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Chapter IV
CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.
Brassica oleracea, crossed and self-fertilised plants.
Great effect of a cross with a fresh stock on the weight of the
offspring.
Iberis umbellata.
Papaver vagum.
Eschscholtzia californica, seedlings from a cross with a fresh stock not
more vigorous, but more fertile than the self-fertilised seedlings.
Reseda lutea and odorata, many individuals sterile with their own pollen.
Viola tricolor, wonderful effects of a cross.
Adonis aestivalis.
Delphinium consolida.
Viscaria oculata, crossed plants hardly taller, but more fertile than
the self-fertilised.
Dianthus caryophyllus, crossed and self-fertilised plants compared for
four generations.
Great effects of a cross with a fresh stock.
Uniform colour of the flowers on the self-fertilised plants.
Hibiscus africanus.
[6. CRUCIFERAE.--Brassica oleracea.
VAR. CATTELL'S EARLY BARNES CABBAGE.
The flowers of the common cabbage are adapted, as shown by H. Muller,
for cross-fertilisation, and should this fail, for self-fertilisation.
(4/1. 'Die Befruchtung' etc. page 139.) It is well known that the
varieties are crossed so largely by insects, that it is impossible to
raise pure kinds in the same garden, if more than one kind is in flower
at the same time. Cabbages, in one respect, were not well fitted for my
experiments, as, after they had formed heads, they were often difficult
to measure. The flower-stems also differ much in height; and a poor
plant will sometimes throw up a higher stem than that of a fine plant.
In the later experiments, the fully-grown plants were cut down and
weighed, and then the immense advantage from a cross became manifest.
A single plant of the above variety was covered with a net just before
flowering, and was crossed with pollen from another plant of the same
variety growing close by; and the seven capsules thus produced contained
on an average 16.3 seeds, with a maximum of twenty in one capsule. Some
flowers were artificially self-fertilised, but their capsules did not
contain so many seeds as those from flowers spontaneously
self-fertilised under the net, of which a considerable number were
produced. Fourteen of these latter capsules contained on an average 4.1
seeds, with a maximum in one of ten seeds; so that the seeds in the
crossed capsules were in number to those in the self-fertilised capsules
as 100 to 25. The self-fertilised seeds, fifty-eight of which weighed
3.88 grains, were, however, a little finer than those from the crossed
capsules, fifty-eight of which weighed 3.76 grains. When few seeds are
produced, these seem often to be better nourished and to be heavier than
when many are produced.
The two lots of seeds in an equal state of germination were planted,
some on opposite sides of a single pot, and some in the open ground. The
young crossed plants in the pot at first exceeded by a little in height
the self-fertilised; then equalled them; were then beaten; and lastly
were again victorious. The plants, without being disturbed, were turned
out of the pot, and planted in the open ground; and after growing for
some time, the crossed plants, which were all of nearly the same height,
exceeded the self-fertilised ones by 2 inches. When they flowered, the
flower-stems of the tallest crossed plant exceeded that of the tallest
self-fertilised plant by 6 inches. The other seedlings which were
planted in the open ground stood separate, so that they did not compete
with one another; nevertheless the crossed plants certainly grew to a
rather greater height than the self-fertilised; but no measurements were
made. The crossed plants which had been raised in the pot, and those
planted in the open ground, all flowered a little before the
self-fertilised plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Some flowers on the crossed plants of the last generation were again
crossed with pollen from another crossed plant, and produced fine
capsules. The flowers on the self-fertilised plants of the last
generation were allowed to fertilise themselves spontaneously under a
net, and they produced some remarkably fine capsules. The two lots of
seeds thus produced germinated on sand, and eight pairs were planted on
opposite sides of four pots. These plants were measured to the tips of
their leaves on the 20th of October of the same year, and the eight
crossed plants averaged in height 8.4 inches, whilst the self-fertilised
averaged 8.53 inches, so that the crossed were a little inferior in
height, as 100 to 101.5. By the 5th of June of the following year these
plants had grown much bulkier, and had begun to form heads. The crossed
had now acquired a marked superiority in general appearance, and
averaged 8.02 inches in height, whilst the self-fertilised averaged 7.31
inches; or as 100 to 91. The plants were then turned out of their pots
and planted undisturbed in the open ground. By the 5th of August their
heads were fully formed, but several had grown so crooked that their
heights could hardly be measured with accuracy. The crossed plants,
however, were on the whole considerably taller than the self-fertilised.
In the following year they flowered; the crossed plants flowering before
the self-fertilised in three of the pots, and at the same time in Pot 2.
The flower-stems were now measured, as shown in Table 4/29.
TABLE 3/29. Brassica oleracea.
Measured in inches to tops of flower-stems: 0 signifies that a
Flower-stem was not formed.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 49 2/8 : 44.
Pot 1 : 39 4/8 : 41.
Pot 2 : 37 4/8 : 38.
Pot 2 : 33 4/8 : 35 4/8.
Pot 3 : 47 : 51 1/8.
Pot 3 : 40 : 41 2/8.
Pot 3 : 42 : 46 4/8.
Pot 4 : 43 6/8 : 20 2/8.
Pot 4 : 37 2/8 : 33 3/8.
Pot 4 : 0 : 0.
Total : 369.75 : 351.00.
The nine flower-stems on the crossed plants here average 41.08 inches,
and the nine on the self-fertilised plants 39 inches in height, or as
100 to 95. But this small difference, which, moreover, depended almost
wholly on one of the self-fertilised plants being only 20 inches high,
does not in the least show the vast superiority of the crossed over the
self-fertilised plants. Both lots, including the two plants in Pot 4,
which did not flower, were now cut down close to the ground and weighed,
but those in Pot 2 were excluded, for they had been accidentally injured
by a fall during transplantation, and one was almost killed. The eight
crossed plants weighed 219 ounces, whilst the eight self-fertilised
plants weighed only 82 ounces, or as 100 to 37; so that the superiority
of the former over the latter in weight was great.
THE EFFECTS OF A CROSS WITH A FRESH STOCK.
Some flowers on a crossed plant of the last or second generation were
fertilised, without being castrated, by pollen taken from a plant of the
same variety, but not related to my plants, and brought from a nursery
garden (whence my seeds originally came) having a different soil and
aspect. The flowers on the self-fertilised plants of the last or second
generation (Table 4/29) were allowed to fertilise themselves
spontaneously under a net, and yielded plenty of seeds. These latter and
the crossed seeds, after germinating on sand, were planted in pairs on
the opposite sides of six large pots, which were kept at first in a cool
greenhouse. Early in January their heights were measured to the tips of
their leaves. The thirteen crossed plants averaged 13.16 inches in
height, and the twelve (for one had died) self-fertilised plants
averaged 13.7 inches, or as 100 to 104; so that the self-fertilised
plants exceeded by a little the crossed plants.
TABLE 3/30. Brassica oleracea.
Weights in ounces of plants after they had formed heads.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants from Pollen of fresh Stock.
