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Chapter V
GERANIACEAE, LEGUMINOSAE, ONAGRACEAE, ETC.
Pelargonium zonale, a cross between plants propagated by cuttings does
no good.
Tropaeolum minus.
Limnanthes douglasii.
Lupinus luteus and pilosus.
Phaseolus multiflorus and vulgaris.
Lathyrus odoratus, varieties of, never naturally intercross in England.
Pisum sativum, varieties of, rarely intercross, but a cross between them
highly beneficial.
Sarothamnus scoparius, wonderful effects of a cross.
Ononis minutissima, cleistogene flowers of.
Summary on the Leguminosae.
Clarkia elegans.
Bartonia aurea.
Passiflora gracilis.
Apium petroselinum.
Scabiosa atropurpurea.
Lactuca sativa.
Specularia speculum.
Lobelia ramosa, advantages of a cross during two generations.
Lobelia fulgens.
Nemophila insignis, great advantages of a cross.
Borago officinalis.
Nolana prostrata.
13. GERANIACEAE.--Pelargonium zonale.
This plant, as a general rule, is strongly proterandrous, and is
therefore adapted for cross-fertilisation by the aid of insects. (5/1.
Mr. J. Denny, a great raiser of new varieties of pelargoniums, after
stating that this species is proterandrous, adds 'The Florist and
Pomologist' January 1872 page 11, "there are some varieties, especially
those with petals of a pink colour, or which possess a weakly
constitution, where the pistil expands as soon as or even before the
pollen-bag bursts, and in which also the pistil is frequently short, so
when it expands it is smothered as it were by the bursting anthers;
these varieties are great seeders, each pip being fertilised by its own
pollen. I would instance Christine as an example of this fact." We have
here an interesting case of variability in an important functional
point.) Some flowers on a common scarlet variety were self-fertilised,
and other flowers were crossed with pollen from another plant; but no
sooner had I done so, than I remembered that these plants had been
propagated by cuttings from the same stock, and were therefore parts in
a strict sense of the same individual. Nevertheless, having made the
cross I resolved to save the seeds, which, after germinating on sand,
were planted on the opposite sides of three pots. In one pot the
quasi-crossed plant was very soon and ever afterwards taller and finer
than the self-fertilised. In the two other pots the seedlings on both
sides were for a time exactly equal; but when the self-fertilised plants
were about 10 inches in height, they surpassed their antagonists by a
little, and ever afterwards showed a more decided and increasing
advantage; so that the self-fertilised plants, taken altogether, were
somewhat superior to the quasi-crossed plants. In this case, as in that
of the Origanum, if individuals which have been asexually propagated
from the same stock, and which have been long subjected to the same
conditions, are crossed, no advantage whatever is gained.
Several flowers on another plant of the same variety were fertilised
with pollen from the younger flowers on the same plant, so as to avoid
using the old and long-shed pollen from the same flower, as I thought
that this latter might be less efficient than fresh pollen. Other
flowers on the same plant were crossed with fresh pollen from a plant
which, although closely similar, was known to have arisen as a distinct
seedling. The self-fertilised seeds germinated rather before the others;
but as soon as I got equal pairs they were planted on the opposite sides
of four pots.
TABLE 5/49. Pelargonium zonale.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 22 3/8 : 25 5/8.
Pot 1 : 19 6/8 : 12 4/8.
Pot 2 : 15 : 19 6/8.
Pot 2 : 12 2/8 : 22 3/8.
Pot 3 : 30 5/8 : 19 4/8.
Pot 3 : 18 4/8 : 7 4/8.
Pot 4 : 38 : 9 1/8.
Total : 156.50 : 116.38.
When the two lots of seedlings were between 4 and 5 inches in height
they were equal, excepting in Pot 4, in which the crossed plant was much
the tallest. When between 11 and 14 inches in height, they were measured
to the tips of their uppermost leaves; the crossed averaged 13.46, and
the self-fertilised 11.07 inches in height, or as 100 to 82. Five months
later they were again measured in the same manner, and the results are
given in Table 5/49.
The seven crossed plants now averaged 22.35, and the seven
self-fertilised 16.62 inches in height, or as 100 to 74. But from the
great inequality of the several plants, the result is less trustworthy
than in most other cases. In Pot 2 the two self-fertilised plants always
had an advantage, except whilst quite young over the two crossed plants.
As I wished to ascertain how these plants would behave during a second
growth, they were cut down close to the ground whilst growing freely.
The crossed plants now showed their superiority in another way, for only
one out of the seven was killed by the operation, whilst three of the
self-fertilised plants never recovered. There was, therefore, no use in
keeping any of the plants excepting those in Pots 1 and 3; and in the
following year the crossed plants in these two pots showed during their
second growth nearly the same relative superiority over the
self-fertilised plants as before.
Tropaeolum minus.
The flowers are proterandrous, and are manifestly adapted for
cross-fertilisation by insects, as shown by Sprengel and Delpino. Twelve
flowers on some plants growing out of doors were crossed with pollen
from a distinct plant and produced eleven capsules, containing
altogether twenty-four good seeds. Eighteen flowers were fertilised with
their own pollen and produced only eleven capsules, containing
twenty-two good seeds; so that a much larger proportion of the crossed
than of the self-fertilised flowers produced capsules, and the crossed
capsules contained rather more seed than the self-fertilised in the
ratio of 100 to 92. The seeds from the self-fertilised capsules were
however the heavier of the two, in the ratio of 100 to 87.
Seeds in an equal state of germination were planted on the opposite
sides of four pots, but only the two tallest plants on each side of each
pot were measured to the tops of their stems. The pots were placed in
the greenhouse, and the plants trained up sticks, so that they ascended
to an unusual height. In three of the pots the crossed plants flowered
first, but in the fourth at the same time with the self-fertilised. When
the seedlings were between 6 and 7 inches in height, the crossed began
to show a slight advantage over their opponents. When grown to a
considerable height the eight tallest crossed plants averaged 44.43, and
the eight tallest self-fertilised plants 37.34 inches, or as 100 to 84.
When their growth was completed they were again measured, as shown in
Table 5/50.
TABLE 5/50. Tropaeolum minus.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 65 : 31.
Pot 1 : 50 : 45.
Pot 2 : 69 : 42.
Pot 2 : 35 : 45.
Pot 3 : 70 : 50 4/8.
Pot 3 : 59 4/8 : 55 4/8.
Pot 4 : 61 4/8 : 37 4/8.
Pot 4 : 57 4/8 : 61 4/8.
Total : 467.5 : 368.0.
