeds. Some of the capsules
contained no seeds, or only two or three, and these are excluded in the column
headed "good capsules" in several of the following tables:--
TABLE 1.6. Primula veris.
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Capsules produced.
Column 4: Number of good Capsules.
Column 5: Weight of Seed in grains.
Column 6: Calculated Weight of Seed from 100 good Capsules.
Long-styled by pollen of short-styled. Legitimate union :
22 : 15 : 14 : 8.8 : 62.
Long-styled by own-form pollen. Illegitimate union :
20 : 8 : 5 : 2.1 : 42.
Short-styled by pollen of long-styled. Legitimate union :
13 : 12 : 11 : 4.9 : 44.
Short-styled by own-form pollen. Illegitimate union :
15 : 8 : 6 : 1.8 : 30.
SUMMARY:
The two legitimate unions :
35 : 27 : 25 : 13.7 : 54.
The two illegitimate unions :
35 : 16 : 11 : 3.9 : 35.
The results may be given in another form (Table 1.7) by comparing, first, the
number of capsules, whether good or bad, or of the good alone, produced by 100
flowers of both forms when legitimately and illegitimately fertilised; secondly,
by comparing the weight of seed in 100 of these capsules, whether good or bad;
or, thirdly, in 100 of the good capsules.
TABLE 1.7. Primula veris.
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Capsules.
Column 4: Number of good Capsules.
Column 5: Weight of Seed in grains.
...
Column 6: Number of Capsules.
Column 7: Weight of Seed in grains.
...
Column 8: Number of good Capsules.
Column 9: Weight of Seed in grains.
The two legitimate unions :
100 : 77 : 71 : 39 :: 100 : 50 :: 100 : 54.
The two illegitimate unions :
100 : 45 : 31 : 11 :: 100 : 24 :: 100 : 35.
We here see that the long-styled flowers fertilised with pollen from the short-
styled yield more capsules, especially good ones (i.e. containing more than one
or two seeds), and that these capsules contain a greater proportional weight of
seeds than do the flowers of the long-styled when fertilised with pollen from a
distinct plant of the same form. So it is with the short-styled flowers, if
treated in an analogous manner. Therefore I have called the former method of
fertilisation a legitimate union, and the latter, as it fails to yield the full
complement of capsules and seeds, an illegitimate union. These two kinds of
union are graphically represented in Figure 1.2.
(FIGURE 1.2. Primula veris.
Graphic representation of two kinds of union between:
Left: Long-styled form.
Right: Short-styled form.)
If we consider the results of the two legitimate unions taken together and the
two illegitimate ones, as shown in Table 1.7, we see that the former compared
with the latter yielded capsules, whether containing many seeds or only a few,
in the proportion of 77 to 45, or as 100 to 58. But the inferiority of the
illegitimate unions is here perhaps too great, for on a subsequent occasion 100
long-styled and short-styled flowers were illegitimately fertilised, and they
together yielded 53 capsules: therefore the rate of 77 to 53, or as 100 to 69,
is a fairer one than that of 100 to 58. Returning to Table 1.7, if we consider
only the good capsules, those from the two legitimate unions were to those from
the two illegitimate in number as 71 to 31, or as 100 to 44. Again, if we take
an equal number of capsules, whether good or bad, from the legitimately and
illegitimately fertilised flowers, we find that the former contained seeds by
weight compared with the latter as 50 to 24, or as 100 to 48; but if all the
poor capsules are rejected, of which many were produced by the illegitimately
fertilised flowers, the proportion is 54 to 35, or as 100 to 65. In this and all
other cases, the relative fertility of the two kinds of union can, I think, be
judged of more truly by the average number of seeds per capsule than by the
proportion of flowers which yield capsules. The two methods might have been
combined by giving the average number of seeds produced by all the flowers which
were fertilised, whether they yielded capsules or not; but I have thought that
it would be more instructive always to show separately the proportion of flowers
which produced capsules, and the average number of apparently good seeds which
the capsules contained.
Flowers legitimately fertilised set seeds under conditions which cause the
almost complete failure of illegitimately fertilised flowers. Thus in the spring
of 1862 forty flowers were fertilised at the same time in both ways. The plants
were accidentally exposed in the greenhouse to too hot a sun, and a large number
of umbels perished. Some, however, remained in moderately good health, and on
these there were twelve flowers which had been fertilised legitimately, and
eleven which had been fertilised illegitimately. The twelve legitimate unions
yielded seven fine capsules, containing on an average each 57.3 good seeds;
whilst the eleven illegitimate unions yielded only two capsules, of which one
contained 39 seeds, but so poor, that I do not suppose one would have
germinated, and the other contained 17 fairly good seeds.