Column 3: Self-fertilised Plants of the Third Generation.
Pot 1 : 130 : 18 2/4.
Pot 2 : 74 : 34 3/4.
Pot 3 : 121 : 17 2/4.
Pot 4 : 127 2/4 : 14.
Pot 5 : 90 : 11 2/4.
Pot 6 : 106 2/4 : 46.
Total : 649.00 : 142.25.
Early in the spring the plants were gradually hardened, and turned out
of their pots into the open ground without being disturbed. By the end
of August the greater number had formed fine heads, but several grew
extremely crooked, from having been drawn up to the light whilst in the
greenhouse. As it was scarcely possible to measure their heights, the
finest plant on each side of each pot was cut down close to the ground
and weighed. In Table 4/30 we have the result.
The six finest crossed plants average 108.16 ounces, whilst the six
finest self-fertilised plants average only 23.7 ounces, or as 100 to 22.
This difference shows in the clearest manner the enormous benefit which
these plants derived from a cross with another plant belonging to the
same sub-variety, but to a fresh stock, and grown during at least the
three previous generations under somewhat different conditions.
THE OFFSPRING FROM A CUT-LEAVED, CURLED, AND VARIEGATED WHITE-GREEN
CABBAGE CROSSED WITH A CUT-LEAVED, CURLED, AND VARIEGATED CRIMSON-GREEN
CABBAGE, COMPARED WITH THE SELF-FERTILISED OFFSPRING FROM THE TWO
VARIETIES.
These trials were made, not for the sake of comparing the growth of the
crossed and self-fertilised seedlings, but because I had seen it stated
that these varieties would not naturally intercross when growing
uncovered and near one another. This statement proved quite erroneous;
but the white-green variety was in some degree sterile in my garden,
producing little pollen and few seeds. It was therefore no wonder that
seedlings raised from the self-fertilised flowers of this variety were
greatly exceeded in height by seedlings from a cross between it and the
more vigorous crimson-green variety; and nothing more need be said about
this experiment.
The seedlings from the reciprocal cross, that is, from the crimson-green
variety fertilised with pollen from the white-green variety, offer a
somewhat more curious case. A few of these crossed seedlings reverted to
a pure green variety with their leaves less cut and curled, so that they
were altogether in a much more natural state, and these plants grew more
vigorously and taller than any of the others. Now it is a strange fact
that a much larger number of the self-fertilised seedlings from the
crimson-green variety than of the crossed seedlings thus reverted; and
as a consequence the self-fertilised seedlings grew taller by 2 1/2
inches on an average than the crossed seedlings, with which they were
put into competition. At first, however, the crossed seedlings exceeded
the self-fertilised by an average of a quarter of an inch. We thus see
that reversion to a more natural condition acted more powerfully in
favouring the ultimate growth of these plants than did a cross; but it
should be remembered that the cross was with a semi-sterile variety
having a feeble constitution.
Iberis umbellata.
VAR. KERMESIANA.
This variety produced plenty of spontaneously self-fertilised seed under
a net. Other plants in pots in the greenhouse were left uncovered, and
as I saw small flies visiting the flowers, it seemed probable that they
would be intercrossed. Consequently seeds supposed to have been thus
crossed and spontaneously self-fertilised seeds were sown on opposite
sides of a pot. The self-fertilised seedlings grew from the first
quicker than the supposed crossed seedlings, and when both lots were in
full flower the former were from 5 to 6 inches higher than the crossed!
I record in my notes that the self-fertilised seeds from which these
self-fertilised plants were raised were not so well ripened as the
crossed; and this may possibly have caused the great difference in their
growth, in a somewhat analogous manner as occurred with the
self-fertilised plants of the eighth generation of Ipomoea raised from
unhealthy parents. It is a curious circumstance, that two other lots of
the above seeds were sown in pure sand mixed with burnt earth, and
therefore without any organic matter; and here the supposed crossed
seedlings grew to double the height of the self-fertilised, before both
lots died, as necessarily occurred at an early period. We shall
hereafter meet with another case apparently analogous to this of Iberis
in the third generation of Petunia.
The above self-fertilised plants were allowed to fertilise themselves
again under a net, yielding self-fertilised plants of the second
generation, and the supposed crossed plants were crossed by pollen of a
distinct plant; but from want of time this was done in a careless
manner, namely, by smearing one head of expanded flowers over another. I
should have thought that this would have succeeded, and perhaps it did
so; but the fact of 108 of the self-fertilised seeds weighing 4.87
grains, whilst the same number of the supposed crossed seeds weighed
only 3.57 grains, does not look like it. Five seedlings from each lot of
seeds were raised, and the self-fertilised plants, when fully grown,
exceeded in average height by a trifle (namely .4 of an inch) the five
probably crossed plants. I have thought it right to give this case and
the last, because had the supposed crossed plants proved superior to the
self-fertilised in height, I should have assumed without doubt that the
former had really been crossed. As it is, I do not know what to
conclude.
Being much surprised at the two foregoing trials, I determined to make
another, in which there should be no doubt about the crossing. I
therefore fertilised with great care (but as usual without castration)
twenty-four flowers on the supposed crossed plants of the last
generation with pollen from distinct plants, and thus obtained
twenty-one capsules. The self-fertilised plants of the last generation
were allowed to fertilise themselves again under a net, and the
seedlings reared from these seeds formed the third self-fertilised
generation. Both lots of seeds, after germinating on bare sand, were
planted in pairs on the opposite sides of two pots. All the remaining
seeds were sown crowded on opposite sides of a third pot; but as all the
self-fertilised seedlings in this latter pot died before they grew to
any considerable height, they were not measured. The plants in Pots 1
and 2 were measured when between 7 and 8 inches in height, and the
crossed exceeded the self-fertilised in average height by 1.57 inches.
When fully grown they were again measured to the summits of their
flower-heads, with the following result:--
TABLE 4/31. Iberis umbellata.
Heights of plants to the summits of their flower-heads, in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants of the Third Generation.
Pot 1 : 18 : 19.
Pot 1 : 21 : 21.
Pot 1 : 18 2/8 : 19 4/8.
Pot 2 : 19 : 16 6/8.
Pot 2 : 18 4/8 : 7 4/8.
Pot 2 : 17 6/8 : 14 4/8.
Pot 2 : 21 3/8 : 16 4/8.
Total : 133.88 : 114.75.
The average height of the seven crossed plants is here 19.12 inches, and
that of the seven self-fertilised plants 16.39, or as 100 to 86. But as
the plants on the self-fertilised side grew very unequally, this ratio
cannot be fully trusted, and is probably too high. In both pots a
crossed plant flowered before any one of the self-fertilised. These
plants were left uncovered in the greenhouse; but from being too much
crowded they were not very productive. The seeds from all seven plants
of both lots were counted; the crossed produced 206, and the
self-fertilised 154; or as 100 to 75.
CROSS BY A FRESH STOCK.