The eight tallest crossed plants now averaged 58.43, and the eight
tallest self-fertilised plants 46 inches in height, or as 100 to 79.
There was also a great difference in the fertility of the two lots which
were left uncovered in the greenhouse. On the 17th of September the
capsules from all the plants were gathered, and the seeds counted. The
crossed plants yielded 243, whilst the same number of self-fertilised
plants yielded only 155 seeds, or as 100 to 64.
Limnanthes douglasii.
Several flowers were crossed and self-fertilised in the usual manner,
but there was no marked difference in the number of seeds which they
yielded. A vast number of spontaneously self-fertilised capsules were
also produced under the net. Seedlings were raised in five pots from the
above seeds, and when the crossed were about 3 inches in height they
showed a slight advantage over the self-fertilised. When double this
height, the sixteen crossed and sixteen self-fertilised plants were
measured to the tips of their leaves; the former averaged 7.3 inches,
and the self-fertilised 6.07 inches in height, or as 100 to 83. In all
the pots, excepting 4, a crossed plant flowered before any one of the
self-fertilised plants. The plants, when fully grown, were again
measured to the summits of their ripe capsules, with the result in Table
5/51.
TABLE 5/51. Limnanthes douglasii.
Heights of plants to the summits of their ripe capsules, measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 17 7/8 : 15 1/8.
Pot 1 : 17 6/8 : 16 4/8.
Pot 1 : 13 : 11.
Pot 2 : 20 : 14 4/8.
Pot 2 : 22 : 15 6/8.
Pot 2 : 21 : 16 1/8.
Pot 2 : 18 4/8 : 17.
Pot 3 : 15 6/8 : 11 4/8.
Pot 3 : 17 2/8 : 10 4/8.
Pot 3 : 14 : 0.
Pot 4 : 20 4/8 : 13 4/8.
Pot 4 : 14 : 13.
Pot 4 : 18 : 12 2/8.
Pot 5 : 17 : 14 2/8.
Pot 5 : 18 5/8 : 14 1/8.
Pot 5 : 14 2/8 : 12 5/8.
Total : 279.50 : 207.75.
The sixteen crossed plants now averaged 17.46, and the fifteen (for one
had died) self-fertilised plants 13.85 inches in height, or as 100 to
79. Mr. Galton considers that a higher ratio would be fairer, namely,
100 to 76. He made a graphical representation of the above measurements,
and adds the words "very good" to the curvature thus formed. Both lots
of plants produced an abundance of seed-capsules, and, as far as could
be judged by the eye, there was no difference in their fertility.]
14. LEGUMINOSAE.
In this family I experimented on the following six genera, Lupinus,
Phaseolus, Lathyrus, Pisum, Sarothamnus, and Ononis.
[Lupinus luteus. (5/2. The structure of the flowers of this plant, and
their manner of fertilisation, have been described by H. Muller
'Befruchtung' etc. page 243. The flowers do not secrete free nectar, and
bees generally visit them for their pollen. Mr. Farrer, however, remarks
'Nature' 1872 page 499, that "there is a cavity at the back and base of
the vexillum, in which I have not been able to find nectar. But the
bees, which constantly visit these flowers, certainly go to this cavity
for what they want, and not to the staminal tube.")
A few flowers were crossed with pollen from a distinct plant, but owing
to the unfavourable season only two crossed seeds were produced. Nine
seeds were saved from flowers spontaneously self-fertilised under a net,
on the same plant which yielded the two crossed seeds. One of these
crossed seeds was sown in a pot with two self-fertilised seeds on the
opposite side; the latter came up between two and three days before the
crossed seed. The second crossed seed was sown in like manner with two
self-fertilised seeds on the opposite side; these latter also came up
about a day before the crossed one. In both pots, therefore, the crossed
seedlings from germinating later, were at first completely beaten by the
self-fertilised; nevertheless, this state of things was afterwards
completely reversed. The seeds were sown late in the autumn, and the
pots, which were much too small, were kept in the greenhouse. The plants
in consequence grew badly, and the self-fertilised suffered most in both
pots. The two crossed plants when in flower during the following spring
were 9 inches in height; one of the self-fertilised plants was 8, and
the three others only 3 inches in height, being thus mere dwarfs. The
two crossed plants produced thirteen pods, whilst the four
self-fertilised plants produced only a single one. Some other
self-fertilised plants which had been raised separately in larger pots
produced several spontaneously self-fertilised pods under a net, and
seeds from these were used in the following experiment.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
The spontaneously self-fertilised seeds just mentioned, and crossed
seeds obtained by intercrossing the two crossed plants of the last
generation, after germinating on sand, were planted in pairs on the
opposite sides of three large pots. When the seedlings were only 4
inches in height, the crossed had a slight advantage over their
opponents. When grown to their full height, every one of the crossed
plants exceeded its opponent in height. Nevertheless the self-fertilised
plants in all three pots flowered before the crossed! The measurements
are given in Table 5/52.
TABLE 5/52. Lupinus luteus.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 33 2/8 : 24 4/8.
Pot 1 : 30 4/8 : 18 4/8.
Pot 1 : 30 : 28.
Pot 2 : 29 4/8 : 26.
Pot 2 : 30 : 25.
Pot 3 : 30 4/8 : 28.
Pot 3 : 31 : 27 2/8.
Pot 3 : 31 4/8 : 24 4/8.
Total : 246.25 : 201.75.
The eight crossed plants here average 30.78, and the eight
self-fertilised 25.21 inches in height; or as 100 to 82. These plants
were left uncovered in the greenhouse to set their pods, but they
produced very few good ones, perhaps in part owing to few bees visiting
them. The crossed plants produced nine pods, containing on an average
3.4 seeds, and the self-fertilised plants seven pods, containing on an
average 3 seeds, so that the seeds from an equal number of plants were
as 100 to 88.
Two other crossed seedlings, each with two self-fertilised seedlings on
the opposite sides of the same large pot, were turned out of their pots
early in the season, without being disturbed, into open ground of good
quality. They were thus subjected to but little competition with one
another, in comparison with the plants in the above three pots. In the
autumn the two crossed plants were about 3 inches taller than the four
self-fertilised plants; they looked also more vigorous and produced many
more pods.
Two other crossed and self-fertilised seeds of the same lot, after
germinating on sand, were planted on the opposite sides of a large pot,
in which a Calceolaria had long been growing, and were therefore exposed
to unfavourable conditions: the two crossed plants ultimately attained a
height of 20 1/2 and 20 inches, whilst the two self-fertilised were only
18 and 9 1/2 inches high.
Lupinus pilosus.