From the facts now given the superiority of a legitimate over an illegitimate
union admits of not the least doubt; and we have here a case to which no
parallel exists in the vegetable or, indeed, in the animal kingdom. The
individual plants of the present species, and as we shall see of several other
species of Primula, are divided into two sets or bodies, which cannot be called
distinct sexes, for both are hermaphrodites; yet they are to a certain extent
sexually distinct, for they require reciprocal union for perfect fertility. As
quadrupeds are divided into two nearly equal bodies of different sexes, so here
we have two bodies, approximately equal in number, differing in their sexual
powers and related to each other like males and females. There are many
hermaphrodite animals which cannot fertilise themselves, but most unite with
another hermaphrodite. So it is with numerous plants; for the pollen is often
mature and shed, or is mechanically protruded, before the flower's own stigma is
ready; and such flowers absolutely require the presence of another hermaphrodite
for sexual union. But with the cowslip and various other species of Primula
there is this wide difference, that one individual, though it can fertilise
itself imperfectly, must unite with another individual for full fertility; it
cannot, however, unite with any other individual in the same manner as an
hermaphrodite plant can unite with any other one of the same species; or as one
snail or earth-worm can unite with any other hermaphrodite individual. On the
contrary, an individual belonging to one form of the cowslip in order to be
perfectly fertile must unite with one of the other form, just as a male
quadruped must and can unite only with the female.
I have spoken of the legitimate unions as being fully fertile; and I am fully
justified in doing so, for flowers artificially fertilised in this manner
yielded rather more seeds than plants naturally fertilised in a state of nature.
The excess may be attributed to the plants having been grown separately in good
soil. With respect to the illegitimate unions, we shall best appreciate their
degree of lessened fertility by the following facts. Gartner estimated the
sterility of the unions between distinct species, in a manner which allows of a
strict comparison with the results of the legitimate and illegitimate unions of
Primula. (1/6. 'Versuche uber die Bastarderzeugung' 1849 page 216.) With P.
veris, for every 100 seeds yielded by the two legitimate unions, only 64 were
yielded by an equal number of good capsules from the two illegitimate unions.
With P. Sinensis, as we shall hereafter see, the proportion was nearly the same-
-namely, as 100 to 62. Now Gartner has shown that, on the calculation of
Verbascum lychnitis yielding with its own pollen 100 seeds, it yielded when
fertilised by the pollen of Verbascum Phoeniceum 90 seeds; by the pollen of
Verbascum nigrum, 63 seeds; by that of Verbascum blattaria, 62 seeds. So again,
Dianthus barbatus fertilised by the pollen of D. superbus yielded 81 seeds, and
by the pollen of D. japonicus 66 seeds, relatively to the 100 seeds produced by
its own pollen. We thus see--and the fact is highly remarkable--that with
Primula the illegitimate unions relatively to the legitimate are more sterile
than crosses between distinct species of other genera relatively to their pure
unions. Mr. Scott has given a still more striking illustration of the same fact:
he crossed Primula auricula with pollen of four other species (P. palinuri,
viscosa, hirsuta, and verticillata), and these hybrid unions yielded a larger
average number of seeds than did P. auricula when fertilised illegitimately with
its own-form pollen. (1/7. 'Journal of the Linnean Society Botany' volume 8 1864
page 93.)
The benefit which heterostyled dimorphic plants derive from the existence of the
two forms is sufficiently obvious, namely, the intercrossing of distinct plants
being thus ensured. (1/8. I have shown in my work on the 'Effects of Cross and
Self-fertilisation' how greatly the offspring from intercrossed plants profit in
height, vigour, and fertility.) Nothing can be better adapted for this end than
the relative positions of the anthers and stigmas in the two forms, as shown in
Figure 1.2; but to this whole subject I shall recur. No doubt pollen will
occasionally be placed by insects or fall on the stigma of the same flower; and
if cross-fertilisation fails, such self-fertilisation will be advantageous to
the plant, as it will thus be saved from complete barrenness. But the advantage
is not so great as might at first be thought, for the seedlings from
illegitimate unions do not generally consist of both forms, but all belong to
the parent form; they are, moreover, in some degree weakly in constitution, as
will be shown in a future chapter. If, however, a flower's own pollen should
first be placed by insects or fall on the stigma, it by no means follows that
cross-fertilisation will be thus prevented. It is well known that if pollen from
a distinct species be placed on the stigma of a plant, and some hours afterwards
its own pollen be placed on it, the latter will be prepotent and will quite
obliterate any effect from the foreign pollen; and there can hardly be a doubt
that with heterostyled dimorphic plants, pollen from the other form will
obliterate the effects of pollen from the same form, even when this has been
placed on the stigma a considerable time before. To test this belief, I placed
on several stigmas of a long-styled cowslip plenty of pollen from the same
plant, and after twenty-four hours added some from a short-styled dark-red
Polyanthus, which is a variety of the cowslip. From the flowers thus treated 30
seedlings were raised, and all these, without exception, bore reddish flowers;
so that the effect of pollen from the same form, though placed on the stigmas
twenty-four hours previously, was quite destroyed by that of pollen from a plant
belonging to the other form.