From the doubts caused by the two first trials, in which it was not
known with certainty that the plants had been crossed; and from the
crossed plants in the last experiment having been put into competition
with plants self-fertilised for three generations, which moreover grew
very unequally, I resolved to repeat the trial on a larger scale, and in
a rather different manner. I obtained seeds of the same crimson variety
of Iberis umbellata from another nursery garden, and raised plants from
them. Some of these plants were allowed to fertilise themselves
spontaneously under a net; others were crossed by pollen taken from
plants raised from seed sent me by Dr. Durando from Algiers, where the
parent-plants had been cultivated for some generations. These latter
plants differed in having pale pink instead of crimson flowers, but in
no other respect. That the cross had been effective (though the flowers
on the crimson mother-plant had NOT been castrated) was well shown when
the thirty crossed seedlings flowered, for twenty-four of them produced
pale pink flowers, exactly like those of their father; the six others
having crimson flowers exactly like those of their mother and like those
of all the self-fertilised seedlings. This case offers a good instance
of a result which not rarely follows from crossing varieties having
differently coloured flowers; namely, that the colours do not blend, but
resemble perfectly those either of the father or mother plant. The seeds
of both lots, after germinating on sand, were planted on opposite sides
of eight pots. When fully grown, the plants were measured to the summits
of the flower-heads, as shown in Table 4/32.
TABLE 4/32. Iberis umbellata.
Height of Plants to the summits of the flower-heads, measured in inches:
0 signifies that the Plant died.
Column 1: Number (Name) of Pot.
Column 2: Plants from a Cross with a fresh Stock.
Column 3: Plants from Spontaneously Self-fertilised Seeds.
Pot 1 : 18 6/8 : 17 3/8.
Pot 1 : 17 5/8 : 16 7/8.
Pot 1 : 17 6/8 : 13 1/8.
Pot 1 : 20 1/8 : 15 3/8.
Pot 2 : 20 2/8 : 0.
Pot 2 : 15 7/8 : 16 6/8.
Pot 2 : 17 : 15 2/8.
Pot 3 : 19 2/8 : 13 6/8.
Pot 3 : 18 1/8 : 14 2/8.
Pot 3 : 15 2/8 : 13 4/8.
Pot 4 : 17 1/8 : 16 4/8.
Pot 4 : 18 7/8 : 14 4/8.
Pot 4 : 17 5/8 : 16.
Pot 4 : 15 6/8 : 15 3/8.
Pot 4 : 14 4/8 : 14 7/8.
Pot 5 : 18 1/8 : 16 4/8.
Pot 5 : 14 7/8 : 16 2/8.
Pot 5 : 16 2/8 : 14 2/8.
Pot 5 : 15 5/8 : 14 2/8.
Pot 5 : 12 4/8 : 16 1/8.
Pot 6 : 18 6/8 : 16 1/8.
Pot 6 : 18 6/8 : 15.
Pot 6 : 17 3/8 : 15 2/8.
Pot 7 : 18 : 16 3/8.
Pot 7 : 16 4/8 : 14 4/8.
Pot 7 : 18 2/8 : 13 5/8.
Pot 8 : 20 6/8 : 15 6/8.
Pot 8 : 17 7/8 : 16 3/8.
Pot 8 : 13 5/8 : 20 2/8.
Pot 8 : 19 2/8 : 15 6/8.
Total : 520.38 : 449.88.
The average height of the thirty crossed plants is here 17.34, and that
of the twenty-nine self-fertilised plants (one having died) 15.51, or as
100 to 89. I am surprised that the difference did not prove somewhat
greater, considering that in the last experiment it was as 100 to 86;
but this latter ratio, as before explained, was probably too great. It
should, however, be observed that in the last experiment (Table 4/31),
the crossed plants competed with plants of the third self-fertilised
generation; whilst in the present case, plants derived from a cross with
a fresh stock competed with self-fertilised plants of the first
generation.
The crossed plants in the present case, as in the last, were more
fertile than the self-fertilised, both lots being left uncovered in the
greenhouse. The thirty crossed plants produced 103 seed-bearing
flowers-heads, as well as some heads which yielded no seeds; whereas the
twenty-nine self-fertilised plants produced only 81 seed-bearing heads;
therefore thirty such plants would have produced 83.7 heads. We thus get
the ratio of 100 to 81, for the number of seed-bearing flower-heads
produced by the crossed and self-fertilised plants. Moreover, a number
of seed-bearing heads from the crossed plants, compared with the same
number from the self-fertilised, yielded seeds by weight, in the ratio
of 100 to 92. Combining these two elements, namely, the number of
seed-bearing heads and the weight of seeds in each head, the
productiveness of the crossed to the self-fertilised plants was as 100
to 75.
The crossed and self-fertilised seeds, which remained after the above
pairs had been planted, (some in a state of germination and some not
so), were sown early in the year out of doors in two rows. Many of the
self-fertilised seedlings suffered greatly, and a much larger number of
them perished than of the crossed. In the autumn the surviving
self-fertilised plants were plainly less well-grown than the crossed
plants.
7. PAPAVERACEAE.--Papaver vagum.
A SUB-SPECIES OF Papaver dubium, FROM THE SOUTH OF FRANCE.
The poppy does not secrete nectar, but the flowers are highly
conspicuous and are visited by many pollen-collecting bees, flies and
beetles. The anthers shed their pollen very early, and in the case of
Papaver rhoeas, it falls on the circumference of the radiating stigmas,
so that this species must often be self-fertilised; but with Papaver
dubium the same result does not follow (according to H. Muller 'Die
Befruchtung' page 128), owing to the shortness of the stamens, unless
the flower happens to stand inclined. The present species, therefore,
does not seem so well fitted for self-fertilisation as most of the
others. Nevertheless Papaver vagum produced plenty of capsules in my
garden when insects were excluded, but only late in the season. I may
here add that Papaver somniferum produces an abundance of spontaneously
self-fertilised capsules, as Professor H. Hoffmann likewise found to be
the case. (4/2. 'Zur Speciesfrage' 1875 page 53.) Some species of
Papaver cross freely when growing in the same garden, as I have known to
be the case with Papaver bracteatum and orientale.
Plants of Papaver vagum were raised from seeds sent me from Antibes
through the kindness of Dr. Bornet. Some little time after the flowers
had expanded, several were fertilised with their own pollen, and others
(not castrated) with pollen from a distinct individual; but I have
reason to believe, from observations subsequently made, that these
flowers had been already fertilised by their own pollen, as this process
seems to take place soon after their expansion. (4/3. Mr. J. Scott found
'Report on the Experimental Culture of the Opium Poppy' Calcutta 1874
page 47, in the case of Papaver somniferum, that if he cut away the
stigmatic surface before the flower had expanded, no seeds were
produced; but if this was done "on the second day, or even a few hours
after the expansion of the flower on the first day, a partial
fertilisation had already been effected, and a few good seeds were
almost invariably produced." This proves at how early a period
fertilisation takes place.) I raised, however, a few seedlings of both
lots, and the self-fertilised rather exceeded the crossed plants in
height.