From a series of accidents I was again unfortunate in obtaining a
sufficient number of crossed seedlings; and the following results would
not be worth giving, did they not strictly accord with those just given
with respect to Lupinus luteus. I raised at first only a single crossed
seedling, which was placed in competition with two self-fertilised ones
on the opposite side of the same pot. These plants, without being
disturbed, were soon afterwards turned into the open ground. By the
autumn the crossed plant had grown to so large a size that it almost
smothered the two self-fertilised plants, which were mere dwarfs; and
the latter died without maturing a single pod. Several self-fertilised
seeds had been planted at the same time separately in the open ground;
and the two tallest of these were 33 and 32 inches, whereas the one
crossed plant was 38 inches in height. This latter plant also produced
many more pods than did any one of the self-fertilised plants, although
growing separately. A few flowers on the one crossed plant were crossed
with pollen from one of the self-fertilised plants, for I had no other
crossed plant from which to obtain pollen. One of the self-fertilised
plants having been covered by a net produced plenty of spontaneously
self-fertilised pods.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
From crossed and self-fertilised seeds obtained in the manner just
described, I succeeded in raising to maturity only a pair of plants,
which were kept in a pot in the greenhouse. The crossed plant grew to a
height of 33 inches, and the self-fertilised to that of 26 1/2 inches.
The former produced, whilst still kept in the greenhouse, eight pods,
containing on an average 2.77 seeds; and the latter only two pods,
containing on an average 2.5 seeds. The average height of the two
crossed plants of the two generations taken together was 35.5, and that
of the three self-fertilised plants of the same two generations 30.5; or
as 100 to 86. (5/3. We here see that both Lupinus luteus and pilosus
seed freely when insects are excluded; but Mr. Swale, of Christchurch,
in New Zealand, informs me 'Gardeners' Chronicle' 1858 page 828, that
the garden varieties of the lupine are not there visited by any bees,
and that they seed less freely than any other introduced leguminous
plant, with the exception of red clover. He adds "I have, for amusement,
during the summer, released the stamens with a pin, and a pod of seed
has always rewarded me for my trouble, the adjoining flowers not so
served having all proved blind." I do not know to what species this
statement refers.)
Phaseolus multiflorus.
This plant, the scarlet-runner of English gardeners and the Phaseolus
coccineus of Lamarck, originally came from Mexico, as I am informed by
Mr. Bentham. The flowers are so constructed that hive and humble-bees,
which visit them incessantly, almost always alight on the left
wing-petal, as they can best suck the nectar from this side. Their
weight and movements depress the petal, and this causes the stigma to
protrude from the spirally-wound keel, and a brush of hairs round the
stigma pushes out the pollen before it. The pollen adheres to the head
or proboscis of the bee which is at work, and is thus placed either on
the stigma of the same flower, or is carried to another flower. (5/4.
The flowers have been described by Delpino, and in an admirable manner
by Mr. Farrer in the 'Annals and Magazine of Natural History' volume 2
4th series October 1868 page 256. My son Francis has explained 'Nature'
January 8, 1874 page 189, the use of one peculiarity in their structure,
namely, a little vertical projection on the single free stamen near its
base, which seems placed as if to guard the entrance into the two
nectar-holes in the staminal sheath. He shows that this projection
prevents the bees reaching the nectar, unless they go to the left side
of the flower, and it is absolutely necessary for cross-fertilisation
that they should alight on the left wing-petal.) Several years ago I
covered some plants under a large net, and these produced on one
occasion about one-third, and on another occasion about one-eighth, of
the number of pods which the same number of uncovered plants growing
close alongside produced. (5/5. 'Gardeners' Chronicle' 1857 page 725 and
more especially ibid 1858 page 828. Also 'Annals and Magazine of Natural
History' 3rd series volume 2 1858 page 462.) This lessened fertility was
not caused by any injury from the net, as I moved the wing-petals of
several protected flowers, in the same manner as bees do, and these
produced remarkably fine pods. When the net was taken off, the flowers
were immediately visited by bees, and it was interesting to observe how
quickly the plants became covered with young pods. As the flowers are
much frequented by Thrips, the self-fertilisation of most of the flowers
under the net may have been due to the action of these minute insects.
Dr. Ogle likewise covered up a large portion of a plant, and "out of a
vast number of blossoms thus protected not a single one produced a pod,
while the unprotected blossoms were for the most part fruitful." Mr.
Belt gives a more curious case; this plant grows well and flowers in
Nicaragua; but as none of the native bees visit the flowers, not a
single pod is ever produced. (5/6. Dr. Ogle 'Popular Science Review'
1870 page 168. Mr. Belt 'The Naturalist in Nicaragua' 1874 page 70. The
latter author gives a case 'Nature' 1875 page 26, of a late crop of
Phaseolus multiflorus near London which "was rendered barren" by the
humble-bees cutting, as they frequently do, holes at the bases of the
flowers instead of entering them in the proper manner.)
From the facts now given we may feel nearly sure that individuals of the
same variety or of different varieties, if growing near each other and
in flower at the same time, would intercross; but I cannot myself
advance any direct evidence of such an occurrence, as only a single
variety is commonly cultivated in England. I have, however, received an
account from the Reverend W.A. Leighton, that plants raised by him from
ordinary seed produced seeds differing in an extraordinary manner in
colour and shape, leading to the belief that their parents must have
been crossed. In France M. Fermond more than once planted close together
varieties which ordinarily come true and which bear differently coloured
flowers and seeds; and the offspring thus raised varied so greatly that
there could hardly be a doubt that they had intercrossed. (5/7.
'Fécondation chez les Végétaux' 1859 pages 34-40. He adds that M.
Villiers has described a spontaneous hybrid, which he calls Phaseolus
coccineus hybridus, in the 'Annales de la Soc. R. de Horticulture' June
1844.) On the other hand, Professor H. Hoffman does not believe in the
natural crossing of the varieties; for although seedlings raised from
two varieties growing close together produced plants which yielded seeds
of a mixed character, he found that this likewise occurred with plants
separated by a space of from 40 to 150 paces from any other variety; he
therefore attributes the mixed character of the seed to spontaneous
variability. (5/8. 'Bestimmung des Werthes von Species und Varietat'
1869 pages 47-72.) But the above distance would be very far from
sufficient to prevent intercrossing: cabbages have been known to cross
at several times this distance; and the careful Gartner gives many
instances of plants growing at from 600 to 800 yards apart fertilising
one another. (5/9. 'Kenntnis der Befruchtung' 1844 pages 573, 577.)