Finally, I may remark that of the four kinds of unions, that of the short-styled
illegitimately fertilised with its own-form pollen seems to be the most sterile
of all, as judged by the average number of seeds, which the capsules contained.
A smaller proportion, also, of these seeds than of the others germinated, and
they germinated more slowly. The sterility of this union is the more remarkable,
as it has already been shown that the short-styled plants yield a larger number
of seeds than the long-styled, when both forms are fertilised, either naturally
or artificially, in a legitimate manner.
In a future chapter, when I treat of the offspring from heterostyled dimorphic
and trimorphic plants illegitimately fertilised with their own-form pollen, I
shall have occasion to show that with the present species and several others,
equal-styled varieties sometimes appear.
Primula elatior, Jacq.
Bardfield oxlip of English authors.
This plant, as well as the last or cowslip (P. veris, vel officinalis), and the
primrose (P. vulgaris, vel acaulis) have been considered by some botanists as
varieties of the same species. But they are all three undoubtedly distinct, as
will be shown in the next chapter. The present species resembles to a certain
extent in general appearance the common oxlip, which is a hybrid between the
cowslip and primrose. Primula elatior is found in England only in two or three
of the eastern counties; and I was supplied with living plants by Mr. Doubleday,
who, as I believe, first called attention to its existence in England. It is
common in some parts of the Continent; and H. Muller has seen several kinds of
humble-bees and other bees, and Bombylius, visiting the flowers in North
Germany. (1/9. 'Die Befruchtung der Blumen' page 347.)
The results of my trials on the relative fertility of the two forms, when
legitimately and illegitimately fertilised, are given in Table 1.8.
TABLE 1.8. Primula elatior.
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of good Capsules produced.
Column 4: Maximum Number of Seeds in any one Capsule.
Column 5: Minimum Number of Seeds in any one Capsule.
Column 6: Average Number of Seeds per Capsule.
Long-styled by pollen of short-styled. Legitimate union :
10 : 6 : 62 : 34 : 46.5.
Long-styled by own-form pollen. Illegitimate union :
20 : 4 : 49* : 2 : 27.7.
(*These seeds were so poor and small that they could hardly have germinated.)
Short-styled by pollen of long-styled. Legitimate union:
10 : 8 : 61 : 37 : 47.7.
Short-styled by own-form pollen. Illegitimate union :
17 : 3 : 19 : 9 : 12.1.
SUMMARY:
The two legitimate unions together :
20 : 14 : 62 : 37 : 47.1.
The two illegitimate unions together :
37 : 7 : 49* : 2 : 35.5.
(*These seeds were so poor and small that they could hardly have germinated.)
If we compare the fertility of the two legitimate unions taken together with
that of the two illegitimate unions together, as judged by the proportional
number of flowers which when fertilised in the two methods yielded capsules, the
ratio is as 100 to 27; so that by this standard the present species is much more
sterile than P. veris, when both species are illegitimately fertilised. If we
judge of the relative fertility of the two kinds of unions by the average number
of seeds per capsule, the ratio is as 100 to 75. But this latter number is
probably much too high, as many of the seeds produced by the illegitimately
fertilised long-styled flowers were so small that they probably would not have
germinated, and ought not to have been counted. Several long-styled and short-
styled plants were protected from the access of insects, and must have been
spontaneously self-fertilised. They yielded altogether only six capsules,
containing any seeds; and their average number was only 7.8 per capsule. Some,
moreover, of these seeds were so small that they could hardly have germinated.
Herr W. Breitenbach informs me that he examined, in two sites near the Lippe (a
tributary of the Rhine), 894 flowers produced by 198 plants of this species; and
he found 467 of these flowers to be long-styled, 411 short-styled, and 16 equal-
styled. I have heard of no other instance with heterostyled plants of equal-
styled flowers appearing in a state of nature, though far from rare with plants
which have been long cultivated. It is still more remarkable that in eighteen
cases the same plant produced both long-styled and short-styled, or long-styled
and equal-styled flowers; and in two out of the eighteen cases, long-styled,
short-styled, and equal-styled flowers. The long-styled flowers greatly
preponderated on these eighteen plants,--61 consisting of this form, 15 of
equal-styled, and 9 of the short-styled form.
Primula vulgaris (var. acaulis, Linn.)
The primrose of English Writers.
(FIGURE 1.3. Outlines of pollen-grains of Primula vulgaris, distended with
water, much magnified and drawn under the camera lucida. The upper and smaller
grains from the long-styled form; the lower and larger grains from the short-
styled.)