Early in the following year I acted differently, and fertilised seven
flowers, very soon after their expansion, with pollen from another
plant, and obtained six capsules. From counting the seeds in a
medium-sized one, I estimated that the average number in each was at
least 120. Four out of twelve capsules, spontaneously self-fertilised at
the same time, were found to contain no good seeds; and the remaining
eight contained on an average 6.6 seeds per capsule. But it should be
observed that later in the season the same plants produced under a net
plenty of very fine spontaneously self-fertilised capsules.
The above two lots of seeds, after germinating on sand, were planted in
pairs on opposite sides of five pots. The two lots of seedlings, when
half an inch in height, and again when 6 inches high, were measured to
the tips of their leaves, but presented no difference. When fully grown,
the flower-stalks were measured to the summits of the seed capsules,
with the following result:--
TABLE 4/33. Papaver vagum.
Heights of flower-stalks to the summits of the seed capsules measured in
inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 24 2/8 : 21.
Pot 1 : 30 : 26 5/8.
Pot 1 : 18 4/8 : 16.
Pot 2 : 14 4/8 : 15 3/8.
Pot 2 : 22 : 20 1/8.
Pot 2 : 19 5/8 : 14 1/8.
Pot 2 : 21 5/8 : 16 4/8.
Pot 3 : 20 6/8 : 19 2/8.
Pot 3 : 20 2/8 : 13 2/8.
Pot 3 : 20 6/8 : 18.
Pot 4 : 25 3/8 : 23 2/8.
Pot 4 : 24 2/8 : 23.
Pot 5 : 20 : 18 3/8.
Pot 5 : 27 7/8 : 27.
Pot 5 : 19 : 21 2/8.
Total : 328.75 : 293.13.
The fifteen crossed plants here average 21.91 inches, and the fifteen
self-fertilised plants 19.54 inches in height, or as 100 to 89. These
plants did not differ in fertility, as far as could be judged by the
number of capsules produced, for there were seventy-five on the crossed
side and seventy-four on the self-fertilised side.
Eschscholtzia californica.
This plant is remarkable from the crossed seedlings not exceeding in
height or vigour the self-fertilised. On the other hand, a cross greatly
increases the productiveness of the flowers on the parent-plant, and is
indeed sometimes necessary in order that they should produce any seed;
moreover, plants thus derived are themselves much more fertile than
those raised from self-fertilised flowers; so that the whole advantage
of a cross is confined to the reproductive system. It will be necessary
for me to give this singular case in considerable detail.
Twelve flowers on some plants in my flower-garden were fertilised with
pollen from distinct plants, and produced twelve capsules; but one of
these contained no good seed. The seeds of the eleven good capsules
weighed 17.4 grains. Eighteen flowers on the same plants were fertilised
with their own pollen and produced twelve good capsules, which contained
13.61 grains weight of seed. Therefore an equal number of crossed and
self-fertilised capsules would have yielded seed by weight as 100 to 71.
(4/4. Professor Hildebrand experimented on plants in Germany on a larger
scale than I did, and found them much more self-fertile. Eighteen
capsules, produced by cross-fertilisation, contained on an average
eighty-five seeds, whilst fourteen capsules from self-fertilised flowers
contained on an average only nine seeds; that is, as 100 to 11: 'Jahrb.
fur Wissen Botanik.' B. 7 page 467.) If we take into account of the fact
that a much greater proportion of flowers produced capsules when crossed
than when self-fertilised, the relative fertility of the crossed to the
self-fertilised flowers was as 100 to 52. Nevertheless these plants,
whilst still protected by the net, spontaneously produced a considerable
number of self-fertilised capsules.
The seeds of the two lots after germinating on sand were planted in
pairs on the opposite sides of four large pots. At first there was no
difference in their growth, but ultimately the crossed seedlings
exceeded the self-fertilised considerably in height, as shown in Table
4/34. But I believe from the cases which follow that this result was
accidental, owing to only a few plants having been measured, and to one
of the self-fertilised plants having grown only to a height of 15
inches. The plants had been kept in the greenhouse, and from being drawn
up to the light had to be tied to sticks in this and the following
trials. They were measured to the summits of their flower-stems.
TABLE 4/34. Eschscholtzia californica.
Heights of Plants to the summits of their flower-stems measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 33 4/8 : 25.
Pot 2 : 34 2/8 : 35.
Pot 3 : 29 : 27 2/8.
Pot 4 : 22 : 15.
Total : 118.75 : 102.25.
The four crossed plants here average 29.68 inches, and the four
self-fertilised 25.56 in height; or as 100 to 86. The remaining seeds
were sown in a large pot in which a Cineraria had long been growing; and
in this case again the two crossed plants on the one side greatly
exceeded in height the two self-fertilised plants on the opposite side.
The plants in the above four pots from having been kept in the
greenhouse did not produce on this or any other similar occasion many
capsules; but the flowers on the crossed plants when again crossed were
much more productive than the flowers on the self-fertilised plants when
again self-fertilised. These plants after seeding were cut down and kept
in the greenhouse; and in the following year, when grown again, their
relative heights were reversed, as the self-fertilised plants in three
out of the four pots were now taller than and flowered before the
crossed plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
The fact just given with respect to the growth of the cut-down plants
made me doubtful about my first trial, so I determined to make another
on a larger scale with crossed and self-fertilised seedlings raised from
the crossed and self-fertilised plants of the last generation. Eleven
pairs were raised and grown in competition in the usual manner; and now
the result was different, for the two lots were nearly equal during
their whole growth. It would therefore be superfluous to give a table of
their heights. When fully grown and measured, the crossed averaged
32.47, and the self-fertilised 32.81 inches in height; or as 100 to 101.
There was no great difference in the number of flowers and capsules
produced by the two lots when both were left freely exposed to the
visits of insects.
PLANTS RAISED FROM BRAZILIAN SEED.
Fritz Muller sent me from South Brazil seeds of plants which were there
absolutely sterile when fertilised with pollen from the same plant, but
were perfectly fertile when fertilised with pollen from any other plant.
The plants raised by me in England from these seeds were examined by
Professor Asa Gray, and pronounced to belong to E. Californica, with
which they were identical in general appearance. Two of these plants
were covered by a net, and were found not to be so completely
self-sterile as in Brazil. But I shall recur to this subject in another
part of this work. Here it will suffice to state that eight flowers on
these two plants, fertilised with pollen from another plant under the
net, produced eight fine capsules, each containing on an average about
eighty seeds. Eight flowers on these same plants, fertilised with their
own pollen, produced seven capsules, which contained on an average only
twelve seeds, with a maximum in one of sixteen seeds. Therefore the
cross-fertilised capsules, compared with the self-fertilised, yielded
seeds in the ratio of about 100 to 15. These plants of Brazilian
parentage differed also in a marked manner from the English plants in
producing extremely few spontaneously self-fertilised capsules under a
net.