Professor Hoffman even maintains that the flowers of the kidney-bean are
specially adapted for self-fertilisation. He enclosed several flowers in
bags; and as the buds often dropped off, he attributes the partial
sterility of these flowers to the injurious effects of the bags, and not
to the exclusion of insects. But the only safe method of experimenting
is to cover up a whole plant, which then never suffers.
Self-fertilised seeds were obtained by moving up and down in the same
manner as bees do the wing-petals of flowers protected by a net; and
crossed seeds were obtained by crossing two of the plants under the same
net. The seeds after germinating on sand were planted on the opposite
sides of two large pots, and equal-sized sticks were given them to twine
up. When 8 inches in height, the plants on the two sides were equal. The
crossed plants flowered before the self-fertilised in both pots. As soon
as one of each pair had grown to the summit of its stick both were
measured.
TABLE 5/53. Phaseolus multiflorus.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 87 : 84 6/8.
Pot 1 : 88 : 87.
Pot 1 : 82 4/8 : 76.
Pot 2 : 90 : 76 4/8.
Pot 2 : 82 4/8 : 87 4/8.
Total : 430.00 : 411.75.
The average height of the five crossed plants is 86 inches, and that of
the five self-fertilised plants 82.35; or as 100 to 96. The pots were
kept in the greenhouse, and there was little or no difference in the
fertility of the two lots. Therefore as far as these few observations
serve, the advantage gained by a cross is very small.
Phaseolus vulgaris.
With respect to this species, I merely ascertained that the flowers were
highly fertile when insects were excluded, as indeed must be the case,
for the plants are often forced during the winter when no insects are
present. Some plants of two varieties (namely Canterbury and Fulmer's
Forcing Bean) were covered with a net, and they seemed to produce as
many pods, containing as many beans, as some uncovered plants growing
alongside; but neither the pods nor the beans were actually counted.
This difference in self-fertility between Phaseolus vulgaris and
multifloris is remarkable, as these two species are so closely related
that Linnaeus thought that they formed one. When the varieties of
Phaseolus vulgaris grow near one another in the open ground, they
sometimes cross largely, notwithstanding their capacity for
self-fertilisation. Mr. Coe has given me a remarkable instance of this
fact with respect to the negro and a white-seeded and a brown-seeded
variety, which were all grown together. The diversity of character in
the seedlings of the second generation raised by me from his plants was
wonderful. I could add other analogous cases, and the fact is well-known
to gardeners. (5/10. I have given Mr. Coe's case in the 'Gardeners'
Chronicle' 1858 page 829. See also for another case ibid page 845.)
Lathyrus odoratus.
Almost everyone who has studied the structure of papilionaceous flowers
has been convinced that they are specially adapted for
cross-fertilisation, although many of the species are likewise capable
of self-fertilisation. The case therefore of Lathyrus odoratus or the
sweet-pea is curious, for in this country it seems invariably to
fertilise itself. I conclude that this is so, as five varieties,
differing greatly in the colour of their flowers but in no other
respect, are commonly sold and come true; yet on inquiry from two great
raisers of seed for sale, I find that they take no precautions to insure
purity--the five varieties being habitually grown close together. (5/11.
See Mr. W. Earley in 'Nature' 1872 page 242, to the same effect. He
once, however, saw bees visiting the flowers, and supposed that on this
occasion they would have been intercrossed.) I have myself purposely
made similar trials with the same result. Although the varieties always
come true, yet, as we shall presently see, one of the five well-known
varieties occasionally gives birth to another, which exhibits all its
usual characters. Owing to this curious fact, and to the darker-coloured
varieties being the most productive, these increase, to the exclusion of
the others, as I was informed by the late Mr. Masters, if there be no
selection.
In order to ascertain what would be the effect of crossing two
varieties, some flowers on the Purple sweet-pea, which has a dark
reddish-purple standard-petal with violet-coloured wing-petals and keel,
were castrated whilst very young, and were fertilised with pollen of the
Painted Lady. This latter variety has a pale cherry-coloured standard,
with almost white wings and keel. On two occasions I raised from a
flower thus crossed plants perfectly resembling both parent-forms; but
the greater number resembled the paternal variety. So perfect was the
resemblance, that I should have suspected some mistake in the label, had
not the plants, which were at first identical in appearance with the
father or Painted Lady, later in the season produced flowers blotched
and streaked with dark purple. This is an interesting example of partial
reversion in the same individual plant as it grows older. The
purple-flowered plants were thrown away, as they might possibly have
been the product of the accidental self-fertilisation of the
mother-plant, owing to the castration not having been effectual. But the
plants which resembled in the colour of their flowers the paternal
variety or Painted Lady were preserved, and their seeds saved. Next
summer many plants were raised from these seeds, and they generally
resembled their grandfather the Painted Lady, but most of them had their
wing-petals streaked and stained with dark pink; and a few had pale
purple wings with the standard of a darker crimson than is natural to
the Painted Lady, so that they formed a new sub-variety. Amongst these
plants a single one appeared having purple flowers like those of the
grandmother, but with the petals slightly streaked with a paler tint:
this was thrown away. Seeds were again saved from the foregoing plants,
and the seedlings thus raised still resembled the Painted Lady, or
great-grandfather; but they now varied much, the standard petal varying
from pale to dark red, in a few instances with blotches of white; and
the wing-petals varied from nearly white to purple, the keel being in
all nearly white.
As no variability of this kind can be detected in plants raised from
seeds, the parents of which have grown during many successive
generations in close proximity, we may infer that they cannot have
intercrossed. What does occasionally occur is that in a row of plants
raised from seeds of one variety, another variety true of its kind
appears; for instance, in a long row of Scarlets (the seeds of which had
been carefully gathered from Scarlets for the sake of this experiment)
two Purples and one Painted Lady appeared. Seeds from these three
aberrant plants were saved and sown in separate beds. The seedlings from
both the Purples were chiefly Purples, but with some Painted Ladies and
some Scarlets. The seedlings from the aberrant Painted Lady were chiefly
Painted Ladies with some Scarlets. Each variety, whatever its parentage
may have been, retained all its characters perfect, and there was no
streaking or blotching of the colours, as in the foregoing plants of
crossed origin. Another variety, however, is often sold, which is
striped and blotched with dark purple; and this is probably of crossed
origin, for I found, as well as Mr. Masters, that it did not transmit
its characters at all truly.