Mr. J. Scott examined 100 plants growing near Edinburgh, and found 44 to be
long-styled, and 56 short-styled; and I took by chance 79 plants in Kent, of
which 39 were long-styled and 40 short-styled; so that the two lots together
consisted of 83 long-styled and 96 short-styled plants. In the long-styled form
the pistil is to that of the short-styled in length, from an average of five
measurements, as 100 to 51. The stigma in the long-styled form is conspicuously
more globose and much more papillose than in the short-styled, in which latter
it is depressed on the summit; it is equally broad in the two forms. In both it
stands nearly, but not exactly, on a level with the anthers of the opposite
form; for it was found, from an average of 15 measurements, that the distance
between the middle of the stigma and the middle of the anthers in the short-
styled form is to that in the long-styled as 100 to 93. The anthers do not
differ in size in the two forms. The pollen-grains from the short-styled flowers
before they were soaked in water were decidedly broader, in proportion to their
length, than those from the long-styled; after being soaked they were relatively
to those from the long-styled as 100 to 71 in diameter, and more transparent. A
large number of flowers from the two forms were compared, and 12 of the finest
flowers from each lot were measured, but there was no sensible difference
between them in size. Nine long-styled and eight short-styled plants growing
together in a state of nature were marked, and their capsules collected after
they had been naturally fertilised; and the seeds from the short-styled weighed
exactly twice as much as those from an equal number of long-styled plants. So
that the primrose resembles the cowslip in the short-styled plants, being the
more productive of the two forms. The results of my trials on the fertility of
the two forms, when legitimately and illegitimately fertilised, are given in
Table 1.9.
TABLE 1.9. Primula vulgaris.
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of good Capsules produced.
Column 4: Maximum Number of Seeds in any one Capsule.
Column 5: Minimum Number of Seeds in any one Capsule.
Column 6: Average Number of Seeds per Capsule.
Long-styled by pollen of short-styled. Legitimate union :
12 : 11 : 77 : 47 : 66.9.
Long-styled by own-form pollen. Illegitimate union :
21 : 14 : 66 : 30 : 52.2.
Short-styled by pollen of long-styled. Legitimate union:
8 : 7 : 75 : 48 : 65.0.
Short-styled by own-form pollen. Illegitimate union :
18 : 7 : 43 : 5 : 18.8 (This average is perhaps rather too low).
SUMMARY:
The two legitimate unions together :
20 : 18 : 77 : 47 : 66.0.
The two illegitimate unions together :
39 : 21 : 66 : 5 : 35.5 (This average is perhaps rather too low).
We may infer from this table that the fertility of the two legitimate unions
taken together is to that of the two illegitimate unions together, as judged by
the proportional number of flowers which when fertilised in the two methods
yielded capsules, as 100 to 60. If we judge by the average number of seeds per
capsule produced by the two kinds of unions, the ratio is as 100 to 54; but this
latter figure is perhaps rather too low. It is surprising how rarely insects can
be seen during the day visiting the flowers, but I have occasionally observed
small kinds of bees at work; I suppose, therefore, that they are commonly
fertilised by nocturnal Lepidoptera. The long-styled plants when protected from
insects yield a considerable number of capsules, and they thus differ remarkably
from the same form of the cowslip, which is quite sterile under the same
circumstances. Twenty-three spontaneously self-fertilised capsules from this
form contained, on an average, 19.2 seeds. The short-styled plants produced
fewer spontaneously self-fertilised capsules, and fourteen of them contained
only 6.2 seeds per capsule. The self-fertilisation of both forms was probably
aided by Thrips, which abounded within the flowers; but these minute insects
could not have placed nearly sufficient pollen on the stigmas, as the
spontaneously self-fertilised capsules contained much fewer seeds, on an
average, than those (as may be seen in Table 1.9.) which were artificially
fertilised with their own-form pollen. But this difference may perhaps be
attributed in part to the flowers in the table having been fertilised with
pollen from a distinct plant belonging to the same form; whilst those which were
spontaneously self-fertilised no doubt generally received their own pollen. In a
future part of this volume some observations will be given on the fertility of a
red-coloured variety of the primrose.
Primula Sinensis.
In the long-styled form the pistil is about twice as long as that of the short-
styled, and the stamens differ in a corresponding, but reversed, manner. The
stigma is considerably more elongated and rougher than that of the short-styled,
which is smooth and almost spherical, being somewhat depressed on the summit;
but the stigma varies much in all its characters, the result, probably, of
cultivation. The pollen-grains of the short-styled form, according to
Hildebrand, are 7 divisions of the micrometer in length and 5 in breadth;
whereas those of the long-styled are only 4 in length and 3 in breadth. (1/10.