Crossed and self-fertilised seeds from the above plants, after
germinating on bare sand, were planted in pairs on the opposite sides of
five large pots. The seedlings thus raised were the grandchildren of the
plants which grew in Brazil; the parents having been grown in England.
As the grandparents in Brazil absolutely require cross-fertilisation in
order to yield any seeds, I expected that self-fertilisation would have
proved very injurious to these seedlings, and that the crossed ones
would have been greatly superior in height and vigour to those raised
from self-fertilised flowers. But the result showed that my anticipation
was erroneous; for as in the last experiment with plants of the English
stock, so in the present one, the self-fertilised plants exceeded the
crossed by a little in height. It will be sufficient to state that the
fourteen crossed plants averaged 44.64, and the fourteen self-fertilised
45.12 inches in height; or as 100 to 101.
THE EFFECTS OF A CROSS WITH A FRESH STOCK.
I now tried a different experiment. Eight flowers on the self-fertilised
plants of the last experiment (i.e., grandchildren of the plants which
grew in Brazil) were again fertilised with pollen from the same plant,
and produced five capsules, containing on an average 27.4 seeds, with a
maximum in one of forty-two seeds. The seedlings raised from these seeds
formed the second SELF-FERTILISED generation of the Brazilian stock.
Eight flowers on one of the crossed plants of the last experiment were
crossed with pollen from another grandchild, and produced five capsules.
These contained on an average 31.6 seeds, with a maximum in one of
forty-nine seeds. The seedlings raised from these seeds may be called
the INTERCROSSED.
Lastly, eight other flowers on the crossed plants of the last experiment
were fertilised with pollen from a plant of the English stock, growing
in my garden, and which must have been exposed during many previous
generations to very different conditions from those to which the
Brazilian progenitors of the mother-plant had been subjected. These
eight flowers produced only four capsules, containing on an average 63.2
seeds, with a maximum in one of ninety. The plants raised from these
seeds may be called the ENGLISH-CROSSED. As far as the above averages
can be trusted from so few capsules, the English-crossed capsules
contained twice as many seeds as the intercrossed, and rather more than
twice as many as the self-fertilised capsules. The plants which yielded
these capsules were grown in pots in the greenhouse, so that their
absolute productiveness must not be compared with that of plants growing
out of doors.
The above three lots of seeds, namely, the self-fertilised,
intercrossed, and English-crossed, were planted in an equal state of
germination (having been as usual sown on bare sand) in nine large pots,
each divided into three parts by superficial partitions. Many of the
self-fertilised seeds germinated before those of the two crossed lots,
and these were of course rejected. The seedlings thus raised are the
great-grandchildren of the plants which grew in Brazil. When they were
from 2 to 4 inches in height, the three lots were equal. They were
measured when four-fifths grown, and again when fully grown, and as
their relative heights were almost exactly the same at these two ages, I
will give only the last measurements. The average height of the nineteen
English-crossed plants was 45.92 inches; that of the eighteen
intercrossed plants (for one died), 43.38; and that of the nineteen
self-fertilised plants, 50.3 inches. So that we have the following
ratios in height:--
The English-crossed to the self-fertilised plants, as 100 to 109.
The English-crossed to the intercrossed plants, as 100 to 94.
The intercrossed to the self-fertilised plants, as 100 to 116.
After the seed-capsules had been gathered, all these plants were cut
down close to the ground and weighed. The nineteen English crossed
plants weighed 18.25 ounces; the intercrossed plants (with their weight
calculated as if there had been nineteen) weighed 18.2 ounces; and the
nineteen self-fertilised plants, 21.5 ounces. We have therefore for the
weights of the three lots of plants the following ratios:--
The English-crossed to the self-fertilised plants, as 100 to 118.
The English-crossed to the intercrossed plants, as 100 to 100.
The intercrossed to the self-fertilised plants, as 100 to 118.
We thus see that in weight, as in height, the self-fertilised plants had
a decided advantage over the English-crossed and intercrossed plants.
The remaining seeds of the three kinds, whether or not in a state of
germination, were sown in three long parallel rows in the open ground;
and here again the self-fertilised seedlings exceeded in height by
between 2 and 3 inches the seedlings in the two other rows, which were
of nearly equal heights. The three rows were left unprotected throughout
the winter, and all the plants were killed, with the exception of two of
the self-fertilised; so that as far as this little bit of evidence goes,
some of the self-fertilised plants were more hardy than any of the
crossed plants of either lot.
We thus see that the self-fertilised plants which were grown in the nine
pots were superior in height (as 116 to 100), and in weight (as 118 to
100), and apparently in hardiness, to the intercrossed plants derived
from a cross between the grandchildren of the Brazilian stock. The
superiority is here much more strongly marked than in the second trial
with the plants of the English stock, in which the self-fertilised were
to the crossed in height as 101 to 100. It is a far more remarkable
fact--if we bear in mind the effects of crossing plants with pollen from
a fresh stock in the cases of Ipomoea, Mimulus, Brassica, and
Iberis--that the self-fertilised plants exceeded in height (as 109 to
100), and in weight (as 118 to 100), the offspring of the Brazilian
stock crossed by the English stock; the two stocks having been long
subjected to widely different conditions.
If we now turn to the fertility of the three lots of plants we find a
very different result. I may premise that in five out of the nine pots
the first plant which flowered was one of the English-crossed; in four
of the pots it was a self-fertilised plant; and in not one did an
intercrossed plant flower first; so that these latter plants were beaten
in this respect, as in so many other ways. The three closely adjoining
rows of plants growing in the open ground flowered profusely, and the
flowers were incessantly visited by bees, and certainly thus
intercrossed. The manner in which several plants in the previous
experiments continued to be almost sterile as long as they were covered
by a net, but set a multitude of capsules immediately that they were
uncovered, proves how effectually the bees carry pollen from plant to
plant. My gardener gathered, at three successive times, an equal number
of ripe capsules from the plants of the three lots, until he had
collected forty-five from each lot. It is not possible to judge from
external appearance whether or not a capsule contains any good seeds; so
that I opened all the capsules. Of the forty-five from the
English-crossed plants, four were empty; of those from the intercrossed,
five were empty; and of those from the self-fertilised, nine were empty.
The seeds were counted in twenty-one capsules taken by chance out of
each lot, and the average number of seeds in the capsules from the
English-crossed plants was 67; from the intercrossed, 56; and from the
self-fertilised, 48.52. It therefore follows that:--
The forty-five capsules (the four empty ones included) from the
English-crossed plants contained 2747 seeds.
The forty-five capsules (the five empty ones included) from the
intercrossed plants contained 2240 seeds.
The forty-five capsules (the nine empty ones included) from the
self-fertilised plants contained 1746.7 seeds.