From the evidence now given, we may conclude that the varieties of the
sweet-pea rarely or never intercross in this country; and this is a
highly remarkable fact, considering, firstly, the general structure of
the flowers; secondly, the large quantity of pollen produced, far more
than is requisite for self-fertilisation; and thirdly, the occasional
visit of insects. That insects should sometimes fail to cross-fertilise
the flowers is intelligible, for I have thrice seen humble-bees of two
kinds, as well as hive-bees, sucking the nectar, and they did not
depress the keel-petals so as to expose the anthers and stigma; they
were therefore quite inefficient for fertilising the flowers. One of
these bees, namely, Bombus lapidarius, stood on one side at the base of
the standard and inserted its proboscis beneath the single separate
stamen, as I afterwards ascertained by opening the flower and finding
this stamen prised up. Bees are forced to act in this manner from the
slit in the staminal tube being closely covered by the broad membranous
margin of the single stamen, and from the tube not being perforated by
nectar-passages. On the other hand, in the three British species of
Lathyrus which I have examined, and in the allied genus Vicia, two
nectar-passages are present. Therefore British bees might well be
puzzled how to act in the case of the sweet-pea. I may add that the
staminal tube of another exotic species, Lathyrus grandiflorus, is not
perforated by nectar-passages, and this species has rarely set any pods
in my garden, unless the wing-petals were moved up and down, in the same
manner as bees ought to do; and then pods were generally formed, but
from some cause often dropped off afterwards. One of my sons caught an
elephant sphinx-moth whilst visiting the flowers of the sweet-pea, but
this insect would not depress the wing-petals and keel. On the other
hand, I have seen on one occasion hive-bees, and two or three occasions
the Megachile willughbiella in the act of depressing the keel; and these
bees had the under sides of their bodies thickly covered with pollen,
and could not thus fail to carry pollen from one flower to the stigma of
another. Why then do not the varieties occasionally intercross, though
this would not often happen, as insects so rarely act in an efficient
manner? The fact cannot, as it appears, be explained by the flowers
being self-fertilised at a very early age; for although nectar is
sometimes secreted and pollen adheres to the viscid stigma before the
flowers are fully expanded, yet in five young flowers which were
examined by me the pollen-tubes were not exserted. Whatever the cause
may be, we may conclude, that in England the varieties never or very
rarely intercross. But it does not follow from this, that they would not
be cross by the aid of other and larger insects in their native country,
which in botanical works is said to be the south of Europe and the East
Indies. Accordingly I wrote to Professor Delpino, in Florence, and he
informs me "that it is the fixed opinion of gardeners there that the
varieties do intercross, and that they cannot be preserved pure unless
they are sown separately."
It follows also from the foregoing facts that the several varieties of
the sweet-pea must have propagated themselves in England by
self-fertilisation for very many generations, since the time when each
new variety first appeared. From the analogy of the plants of Mimulus
and Ipomoea, which had been self-fertilised for several generations, and
from trials previously made with the common pea, which is in nearly the
same state as the sweet-pea, it appeared to me very improbable that a
cross between the individuals of the same variety would benefit the
offspring. A cross of this kind was therefore not tried, which I now
regret. But some flowers of the Painted Lady, castrated at an early age,
were fertilised with pollen from the Purple sweet-pea; and it should be
remembered that these varieties differ in nothing except in the colour
of their flowers. The cross was manifestly effectual (though only two
seeds were obtained), as was shown by the two seedlings, when they
flowered, closely resembling their father, the Purple pea, excepting
that they were a little lighter coloured, with their keels slightly
streaked with pale purple. Seeds from flowers spontaneously
self-fertilised under a net were at the same time saved from the same
mother-plant, the Painted Lady. These seeds unfortunately did not
germinate on sand at the same time with the crossed seeds, so that they
could not be planted simultaneously. One of the two crossed seeds in a
state of germination was planted in a pot (Number 1) in which a
self-fertilised seed in the same state had been planted four days
before, so that this latter seedling had a great advantage over the
crossed one. In Pot 2 the other crossed seed was planted two days before
a self-fertilised one; so that here the crossed seedling had a
considerable advantage over the self-fertilised one. But this crossed
seedling had its summit gnawed off by a slug, and was in consequence for
a time quite beaten by the self-fertilised plant. Nevertheless I allowed
it to remain, and so great was its constitutional vigour that it
ultimately beat its uninjured self-fertilised rival. When all four
plants were almost fully grown they were measured, as here shown:--
TABLE 5/54. Lathyrus odoratus.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 80 : 64 4/8.
Pot 2 : 78 4/8 : 63.
Total : 158.5 : 127.5.
The two crossed plants here average 79.25, and the two self-fertilised
63.75 inches in height, or as 100 to 80. Six flowers on these two
crossed plants were reciprocally crossed with pollen from the other
plant, and the six pods thus produced contained on an average six peas,
with a maximum in one of seven. Eighteen spontaneously self-fertilised
pods from the Painted Lady, which, as already stated, had no doubt been
self-fertilised for many previous generations, contained on an average
only 3.93 peas, with a maximum in one of five peas; so that the number
of peas in the crossed and self-fertilised pods was as 100 to 65. The
self-fertilised peas were, however, quite as heavy as those from the
crossed pods. From these two lots of seeds, the plants of the next
generation were raised.
PLANTS OF THE SECOND GENERATION.
Many of the self-fertilised peas just referred to germinated on sand
before any of the crossed ones, and were rejected. As soon as I got
equal pairs, they were planted on the opposite sides of two large pots,
which were kept in the greenhouse. The seedlings thus raised were the
grandchildren of the Painted Lady, which was first crossed by the Purple
variety. When the two lots were from 4 to 6 inches in height there was
no difference between them. Nor was there any marked difference in the
period of their flowering. When fully grown they were measured, as
follows:--
TABLE 5/55. Lathyrus odoratus (Second Generation).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Seedlings from Plants Crossed during the two previous
Generations.
Column 3: Seedlings from Plants Self-fertilised during many previous
Generations.
Pot 1 : 72 4/8 : 57 4/8.
Pot 1 : 71 : 67.
Pot 1 : 52 2/8 : 56 2/8.
Pot 2 : 81 4/8 : 66 2/8.
Pot 2 : 45 2/8 : 38 7/8.
Pot 2 : 55 : 46.
Total : 377.50 : 331.86.
The average height of the six crossed plants is here 62.91, and that of
the six self-fertilised 55.31 inches; or as 100 to 88. There was not
much difference in the fertility of the two lots; the crossed plants
having produced in the greenhouse thirty-five pods, and the
self-fertilised thirty-two pods.
Seeds were saved from the self-fertilised flowers on these two lots of
plants, for the sake of ascertaining whether the seedlings thus raised
would inherit any difference in growth or vigour. It must therefore be
understood that both lots in the following trial are plants of
self-fertilised parentage; but that in the one lot the plants were the
children of plants which had been crossed during two previous
generations, having been before that self-fertilised for many
generations; and that in the other lot they were the children of plants
which had not been crossed for very many previous generations. The seeds
germinated on sand and were planted in pairs on the opposite sides of
four pots. They were measured, when fully grown, with the following
result:--
TABLE 5/56. Lathyrus odoratus.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants from Crossed Plants.