After the appearance of my paper this author published some excellent
observations on the present species 'Botanische Zeitung' January 1, 1864, and he
shows that I erred greatly about the size of the pollen-grains in the two forms.
I suppose that by mistake I measured twice over pollen-grains from the same
form.) The grains, therefore, of the short-styled are to those of the long-
styled in length as 100 to 57. Hildebrand also remarked, as I had done in the
case of P. veris, that the smaller grains from the long-styled are much more
transparent than the larger ones from the short-styled form. We shall hereafter
see that this cultivated plant varies much in its dimorphic condition and is
often equal-styled. Some individuals may be said to be sub-heterostyled; thus in
two white-flowered plants the pistil projected above the stamens, but in one of
them it was longer and had a more elongated and rougher stigma, than in the
other; and the pollen-grains from the latter were to those from the plant with a
more elongated pistil only as 100 to 88 in diameter, instead of as 100 to 57.
The corolla of the long-styled and short-styled form differs in shape, in the
same manner as in P. veris. The long-styled plants tend to flower before the
short-styled. When both forms were legitimately fertilised, the capsules from
the short-styled plants contained, on an average, more seeds than those from the
long-styled, in the ratio of 12.2 to 9.3 by weight, that is, as 100 to 78. In
Table 1.10 we have the results of two sets of experiments tried at different
periods.
TABLE 1.10. Primula Sinensis.
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of good Capsules produced.
Column 4: Average Weight of Seeds per Capsule.
...
Column 5: Average Number of Seeds per Capsule as ascertained on a subsequent
occasion.
Long-styled by pollen of short-styled. Legitimate union :
24 : 16 : 0.58 :: 50.
Long-styled by own-form pollen. Illegitimate union :
20 : 13 : 0.45 :: 35.
Short-styled by pollen of long-styled. Legitimate union:
8 : 8 : 0.76 :: 64.
Short-styled by own-form pollen. Illegitimate union :
7 : 4 : 0.23 :: 25.
SUMMARY:
The two legitimate unions together :
32 : 24 : 0.64 :: 57.
The two illegitimate unions together :
27 : 17 : 0.40 :: 30.
The fertility, therefore, of the two legitimate unions together to that of the
two illegitimate unions, as judged by the proportional number of flowers which
yielded capsules, is as 100 to 84. Judging by the average weight of seeds per
capsule produced by the two kinds of unions, the ratio is as 100 to 63. On
another occasion a large number of flowers of both forms were fertilised in the
same manner, but no account of their number was kept. The seeds, however, were
carefully counted, and the averages are shown in the right hand column. The
ratio for the number of seeds produced by the two legitimate compared with the
two illegitimate unions is here 100 to 53, which is probably more accurate than
the foregoing one of 100 to 63.
TABLE 1.11. Primula Sinensis (from Hildebrand).
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of good Capsules produced.
Column 4: Average Number of Seeds per Capsule.
Long-styled by pollen of short-styled. Legitimate union :
14 : 14 : 41.
Long-styled by own-form pollen, from a distinct plant. Illegitimate union :
26 : 26 : 18.
Long-styled by pollen from same flower. Illegitimate union :
27 : 21 : 17.
Short-styled by pollen of long-styled. Legitimate union:
14 : 14 : 44.
Short-styled by own-form pollen, from a distinct plant. Illegitimate union :
16 : 16 : 20.
Short-styled by pollen from the same flower. Illegitimate union :
21 : 11 : 8.
SUMMARY:
The two legitimate unions together :
28 : 28 : 43.
The two illegitimate unions together (own-form pollen):
42 : 42 : 18.
The two illegitimate unions together (pollen from the same flower ):
48 : 32 : 13.
Hildebrand in the paper above referred to gives the results of his experiments
on the present species; and these are shown in a condensed form in Table 1.11.
Besides using for the illegitimate unions pollen from a distinct plant of the
same form, as was always done by me, he tried, in addition, the effects of the
plant's own pollen. He counted the seeds.
It is remarkable that here all the flowers which were fertilised legitimately,
as well as those fertilised illegitimately with pollen from a distinct plant
belonging to the same form, yielded capsules; and from this fact it might be
inferred that the two forms were reciprocally much more fertile in his case than
in mine. But his illegitimately fertilised capsules from both forms contained
fewer seeds relatively to the legitimately fertilised capsules than in my
experiments; for the ratio in his case is as 42 to 100, instead of, as in mine,
as 53 to 100. Fertility is a very variable element with most plants, being
determined by the conditions to which they are subjected, of which fact I have
observed striking instances with the present species; and this may account for
the difference between my results and those of Hildebrand. His plants were kept
in a room, and perhaps were grown in too small pots or under some other
unfavourable conditions, for his capsules in almost every case contained a
smaller number of seeds than mine, as may be seen by comparing the right hand
columns in Tables 1.10 and 1.11.