The reader should remember that these capsules are the product of
cross-fertilisation, effected by the bees; and that the difference in
the number of the contained seeds must depend on the constitution of the
plants;--that is, on whether they were derived from a cross with a
distinct stock, or from a cross between plants of the same stock, or
from self-fertilisation. From the above facts we obtain the following
ratios:--
Number of seeds contained in an equal number of naturally fertilised
capsules produced:--
By the English-crossed and self-fertilised plants, as 100 to 63.
By the English-crossed and intercrossed plants, as 100 to 81.
By the intercrossed and self-fertilised plants, as 100 to 78.
But to have ascertained the productiveness of the three lots of plants,
it would have been necessary to know how many capsules were produced by
the same number of plants. The three long rows, however, were not of
quite equal lengths, and the plants were much crowded, so that it would
have been extremely difficult to have ascertained how many capsules were
produced by them, even if I had been willing to undertake so laborious a
task as to collect and count all the capsules. But this was feasible
with the plants grown in pots in the greenhouse; and although these were
much less fertile than those growing out of doors, their relative
fertility appeared, after carefully observing them, to be the same. The
nineteen plants of the English-crossed stock in the pots produced
altogether 240 capsules; the intercrossed plants (calculated as
nineteen) produced 137.22 capsules; and the nineteen self-fertilised
plants, 152 capsules. Now, knowing the number of seeds contained in
forty-five capsules of each lot, it is easy to calculate the relative
numbers of seeds produced by an equal number of the plants of the three
lots.
Number of seeds produced by an equal number of naturally-fertilised
plants:--
Plants of English-crossed and self-fertilised parentage, as 100 to 40
seeds.
Plants of English-crossed and intercrossed parentage, as 100 to 45
seeds.
Plants of intercrossed and self-fertilised parentage, as 100 to 89
seeds.
The superiority in productiveness of the intercrossed plants (that is,
the product of a cross between the grandchildren of the plants which
grew in Brazil) over the self-fertilised, small as it is, is wholly due
to the larger average number of seeds contained in the capsules; for the
intercrossed plants produced fewer capsules in the greenhouse than did
the self-fertilised plants. The great superiority in productiveness of
the English-crossed over the self-fertilised plants is shown by the
larger number of capsules produced, the larger average number of
contained seeds, and the smaller number of empty capsules. As the
English-crossed and intercrossed plants were the offspring of crosses in
every previous generation (as must have been the case from the flowers
being sterile with their own pollen), we may conclude that the great
superiority in productiveness of the English-crossed over the
intercrossed plants is due to the two parents of the former having been
long subjected to different conditions.
The English-crossed plants, though so superior in productiveness, were,
as we have seen, decidedly inferior in height and weight to the
self-fertilised, and only equal to, or hardly superior to, the
intercrossed plants. Therefore, the whole advantage of a cross with a
distinct stock is here confined to productiveness, and I have met with
no similar case.
8. RESEDACEAE.--Reseda lutea.
Seeds collected from wild plants growing in this neighbourhood were sown
in the kitchen-garden; and several of the seedlings thus raised were
covered with a net. Of these, some were found (as will hereafter be more
fully described) to be absolutely sterile when left to fertilise
themselves spontaneously, although plenty of pollen fell on their
stigmas; and they were equally sterile when artificially and repeatedly
fertilised with their own pollen; whilst other plants produced a few
spontaneously self-fertilised capsules. The remaining plants were left
uncovered, and as pollen was carried from plant to plant by the hive and
humble-bees which incessantly visit the flowers, they produced an
abundance of capsules. Of the necessity of pollen being carried from one
plant to another, I had ample evidence in the case of this species and
of R. odorata; for those plants, which set no seeds or very few as long
as they were protected from insects, became loaded with capsules
immediately that they were uncovered.
Seeds from the flowers spontaneously self-fertilised under the net, and
from flowers naturally crossed by the bees, were sown on opposite sides
of five large pots. The seedlings were thinned as soon as they appeared
above ground, so that an equal number were left on the two sides. After
a time the pots were plunged into the open ground. The same number of
plants of crossed and self-fertilised parentage were measured up to the
summits of their flower-stems, with the result given in Table 4/35.
Those which did not produce flower-stems were not measured.
TABLE 4/35. Reseda lutea, in pots.
Heights of plants to the summits of the flower-stems measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 21 : 12 7/8.
Pot 1 : 14 2/8 : 16.
Pot 1 : 19 1/8 : 11 7/8.
Pot 1 : 7 : 15 2/8.
Pot 1 : 15 1/8 : 19 1/8.
Pot 2 : 20 4/8 : 12 4/8.
Pot 2 : 17 3/8 : 16 2/8.
Pot 2 : 23 7/8 : 16 2/8.
Pot 2 : 17 1/8 : 13 3/8.
Pot 2 : 20 6/8 : 13 5/8.
Pot 3 : 16 1/8 : 14 4/8.
Pot 3 : 17 6/8 : 19 4/8.
Pot 3 : 16 2/8 : 20 7/8.
Pot 3 : 10 : 7 7/8.
Pot 3 : 10 : 17 6/8.
Pot 4 : 22 1/8 : 9.
Pot 4 : 19 : 11 4/8.
Pot 4 : 18 7/8 : 11.
Pot 4 : 16 4/8 : 16.
Pot 4 : 19 2/8 : 16 3/8.
Pot 5 : 25 2/8 : 14 6/8.
Pot 5 : 22 : 16.
Pot 5 : 8 6/8 : 14 3/8.
Pot 5 : 14 2/8 : 14 2/8.
Total : 412.25 : 350.86.
The average height of the twenty-four crossed plants is here 17.17
inches, and that of the same number of self-fertilised plants 14.61; or
as 100 to 85. Of the crossed plants all but five flowered, whilst
several of the self-fertilised did not do so. The above pairs, whilst
still in flower, but with some capsules already formed, were afterwards
cut down and weighed. The crossed weighed 90.5 ounces; and an equal
number of the self-fertilised only 19 ounces, or as 100 to 21; and this
is an astonishing difference.
Seeds of the same two lots were also sown in two adjoining rows in the
open ground. There were twenty crossed plants in the one row and
thirty-two self-fertilised plants in the other row, so that the
experiment was not quite fair; but not so unfair as it at first appears,
for the plants in the same row were not crowded so much as seriously to
interfere with each other's growth, and the ground was bare on the
outside of both rows. These plants were better nourished than those in
the pots and grew to a greater height. The eight tallest plants in each
row were measured in the same manner as before, with the following
result:--
TABLE 4/36. Reseda lutea, growing in the open ground.
Heights of plants to the summits of the flower-stems measured in inches.
Column 1: Crossed Plants.
Column 2: Self-fertilised Plants.
28 : 33 2/8.
27 3/8 : 23.
27 5/8 : 21 5/8.
28 6/8 : 20 4/8.
29 7/8 : 21 5/8.
26 6/8 : 22.
26 2/8 : 21 2/8.
30 1/8 : 21 7/8.