Column 3: Self-fertilised Plants from Self-fertilised Plants.
Pot 1 : 72 : 65.
Pot 1 : 72 : 61 4/8.
Pot 2 : 58 : 64.
Pot 2 : 68 : 68 2/8.
Pot 2 : 72 4/8 : 56 4/8.
Pot 3 : 81 : 60 2/8.
Pot 4 : 77 4/8 : 76 4/8.
Total : 501 : 452.
The average height of the seven self-fertilised plants, the offspring of
crossed plants, is 71.57, and that of the seven self-fertilised plants,
the offspring of self-fertilised plants, is 64.57; or as 100 to 90. The
self-fertilised plants from the self-fertilised produced rather more
pods--namely, thirty-six--than the self-fertilised plants from the
crossed, for these produced only thirty-one pods.
A few seeds of the same two lots were sown in the opposite corners of a
large box in which a Brugmansia had long been growing, and in which the
soil was so exhausted that seeds of Ipomoea purpurea would hardly
vegetate; yet the two plants of the sweet-pea which were raised
flourished well. For a long time the self-fertilised plant from the
self-fertilised beat the self-fertilised plant from the crossed plant;
the former flowered first, and was at one time 77 1/2 inches, whilst the
latter was only 68 1/2 in height; but ultimately the plant from the
previous cross showed its superiority and attained a height of 108 1/2
inches, whilst the other was only 95 inches. I also sowed some of the
same two lots of seeds in poor soil in a shady place in a shrubbery.
Here again the self-fertilised plants from the self-fertilised for a
long time exceeded considerably in height those from the previously
crossed plants; and this may probably be attributed, in the present as
in the last case, to these seeds having germinated rather sooner than
those from the crossed plants; but at the close of the season the
tallest of the self-fertilised plants from the crossed plants was 30
inches, whilst the tallest of the self-fertilised from the
self-fertilised was 29 3/8 inches in height.
From the various facts now given we see that plants derived from a cross
between two varieties of the sweet-pea, which differ in no respect
except in the colour of their flowers, exceed considerably in height the
offspring from self-fertilised plants, both in the first and second
generations. The crossed plants also transmit their superiority in
height and vigour to their self-fertilised offspring.
Pisum sativum.
The common pea is perfectly fertile when its flowers are protected from
the visits of insects; I ascertained this with two or three different
varieties, as did Dr. Ogle with another. But the flowers are likewise
adapted for cross-fertilisation; Mr. Farrer specifies the following
points, namely: "The open blossom displaying itself in the most
attractive and convenient position for insects; the conspicuous
vexillum; the wings forming an alighting place; the attachment of the
wings to the keel, by which any body pressing on the former must press
down the latter; the staminal tube enclosing nectar, and affording by
means of its partially free stamen with apertures on each side of its
base an open passage to an insect seeking the nectar; the moist and
sticky pollen placed just where it will be swept out of the apex of the
keel against the entering insect; the stiff elastic style so placed that
on a pressure being applied to the keel it will be pushed upwards out of
the keel; the hairs on the style placed on that side of the style only
on which there is space for the pollen, and in such a direction as to
sweep it out; and the stigma so placed as to meet an entering
insect,--all these become correlated parts of one elaborate mechanism,
if we suppose that the fertilisation of these flowers is effected by the
carriage of pollen from one to the other." (5/12. 'Nature' October 10,
1872 page 479. Hermann Muller gives an elaborate description of the
flowers 'Befruchtung' etc. page 247.) Notwithstanding these manifest
provisions for cross-fertilisation, varieties which have been cultivated
for very many successive generations in close proximity, although
flowering at the same time, remain pure. I have elsewhere given evidence
on this head, and if required could give more. (5/13. 'Variation of
Animals and Plants under Domestication' chapter 9 2nd edition volume 1
page 348.) There can hardly be a doubt that some of Knight's varieties,
which were originally produced by an artificial cross and were very
vigorous, lasted for at least sixty years, and during all these years
were self-fertilised; for had it been otherwise, they would not have
kept true, as the several varieties are generally grown near together.
Most of the varieties, however, endure for a shorter period; and this
may be in part due to their weakness of constitution from long-continued
self-fertilisation.
It is remarkable, considering that the flowers secrete much nectar and
afford much pollen, how seldom they are visited by insects either in
England, or, as H. Muller remarks, in North Germany. I have observed the
flowers for the last thirty years, and in all this time have only thrice
seen bees of the proper kind at work (one of them being Bombus
muscorum), such as were sufficiently powerful to depress the keel, so as
to get the undersides of their bodies dusted with pollen. These bees
visited several flowers, and could hardly have failed to cross-fertilise
them. Hive-bees and other small kinds sometimes collect pollen from old
and already fertilised flowers, but this is of no account. The rarity of
the visits of efficient bees to this exotic plant is, I believe, the
chief cause of the varieties so seldom intercrossing. That a cross does
occasionally take place, as might be expected from what has just been
stated, is certain, from the recorded cases of the direct action of the
pollen of one variety on the seed-coats of another. (5/14. 'Variation of
Animals and Plants under Domestication' chapter 11 2nd edition volume 1
page 428.) The late Mr. Masters, who particularly attended to the
raising of new varieties of peas, was convinced that some of them had
originated from accidental crosses. But as such crosses are rare, the
old varieties would not often be thus deteriorated, more especially as
plants departing from the proper type are generally rejected by those
who collect seed for sale. There is another cause which probably tends
to render cross-fertilisation rare, namely, the early age at which the
pollen-tubes are exserted; eight flowers not fully expanded were
examined, and in seven of these the pollen-tubes were in this state; but
they had not as yet penetrated the stigma. Although so few insects visit
the flowers of the pea in this country or in North Germany, and although
the anthers seem here to open abnormally soon, it does not follow that
the species in its native country would be thus circumstanced.
Owing to the varieties having been self-fertilised for many generations,
and to their having been subjected in each generation to nearly the same
conditions (as will be explained in a future chapter) I did not expect
that a cross between two such plants would benefit the offspring; and so
it proved on trial. In 1867 I covered up several plants of the Early
Emperor pea, which was not then a very new variety, so that it must
already have been propagated by self-fertilisation for at least a dozen
generations. Some flowers were crossed with pollen from a distinct plant
growing in the same row, and others were allowed to fertilise themselves
under a net. The two lots of seeds thus obtained were sown on opposite
sides of two large pots, but only four pairs came up at the same time.