The most interesting point in Hildebrand's experiments is the difference in the
effects of illegitimate fertilisation with a flower's own pollen, and with that
from a distinct plant of the same form. In the latter case all the flowers
produced capsules, whilst only 67 out of 100 of those fertilised with their own
pollen produced capsules. The self-fertilised capsules also contained seeds, as
compared with capsules from flowers fertilised with pollen from a distinct plant
of the same form, in the ratio of 72 to 100.
In order to ascertain how far the present species was spontaneously self-
fertile, five long-styled plants were protected by me from insects; and they
bore up to a given period 147 flowers which set 62 capsules; but many of these
soon fell off, showing that they had not been properly fertilised. At the same
time five short-styled plants were similarly treated, and they bore 116 flowers
which ultimately produced only seven capsules. On another occasion 13 protected
long-styled plants yielded by weight 25.9 grains of spontaneously self-
fertilised seeds. At the same time seven protected short-styled plants yielded
only half-a-grain weight of seeds. Therefore the long-styled plants yielded
nearly 24 times as many spontaneously self-fertilised seeds as did the same
number of short-styled plants. The chief cause of this great difference appears
to be, that when the corolla of a long-styled plant falls off, the anthers, from
being situated near the bottom of the tube are necessarily dragged over the
stigma and leave pollen on it, as I saw when I hastened the fall of nearly
withered flowers; whereas in the short-styled flowers, the stamens are seated at
the mouth of the corolla, and in falling off do not brush over the lowly-seated
stigmas. Hildebrand likewise protected some long-styled and short-styled plants,
but neither ever yielded a single capsule. He thinks that the difference in our
results may be accounted for by his plants having been kept in a room and never
having been shaken; but this explanation seems to me doubtful; his plants were
in a less fertile condition than mine, as shown by the difference in the number
of seeds produced, and it is highly probable that their lessened fertility would
have interfered with especial force with their capacity for producing self-
fertilised seeds.
[Primula auricula. (1/11. According to Kerner our garden auriculas are descended
from P. pubescens, Jacq., which is a hybrid between the true P. auricula and
hirsuta. This hybrid has now been propagated for about 300 years, and produces,
when legitimately fertilised, a large number of seeds; the long-styled forms
yielding an average number of 73, and the short-styled 98 seeds per capsule: see
his "Geschichte der Aurikel" 'Zeitschr. des Deutschen und Oest. Alpen-Vereins'
Band 6 page 52. Also 'Die Primulaceen-Bastarten' 'Oest. Botanische Zeitschrift'
1835 Numbers 3, 4 and 5.)
This species is heterostyled, like the preceding ones; but amongst the varieties
distributed by florists the long-styled form is rare, as it is not valued. There
is a much greater relative inequality in the length of the pistil and stamens in
the two forms of the auricula than in the cowslip; the pistil in the long-styled
being nearly four times as long as that in the short-styled, in which it is
barely longer than the ovarium. The stigma is nearly of the same shape in both
forms, but is rougher in the long-styled, though the difference is not so great
as between the two forms of the cowslip. In the long-styled plants the stamens
are very short, rising but little above the ovarium. The pollen-grains of these
short stamens, when distended with water, were barely 5/6000 of an inch in
diameter, whereas those from the long stamens of the short-styled plants were
barely 7/6000, showing a relative difference of about 71 to 100. The smaller
grains of the long-styled plant are also much more transparent, and before
distention with water more triangular in outline than those of the other form.
Mr. Scott compared ten plants of both forms growing under similar conditions,
and found that, although the long-styled plant produced more umbels and more
capsules than the short-styled, yet they yielded fewer seeds, in the ratio of 66
to 100. (1/12. 'Journal of the Linnean Society Botany' volume 8 1864 page 86.)
Three short-styled plants were protected by me from the access of insects, and
they did not produce a single seed. Mr. Scott protected six plants of both
forms, and found them excessively sterile. The pistil of the long-styled form
stands so high above the anthers, that it is scarcely possible that pollen
should reach the stigma without some aid; and one of Mr. Scott's long-styled
plants which yielded a few seeds (only 18 in number) was infested by aphides,
and he does not doubt that these had imperfectly fertilised it.
I tried a few experiments by reciprocally fertilising the two forms in the same
manner as before, but my plants were unhealthy, so I will give, in a condensed
form, the results of Mr. Scott's experiments. For fuller particulars with
respect to this and the five following species, the paper lately referred to may
be consulted. In each case the fertility of the two legitimate unions, taken
together, is compared with that of the two illegitimate unions together, by the
same two standards as before, namely, by the proportional number of flowers
which produced good capsules, and by the average number of seeds per capsule.
The fertility of the legitimate unions is always taken at 100.