Total : 224.75 : 185.13
The average height of the crossed plants, whilst in full flower, was
here 28.09, and that of the self-fertilised 23.14 inches; or as 100 to
82. It is a singular fact that the tallest plant in the two rows, was
one of the self-fertilised. The self-fertilised plants had smaller and
paler green leaves than the crossed. All the plants in the two rows were
afterwards cut down and weighed. The twenty crossed plants weighed 65
ounces, and twenty self-fertilised (by calculation from the actual
weight of the thirty-two self-fertilised plants) weighed 26.25 ounces;
or as 100 to 40. Therefore the crossed plants did not exceed in weight
the self-fertilised plants in nearly so great a degree as those growing
in the pots, owing probably to the latter having been subjected to more
severe mutual competition. On the other hand, they exceeded the
self-fertilised in height in a slightly greater degree.
Reseda odorata.
Plants of the common mignonette were raised from purchased seed, and
several of them were placed under separate nets. Of these some became
loaded with spontaneously self-fertilised capsules; others produced a
few, and others not a single one. It must not be supposed that these
latter plants produced no seed because their stigmas did not receive any
pollen, for they were repeatedly fertilised with pollen from the same
plant with no effect; but they were perfectly fertile with pollen from
any other plant. Spontaneously self-fertilised seeds were saved from one
of the highly self-fertile plants, and other seeds were collected from
the plants growing outside the nets, which had been crossed by the bees.
These seeds after germinating on sand were planted in pairs on the
opposite sides of five pots. The plants were trained up sticks, and
measured to the summits of their leafy stems--the flower-stems not being
included. We here have the result:--
TABLE 4/37. Reseda odorata (seedlings from a highly self-fertile plant).
Heights of plants to the summits of the leafy stems, flower-stems not
included, measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 20 7/8 : 22 4/8.
Pot 1 : 34 7/8 : 28 5/8.
Pot 1 : 26 6/8 : 23 2/8.
Pot 1 : 32 6/8 : 30 4/8.
Pot 2 : 34 3/8 : 28 5/8.
Pot 2 : 34 5/8 : 30 5/8.
Pot 2 : 11 6/8 : 23.
Pot 2 : 33 3/8 : 30 1/8.
Pot 3 : 17 7/8 : 4 4/8.
Pot 3 : 27 : 25.
Pot 3 : 30 1/8 : 26 3/8.
Pot 3 : 30 2/8 : 25 1/8.
Pot 4 : 21 5/8 : 22 6/8.
Pot 4 : 28 : 25 4/8.
Pot 4 : 32 5/8 : 15 1/8.
Pot 4 : 32 3/8 : 24 6/8.
Pot 5 : 21 : 11 6/8.
Pot 5 : 25 2/8 : 19 7/8.
Pot 5 : 26 6/8 : 10 4/8.
Total : 522.25 : 428.50.
The average height of the nineteen crossed plants is here 27.48, and
that of the nineteen self-fertilised 22.55 inches; or as 100 to 82. All
these plants were cut down in the early autumn and weighed: the crossed
weighed 11.5 ounces, and the self-fertilised 7.75 ounces, or as 100 to
67. These two lots having been left freely exposed to the visits of
insects, did not present any difference to the eye in the number of
seed-capsules which they produced.
The remainder of the same two lots of seeds were sown in two adjoining
rows in the open ground; so that the plants were exposed to only
moderate competition. The eight tallest on each side were measured, as
shown in Table 4/38.
TABLE 4/38. Reseda odorata, growing in the open ground.
Heights of plants measured in inches.
Column 1: Crossed Plants.
Column 2: Self-fertilised Plants.
24 4/8 : 26 5/8.
27 2/8 : 25 7/8.
24 : 25.
26 6/8 : 28 3/8.
25 : 29 7/8.
26 2/8 : 25 7/8.
27 2/8 : 26 7/8.
25 1/8 : 28 2/8.
Total : 206.13 : 216.75
The average height of the eight crossed plants is 25.76, and that of the
eight self-fertilised 27.09; or as 100 to 105.
We here have the anomalous result of the self-fertilised plants being a
little taller than the crossed; of which fact I can offer no
explanation. It is of course possible, but not probable, that the labels
may have been interchanged by accident.
Another experiment was now tried: all the self-fertilised capsules,
though very few in number, were gathered from one of the
semi-self-sterile plants under a net; and as several flowers on this
same plant had been fertilised with pollen from a distinct individual,
crossed seeds were thus obtained. I expected that the seedlings from
this semi-self-sterile plant would have profited in a higher degree from
a cross, than did the seedlings from the fully self-fertile plants. But
my anticipation was quite wrong, for they profited in a less degree. An
analogous result followed in the case of Eschscholtzia, in which the
offspring of the plants of Brazilian parentage (which were partially
self-sterile) did not profit more from a cross, than did the plants of
the far more self-fertile English stock. The above two lots of crossed
and self-fertilised seeds from the same plant of Reseda odorata, after
germinating on sand, were planted on opposite sides of five pots, and
measured as in the last case, with the result in Table 4/39.
TABLE 4/39. Reseda odorata (seedlings from a semi-self-sterile plant).
Heights of plants to the summits of the leafy stems, flower-stems not
included, measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 33 4/8 : 31.
Pot 1 : 30 6/8 : 28.
Pot 1 : 29 6/8 : 13 2/8.
Pot 1 : 20 : 32.
Pot 2 : 22 : 21 6/8.
Pot 2 : 33 4/8 : 26 6/8.
Pot 2 : 31 2/8 : 25 2/8.
Pot 2 : 32 4/8 : 30 4/8.
Pot 3 : 30 1/8 : 17 2/8.
Pot 3 : 32 1/8 : 29 6/8.
Pot 3 : 31 4/8 : 24 6/8.
Pot 3 : 32 2/8 : 34 2/8.
Pot 4 : 19 1/8 : 20 6/8.
Pot 4 : 30 1/8 : 32 6/8.
Pot 4 : 24 3/8 : 31 4/8.
Pot 4 : 30 6/8 : 36 6/8.
Pot 5 : 34 6/8 : 24 5/8.
Pot 5 : 37 1/8 : 34.
Pot 5 : 31 2/8 : 22 2/8.
Pot 5 : 33 : 37 1/8.
Total : 599.75 : 554.25.
The average height of the twenty crossed plants is here 29.98, and that
of the twenty self-fertilised 27.71 inches; or as 100 to 92. These
plants were then cut down and weighed; and the crossed in this case
exceeded the self-fertilised in weight by a mere trifle, namely, in the
ratio of 100 to 99. The two lots, left freely exposed to insects, seemed
to be equally fertile.
The remainder of the seed was sown in two adjoining rows in the open
ground; and the eight tallest plants in each row were measured, with the
result in Table 4/40.
TABLE 4/40. Reseda odorata, (seedlings from a semi-self-sterile plant,
planted in the open ground).
Heights of plants measured in inches.
Column 1: Crossed Plants.
Column 2: Self-fertilised Plants.