The pots were kept in the greenhouse. The seedlings of both lots when
between 6 and 7 inches in height were equal. When nearly full-grown they
were measured, as in Table 5/57.
TABLE 5/57. Pisum sativum.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 35 : 29 6/8.
Pot 2 : 31 4/8 : 51.
Pot 2 : 35 : 45.
Pot 2 : 37 : 33.
Total : 138.50 : 158.75.
The average height of the four crossed plants is here 34.62, and that of
the four self-fertilised plants 39.68, or as 100 to 115. So that the
crossed plants, far from beating the self-fertilised, were completely
beaten by them.
There can be no doubt that the result would have been widely different,
if any two varieties out of the numberless ones which exist had been
crossed. Notwithstanding that both had been self-fertilised for many
previous generations, each would almost certainly have possessed its own
peculiar constitution; and this degree of differentiation would have
been sufficient to make a cross highly beneficial. I have spoken thus
confidently of the benefit which would have been derived from crossing
any two varieties of the pea from the following facts: Andrew Knight in
speaking of the results of crossing reciprocally very tall and short
varieties, says, "I had in this experiment a striking instance of the
stimulative effects of crossing the breeds; for the smallest variety,
whose height rarely exceeded 2 feet, was increased to 6 feet; whilst the
height of the large and luxuriant kind was very little diminished."
(5/15. 'Philosophical Transactions' 1799 page 200.) Recently Mr. Laxton
has made numerous crosses, and everyone had been astonished at the
vigour and luxuriance of the new varieties which he has thus raised and
afterwards fixed by selection. He gave me seed-peas produced from
crosses between four distinct kinds; and the plants thus raised were
extraordinarily vigorous, being in each case from 1 to 2 or even 3 feet
taller than the parent-forms, which were raised at the same time close
alongside. But as I did not measure their actual height I cannot give
the exact ratio, but it must have been at least as 100 to 75. A similar
trial was subsequently made with two other peas from a different cross,
and the result was nearly the same. For instance, a crossed seedling
between the Maple and Purple-podded pea was planted in poor soil and
grew to the extraordinary height of 116 inches; whereas the tallest
plant of either parent variety, namely, a Purple-podded pea, was only 70
inches in height; or as 100 to 60.
Sarothamnus scoparius.
Bees incessantly visit the flowers of the common Broom, and these are
adapted by a curious mechanism for cross-fertilisation. When a bee
alights on the wing-petals of a young flower, the keel is slightly
opened and the short stamens spring out, which rub their pollen against
the abdomen of the bee. If a rather older flower is visited for the
first time (or if the bee exerts great force on a younger flower), the
keel opens along its whole length, and the longer as well as the shorter
stamens, together with the much elongated curved pistil, spring forth
with violence. The flattened, spoon-like extremity of the pistil rests
for a time on the back of the bee, and leaves on it the load of pollen
with which it is charged. As soon as the bee flies away, the pistil
instantly curls round, so that the stigmatic surface is now upturned and
occupies a position, in which it would be rubbed against the abdomen of
another bee visiting the same flower. Thus, when the pistil first
escapes from the keel, the stigma is rubbed against the back of the bee,
dusted with pollen from the longer stamens, either of the same or
another flower; and afterwards against the lower surface of the bee
dusted with pollen from the shorter stamens, which is often shed a day
or two before that from the longer stamens. (5/16. These observations
have been quoted in an abbreviated form by the Reverend G. Henslow, in
the 'Journal of Linnean Society Botany' volume 9 1866 page 358. Hermann
Muller has since published a full and excellent account of the flower in
his 'Befruchtung' etc. page 240.) By this mechanism cross-fertilisation
is rendered almost inevitable, and we shall immediately see that pollen
from a distinct plant is more effective than that from the same flower.
I need only add that, according to H. Muller, the flowers do not secrete
nectar, and he thinks that bees insert their proboscides only in the
hope of finding nectar; but they act in this manner so frequently and
for so long a time that I cannot avoid the belief that they obtain
something palatable within the flowers.
If the visits of bees are prevented, and if the flowers are not dashed
by the wind against any object, the keel never opens, so that the
stamens and pistil remain enclosed. Plants thus protected yield very few
pods in comparison with those produced by neighbouring uncovered bushes,
and sometimes none at all. I fertilised a few flowers on a plant growing
almost in a state of nature with pollen from another plant close
alongside, and the four crossed capsules contained on an average 9.2
seeds. This large number no doubt was due to the bush being covered up,
and thus not exhausted by producing many pods; for fifty pods gathered
from an adjoining plant, the flowers of which had been fertilised by the
bees, contained an average of only 7.14 seeds. Ninety-three pods
spontaneously self-fertilised on a large bush which had been covered up,
but had been much agitated by the wind, contained an average of 2.93
seeds. Ten of the finest of these ninety-three capsules yielded an
average of 4.30 seeds, that is less than half the average number in the
four artificially crossed capsules. The ratio of 7.14 to 2.93, or as 100
to 41, is probably the fairest for the number of seeds per pod, yielded
by naturally-crossed and spontaneously self-fertilised flowers. The
crossed seeds compared with an equal number of the spontaneously
self-fertilised seeds were heavier, in the ratio of 100 to 88. We thus
see that besides the mechanical adaptations for cross-fertilisation, the
flowers are much more productive with pollen from a distinct plant than
with their own pollen.
Eight pairs of the above crossed and self-fertilised seeds, after they
had germinated on sand, were planted (1867) on the opposite sides of two
large pots. When several of the seedlings were an inch and a half in
height, there was no marked difference between the two lots. But even at
this early age the leaves of the self-fertilised seedlings were smaller
and of not so bright a green as those of the crossed seedlings. The pots
were kept in the greenhouse, and as the plants on the following spring
(1868) looked unhealthy and had grown but little, they were plunged,
still in their pots, into the open ground. The plants all suffered much
from the sudden change, especially the self-fertilised, and two of the
latter died. The remainder were measured, and I give the measurements in
Table 5/58, because I have not seen in any other species so great a
difference between the crossed and self-fertilised seedlings at so early
an age.
TABLE 5/58. Sarothamnus scoparius (very young plants).
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 4 4/8 : 2 4/8.
Pot 1 : 6 : 1 4/8.
Pot 1 : 2 : 1.
Pot 2 : 2 : 1 4/8.
Pot 2 : 2 4/8 : 1.
Pot 2 : 0 4/8 : 0 4/8.