By the first standard, the fertility of the two legitimate unions of the
auricula is to that of the two illegitimate unions as 100 to 80; and by the
second standard as 100 to 15.
Primula Sikkimensis.
According to Mr. Scott, the pistil of the long-styled form is fully four times
as long as that of the short-styled, but their stigmas are nearly alike in shape
and roughness. The stamens do not differ so much in relative length as the
pistils. The pollen-grains differ in a marked manner in the two forms; "those of
the long-styled plants are sharply triquetrous, smaller, and more transparent
than those of the short-styled, which are of a bluntly triangular form." The
fertility of the two legitimate unions to that of the two illegitimate unions is
by the first standard as 100 to 95, and by the second standard as 100 to 31.
Primula cortusoides.
The pistil of the long-styled form is about thrice as long as that of the short-
styled, the stigma being double as long and covered with much longer papillae.
The pollen-grains of the short-styled form are, as usual, "larger, less
transparent, and more bluntly triangular than those from the long-styled
plants." The fertility of the two legitimate unions to that of the two
illegitimate unions is by the first standard as 100 to 74, and by the second
standard as 100 to 66.
Primula involucrata.
The pistil of the long-styled form is about thrice as long as that of the short-
styled; the stigma of the former is globular and closely beset with papillae,
whilst that of the short-styled is smooth and depressed on the apex. The pollen-
grains of the two forms differ in size and transparency as before, but not in
shape. The fertility of the two legitimate to that of the two illegitimate
unions is by the first standard as 100 to 72; and by the second standard as 100
to 47.
Primula farinosa.
According to Mr. Scott, the pistil of the long-styled form is only about twice
as long as that of the short-styled. The stigmas of the two forms differ but
little in shape. The pollen-grains differ in the usual manner in size, but not
in form. The fertility of the two legitimate to that of the two illegitimate
unions is by the first standard as 100 to 71, and by the second standard as 100
to 44.]
SUMMARY ON THE FOREGOING HETEROSTYLED SPECIES OF PRIMULA.
TABLE 1.12. Summary on the Fertility of the two Legitimate Unions, compared with
that of the two Illegitimate Unions, in the genus Primula. The former taken at
100.
Column 1: Name of Species.
Column 2: Illegitimate Unions, Judged of by the Proportional Number of Flowers
which produced Capsules.
Column 3: Illegitimate Unions, Judged of by the Average Number (or Weight in
some cases) of Seeds per Capsule.
Primula veris : 69 : 65.
Primula elatior : 27 : 75 (Probably too high).
Primula vulgaris : 60 : 54 (Perhaps too low).
Primula Sinensis : 84 : 63.
Primula Sinensis (second trial) : ? : 53.
Primula Sinensis (after Hildebrand) : 100 : 42.
Primula auricula (Scott) : 80 : 15.
Primula Sikkimensis (Scott): 95 : 31.
Primula cortusoides (Scott): 74 : 66.
Primula involucrata (Scott): 72 : 48.
Primula farinosa (Scott): 71 : 44.
Average of the nine species : 88.4 : 61.8.
The fertility of the long-and short-styled plants of the above species of
Primula, when the two forms are fertilised legitimately, and illegitimately with
pollen of the same form taken from a distinct plant, has now been given. The
results are seen in Table 1.12; the fertility being judged by two standards,
namely, by that of the proportional number of flowers which yielded capsules,
and by that of the average number of seeds per capsule. But for full accuracy
many more observations, under varied conditions, would be requisite.
With plants of all kinds some flowers generally fail to produce capsules, from
various accidental causes; but this source of error has been eliminated, as far
as possible, in all the previous cases, by the manner in which the calculations
have been made. Supposing, for instance, that 20 flowers were fertilised
legitimately and yielded 18 capsules, and that 30 flowers were fertilised
illegitimately and yielded 15 capsules, we may assume that on an average an
equal proportion of the flowers in both lots would fail to produce capsules from
various accidental causes; and the ratio of 18/20 to 15/30, or as 100 to 56 (in
whole numbers), would show the proportional number of capsules due to the two
methods of fertilisation; and the number 56 would appear in the left-hand column
of Table 1.12, and in my other tables. With respect to the average number of
seeds per capsule hardly anything need be said: supposing that the legitimately
fertilised capsules contained, on an average, 50 seeds, and the illegitimately
fertilised capsules 25 seeds; then as 50 is to 25 so is 100 to 50; and the
latter number would appear in the right hand column.