28 2/8 : 22 3/8.
22 4/8 : 24 3/8.
25 7/8 : 23 4/8.
25 3/8 : 21 4/8.
29 4/8 : 22 5/8.
27 1/8 : 27 3/8.
22 4/8 : 27 3/8.
26 2/8 : 19 2/8.
Total : 207.38 : 188.38.
The average height of the eight crossed plants is here 25.92, and that
of the eight self-fertilised plants 23.54 inches; or as 100 to 90.
9. VIOLACEAE.--Viola tricolor.
Whilst the flowers of the common cultivated heartsease are young, the
anthers shed their pollen into a little semi-cylindrical passage, formed
by the basal portion of the lower petal, and surrounded by papillae. The
pollen thus collected lies close beneath the stigma, but can seldom gain
access into its cavity, except by the aid of insects, which pass their
proboscides down this passage into the nectary. (4/5. The flowers of
this plant have been fully described by Sprengel, Hildebrand, Delpino,
and H. Muller. The latter author sums up all the previous observations
in his 'Befruchtung der Blumen' and in 'Nature' November 20, 1873 page
44. See also Mr. A.W. Bennett in 'Nature' May 15, 1873 page 50 and some
remarks by Mr. Kitchener ibid page 143. The facts which follow on the
effects of covering up a plant of V. tricolor have been quoted by Sir J.
Lubbock in his 'British Wild Flowers' etc. page 62.) Consequently when I
covered up a large plant of a cultivated variety, it set only eighteen
capsules, and most of these contained very few good seeds--several from
only one to three; whereas an equally fine uncovered plant of the same
variety, growing close by, produced 105 fine capsules. The few flowers
which produce capsules when insects are excluded, are perhaps fertilised
by the curling inwards of the petals as their wither, for by this means
pollen-grains adhering to the papillae might be inserted into the cavity
of the stigma. But it is more probable that their fertilisation is
effected, as Mr. Bennett suggests, by Thrips and certain minute beetles
which haunt the flowers, and which cannot be excluded by any net.
Humble-bees are the usual fertilisers; but I have more than once seen
flies (Rhingia rostrata) at work, with the under sides of their bodies,
heads and legs dusted with pollen; and having marked the flowers which
they visited, I found them after a few days fertilised. (4/6. I should
add that this fly apparently did not suck the nectar, but was attracted
by the papillae which surround the stigma. Hermann Muller also saw a
small bee, an Andrena, which could not reach the nectar, repeatedly
inserting its proboscis beneath the stigma, where the papillae are
situated; so that these papillae must be in some way attractive to
insects. A writer asserts 'Zoologist' volume 3-4 page 1225, that a moth
(Plusia) frequently visits the flowers of the pansy. Hive-bees do not
ordinarily visit them, but a case has been recorded 'Gardeners'
Chronicle' 1844 page 374, of these bees doing so. Hermann Muller has
also seen the hive-bee at work, but only on the wild small-flowered
form. He gives a list 'Nature' 1873 page 45, of all the insects which he
has seen visiting both the large and small-flowered forms. From his
account, I suspect that the flowers of plants in a state of nature are
visited more frequently by insects than those of the cultivated
varieties. He has seen several butterflies sucking the flowers of wild
plants, and this I have never observed in gardens, though I have watched
the flowers during many years.) It is curious for how long a time the
flowers of the heartsease and of some other plants may be watched
without an insect being seen to visit them. During the summer of 1841, I
observed many times daily for more than a fortnight some large clumps of
heartsease growing in my garden, before I saw a single humble-bee at
work. During another summer I did the same, but at last saw some
dark-coloured humble-bees visiting on three successive days almost every
flower in several clumps; and almost all these flowers quickly withered
and produced fine capsules. I presume that a certain state of the
atmosphere is necessary for the secretion of nectar, and that as soon as
this occurs the insects discover the fact by the odour emitted, and
immediately frequent the flowers.
As the flowers require the aid of insects for their complete
fertilisation, and as they are not visited by insects nearly so often as
most other nectar-secreting flowers, we can understand the remarkable
fact discovered by H. Muller and described by him in 'Nature,' namely,
that this species exists under two forms. One of these bears conspicuous
flowers, which, as we have seen, require the aid of insects, and are
adapted to be cross-fertilised by them; whilst the other form has much
smaller and less conspicuously coloured flowers, which are constructed
on a slightly different plan, favouring self-fertilisation, and are thus
adapted to ensure the propagation of the species. The self-fertile form,
however, is occasionally visited, and may be crossed by insects, though
this is rather doubtful.
In my first experiments on Viola tricolor I was unsuccessful in raising
seedlings, and obtained only one full-grown crossed and self-fertilised
plant. The former was 12 1/2 inches and the latter 8 inches in height.
On the following year several flowers on a fresh plant were crossed with
pollen from another plant, which was known to be a distinct seedling;
and to this point it is important to attend. Several other flowers on
the same plant were fertilised with their own pollen. The average number
of seeds in the ten crossed capsules was 18.7, and in the twelve
self-fertilised capsules 12.83; or as 100 to 69. These seeds, after
germinating on bare sand, were planted in pairs on the opposite sides of
five pots. They were first measured when about a third of their full
size, and the crossed plants then averaged 3.87 inches, and the
self-fertilised only 2.00 inches in height; or as 100 to 52. They were
kept in the greenhouse, and did not grow vigorously. Whilst in flower
they were again measured to the summits of their stems (see Table 4/41),
with the following result:--
TABLE 4/41. Viola tricolor.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 8 2/8 : 0 2/8.
Pot 1 : 7 4/8 : 2 4/8.
Pot 1 : 5 : 1 2/8.
Pot 2 : 5 : 6.
Pot 2 : 4 : 4.
Pot 2 : 4 4/8 : 3 1/8.
Pot 3 : 9 4/8 : 3 1/8.
Pot 3 : 3 3/8 : 1 7/8.
Pot 3 : 8 4/8 : 0 5/8.
Pot 4 : 4 7/8 : 2 1/8.
Pot 4 : 4 2/8 : 1 6/8.
Pot 4 : 4 : 2 1/8.
Pot 5 : 6 : 3.
Pot 5 : 3 3/8 : 1 4/8.
Total : 78.13 : 33.25.
The average height of the fourteen crossed plants is here 5.58 inches,
and that of the fourteen self-fertilised 2.37; or as 100 to 42. In four
out of the five pots, a crossed plant flowered before any one of the
self-fertilised; as likewise occurred with the pair raised during the
previous year. These plants without being disturbed were now turned out
of their pots and planted in the open ground, so as to form five
separate clumps. Early in the following summer (1869) they flowered
profusely, and being visited by humble-bees set many capsules, which
were carefully collected from all the plants on both sides. The crossed
plants produced 167 capsules, and the self-fertilised only 17; or as 100
to 10. So that the crossed plants were more than twice the height of the
self-fertilised, generally flowered first, and produced ten t
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