Total : 17.5 : 8.0.
The six crossed plants here average 2.91, and the six self-fertilised
1.33 inches in height; so that the former were more than twice as high
as the latter, or as 100 to 46.
In the spring of the succeeding year (1869) the three crossed plants in
Pot 1 had all grown to nearly a foot in height, and they had smothered
the three little self-fertilised plants so completely that two were
dead; and the third, only an inch and a half in height, was dying. It
should be remembered that these plants had been bedded out in their
pots, so that they were subjected to very severe competition. This pot
was now thrown away.
The six plants in Pot 2 were all alive. One of the self-fertilised was
an inch and a quarter taller than any one of the crossed plants; but the
other two self-fertilised plants were in a very poor condition. I
therefore resolved to leave these plants to struggle together for some
years. By the autumn of the same year (1869) the self-fertilised plant
which had been victorious was now beaten. The measurements are shown in
Table 5/59.
TABLE 5/59. Pot 2.--Sarothamnus scoparius.
Heights of plants measured in inches.
Column 1: Crossed Plants.
Column 2: Self-fertilised Plants.
: 15 6/8 : 13 1/8.
: 9 6/8 : 3.
: 8 2/8 : 2 4/8.
The same plants were again measured in the autumn of the following year,
1870.
TABLE 5/60. Pot 2.--Sarothamnus scoparius.
Heights of plants measured in inches.
Column 1: Crossed Plants.
Column 2: Self-fertilised Plants.
: 26 2/8 : 14 2/8.
: 16 4/8 : 11 4/8.
: 14 : 9 6/8.
Total : 56.75 : 35.50.
The three crossed plants now averaged 18.91, and the three
self-fertilised 11.83 inches in height; or as 100 to 63. The three
crossed plants in Pot 1, as already shown, had beaten the three
self-fertilised plants so completely, that any comparison between them
was superfluous.
The winter of 1870-1871 was severe. In the spring the three crossed
plants in Pot 2 had not even the tips of their shoots in the least
injured, whereas all three self-fertilised plants were killed half-way
down to the ground; and this shows how much more tender they were. In
consequence not one of these latter plants bore a single flower during
the ensuing summer of 1871, whilst all three crossed plants flowered.
Ononis minutissima.
This plant, of which seeds were sent me from North Italy, produces,
besides the ordinary papilionaceous flowers, minute, imperfect, closed
or cleistogene flowers, which can never be cross-fertilised, but are
highly self-fertile. Some of the perfect flowers were crossed with
pollen from a distinct plant, and six capsules thus produced yielded on
an average 3.66 seeds, with a maximum of five in one. Twelve perfect
flowers were marked and allowed to fertilise themselves spontaneously
under a net, and they yielded eight capsules, containing on an average
2.38 seeds, with a maximum of three seeds in one. So that the crossed
and self-fertilised capsules from the perfect flowers yielded seeds in
the proportion of 100 to 65. Fifty-three capsules produced by the
cleistogene flowers contained on an average 4.1 seeds, so that these
were the most productive of all; and the seeds themselves looked finer
even than those from the crossed perfect flowers.
The seeds from the crossed perfect flowers and from the self-fertilised
cleistogene flowers were allowed to germinate on sand; but unfortunately
only two pairs germinated at the same time. These were planted on the
opposite sides of the same pot, which was kept in the greenhouse. In the
summer of the same year, when the seedlings were about 4 1/2 inches in
height, the two lots were equal. In the autumn of the following year
(1868) the two crossed plants were of exactly the same height, namely,
11 4/8 inches, and the two self-fertilised plants 12 6/8 and 7 2/8
inches; so that one of the self-fertilised exceeded considerably in
height all the others. By the autumn of 1869 the two crossed plants had
acquired the supremacy; their height being 16 4/8 and 15 1/8, whilst
that of the two self-fertilised plants was 14 5/8 and 11 4/8 inches.
By the autumn of 1870, the heights were as follows:--
TABLE 5/61. Ononis minutissima.
Heights of plants measured in inches.
Column 1: Crossed Plants.
Column 2: Self-fertilised Plants.
: 20 3/8 : 17 4/8.
: 19 2/8 : 17 2/8.
Total : 39.63 : 34.75.
So that the mean height of the two crossed plants was 19.81, and that of
the two self-fertilised 17.37 inches; or as 100 to 88. It should be
remembered that the two lots were at first equal in height; that one of
the self-fertilised plants then had the advantage, the two crossed
plants being at last victorious.]
SUMMARY ON THE LEGUMINOSAE.
Six genera in this family were experimented on, and the results are in
some respects remarkable. The crossed plants of the two species of
Lupinus were conspicuously superior to the self-fertilised plants in
height and fertility; and when grown under very unfavourable conditions,
in vigour. The scarlet-runner (Phaseolus multiflorus) is partially
sterile if the visits of bees are prevented, and there is reason to
believe that varieties growing near one another intercross. The five
crossed plants, however, exceeded in height the five self-fertilised
only by a little. Phaseolus vulgaris is perfectly self-sterile;
nevertheless, varieties growing in the same garden sometimes intercross
largely. The varieties of Lathyrus odoratus, on the other hand, appear
never to intercross in this country; and though the flowers are not
often visited by efficient insects, I cannot account for this fact, more
especially as the varieties are believed to intercross in North Italy.
Plants raised from a cross between two varieties, differing only in the
colour of their flowers, grew much taller and were under unfavourable
conditions more vigorous than the self-fertilised plants; they also
transmitted, when self-fertilised, their superiority to their offspring.
The many varieties of the common Pea (Pisum sativum), though growing in
close proximity, very seldom intercross; and this seems due to the
rarity in this country of the visits of bees sufficiently powerful to
effect cross-fertilisation. A cross between the self-fertilised
individuals of the same variety does no good whatever to the offspring;
whilst a cross between distinct varieties, though closely allied, does
great good, of which we have excellent evidence. The flowers of the
Broom (Sarothamnus) are almost sterile if they are not disturbed and if
insects are excluded. The pollen from a distinct plant is more effective
than that from the same flower in producing seeds. The crossed seedlings
have an enormous advantage over the self-fertilised when grown together
in close competition. Lastly, only four plants of the Ononis minutissima
were raised; but as these were observed during their whole growth, the
advantage of the crossed over the self-fertilised plants may, I think,
be fully trusted.
[15. ONAGRACEAE.--Clarkia elegans.
Owing to the season being very unfavourable (1867), few of the flowers
which I fertilised formed capsules; twelve crossed flowers produced only
four, and eighteen self-fertilised flowers yielded only one capsule. The
seeds after germinati
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