It is impossible to look at the above table and doubt that the legitimate unions
between the two forms of the above nine species of Primula are much more fertile
than the illegitimate unions; although in the latter case pollen was always
taken from a distinct plant of the same form. There is, however, no close
correspondence in the two rows of figures, which give, according to the two
standards, the difference of fertility between the legitimate and illegitimate
unions. Thus all the flowers of P. Sinensis which were illegitimately fertilised
by Hildebrand produced capsules; but these contained only 42 per cent of the
number of seeds yielded by the legitimately fertilised capsules. So again, 95
per cent of the illegitimately fertilised flowers of P. Sikkimensis produced
capsules; but these contained only 31 per cent of the number of seeds in the
legitimate capsules. On the other hand, with P. elatior only 27 per cent of the
illegitimately fertilised flowers yielded capsules; but these contained nearly
75 per cent of the legitimate number of seeds. It appears that the setting of
the flowers, that is, the production of capsules whether good or bad, is not so
much influenced by legitimate and illegitimate fertilisation as is the number of
seeds which the capsules contain. For, as may be seen at the bottom of Table
1.12, 88.4 per cent of the illegitimately fertilised flowers yielded capsules;
but these contained only 61.8 per cent of seeds, in comparison, in each case,
with the legitimately fertilised flowers and capsules of the same species.
There is another point which deserves notice, namely, the relative degree of
infertility in the several species of the long-styled and short-styled flowers,
when both are illegitimately fertilised. The data may be found in the earlier
tables, and in those given by Mr. Scott in the Paper already referred to. If we
call the number of seeds per capsule produced by the illegitimately fertilised
long-styled flowers 100, the seeds from the illegitimately fertilised short-
styled flowers will be represented by the following numbers (Table 1.a.):--
TABLE 1.a.
Primula veris : 71.
Primula elatior : 44 (Probably too low).
Primula vulgaris : 36 (Perhaps too low).
Primula Sinensis : 71.
Primula auricula : 119.
Primula Sikkimensis : 57.
Primula cortusoides : 93.
Primula involucrata : 74.
Primula farinosa : 63.
We thus see that, with the exception of P. auricula, the long-styled flowers of
all nine species are more fertile than the short-styled flowers, when both forms
are illegitimately fertilised. Whether P. auricula really differs from the other
species in this respect I can form no opinion, as the result may have been
accidental. The degree of self-fertility of a plant depends on two elements,
namely, on the stigma receiving its own pollen and on its more or less efficient
action when placed there. Now as the anthers of the short-styled flowers of
several species of Primula stand directly above the stigma, their pollen is more
likely to fall on it, or to be carried down to it by insects, than in the case
of the long-styled form. It appears probable, therefore, at first sight, that
the lessened capacity of the short-styled flowers to be fertilised with their
own pollen, is a special adaptation for counteracting their greater liability to
receive their own pollen, and thus for checking self-fertilisation. But from
facts with respect to other species hereafter to be given, this view can hardly
be admitted. In accordance with the above liability, when some of the species of
Primula were allowed to fertilise themselves spontaneously under a net, all
insects being excluded, except such minute ones as Thrips, the short-styled
flowers, notwithstanding their greater innate self-sterility, yielded more seed
than did the long-styled. None of the species, however, when insects were
excluded, made a near approach to full fertility. But the long-styled form of P.
Sinensis gave, under these circumstances, a considerable number of seeds, as the
corolla in falling off drags the anthers, which are seated low down in the tube,
over the stigma, and thus leaves plenty of pollen on it.
HOMOSTYLED SPECIES OF PRIMULA.
It has now been shown that nine of the species in this genus exist under two
forms, which differ not only in structure but in function. Besides these Mr.
Scott enumerates 27 other species which are heterostyled (1/13. H. Muller has
given in 'Nature' December 10, 1874 page 110, a drawing of one of these species,
viz. The alpine P. villosa, and shows that it is fertilised exclusively by
Lepidoptera.); and to these probably others will be hereafter added.
Nevertheless, some species are homostyled; that is, they exist only under a
single form; but much caution is necessary on this head, as several species when
cultivated are apt to become equal-styled. Mr. Scott believes that P. Scotica,
verticillata, a variety of Sibirica, elata, mollis, and longiflora, are truly
homostyled; and to these may be added, according to Axell, P. stricta. (1/14.
Koch was aware that this species was homostyled: see "Treviranus uber Dichogamie
nach Sprengel und Darwin" 'Botanische Zeitung' January 2, 1863 page 4.) Mr.
Scott experimented on P. Scotica, mollis, and verticillata, and found that their
flowers yielded an abundance of seeds when fertilised with their own pollen.
This shows that they are not heterostyled in function. P. Scotica is, however,
only moderately fertile when insects are excluded, but this depends merely on
the coherent pollen not readily falling on the stigma without their aid. Mr.
Scott also found that the capsules of P. verticillata contained rather more seed
when the flowers were fertilised with pollen from a distinct plant than when
with their own pollen; and from this fact he infers that they are sub-
heterostyled in function, though not in stru
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