CONVOLVULACEAE.
Ipomoea purpurea, comparison of the height and fertility of the crossed
and self-fertilised plants during ten successive generations.
Greater constitutional vigour of the crossed plants.
The effects on the offspring of crossing different flowers on the same
plant, instead of crossing distinct individuals.
The effects of a cross with a fresh stock.
The descendants of the self-fertilised plant named Hero.
Summary on the growth, vigour, and fertility of the successive crossed
and self-fertilised generations.
Small amount of pollen in the anthers of the self-fertilised plants of
the later generations, and the sterility of their first-produced
flowers.
Uniform colour of the flowers produced by the self-fertilised plants.
The advantage from a cross between two distinct plants depends on their
differing in constitution.
A plant of Ipomoea purpurea, or as it is often called in England the
convolvulus major, a native of South America, grew in my greenhouse. Ten
flowers on this plant were fertilised with pollen from the same flower;
and ten other flowers on the same plant were crossed with pollen from a
distinct plant. The fertilisation of the flowers with their own pollen
was superfluous, as this convolvulus is highly self-fertile; but I acted
in this manner to make the experiments correspond in all respects.
Whilst the flowers are young the stigma projects beyond the anthers; and
it might have been thought that it could not be fertilised without the
aid of humble-bees, which often visit the flowers; but as the flower
grows older the stamens increase in length, and their anthers brush
against the stigma, which thus receives some pollen. The number of seeds
produced by the crossed and self-fertilised flowers differed very
little.
[Crossed and self-fertilised seeds obtained in the above manner were
allowed to germinate on damp sand, and as often as pairs germinated at
the same time they were planted in the manner described in the
Introduction (Chapter 1), on the opposite sides of two pots. Five pairs
were thus planted; and all the remaining seeds, whether or not in a
state of germination, were planted on the opposite sides of a third pot,
so that the young plants on both sides were here greatly crowded and
exposed to very severe competition. Rods of iron or wood of equal
diameter were given to all the plants to twine up; and as soon as one of
each pair reached the summit both were measured. A single rod was placed
on each side of the crowded pot, Number 3, and only the tallest plant on
each side was measured.
TABLE 2/1. Ipomoea purpurea (First Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Seedlings from Crossed Plants.
Column 3: Seedlings from Self-fertilised Plants.
Pot 1 : 87 4/8 : 69.
Pot 1 : 87 4/8 : 66.
Pot 1 : 89 : 73.
Pot 2 : 88 : 68 4/8.
Pot 2 : 87 : 60 4/8.
Pot 3 : 77 : 57.
Plants crowded; the tallest one measured on each side.
Total : 516 : 394.
The average height of the six crossed plants is here 86 inches, whilst
that of the six self-fertilised plants is only 65.66 inches, so that the
crossed plants are to the self-fertilised in height as 100 to 76. It
should be observed that this difference is not due to a few of the
crossed plants being extremely tall, or to a few of the self-fertilised
being extremely short, but to all the crossed plants attaining a greater
height than their antagonists. The three pairs in Pot 1 were measured at
two earlier periods, and the difference was sometimes greater and
sometimes less than that at the final measuring. But it is an
interesting fact, of which I have seen several other instances, that one
of the self-fertilised plants, when nearly a foot in height, was half an
inch taller than the crossed plant; and again, when two feet high, it
was 1 3/8 of an inch taller, but during the ten subsequent days the
crossed plant began to gain on its antagonist, and ever afterward
asserted its supremacy, until it exceeded its self-fertilised opponent
by 16 inches.
The five crossed plants in Pots 1 and 2 were covered with a net, and
produced 121 capsules; the five self-fertilised plants produced
eighty-four capsules, so that the numbers of capsules were as 100 to 69.
Of the 121 capsules on the crossed plants sixty-five were the product of
flowers crossed with pollen from a distinct plant, and these contained
on an average 5.23 seeds per capsule; the remaining fifty-six capsules
were spontaneously self-fertilised. Of the eighty-four capsules on the
self-fertilised plants, all the product of renewed self-fertilisation,
fifty-five (which were alone examined) contained on an average 4.85
seeds per capsule. Therefore the cross-fertilised capsules, compared
with the self-fertilised capsules, yielded seeds in the proportion of
100 to 93. The crossed seeds were relatively heavier than the
self-fertilised seeds. Combining the above data (i.e., number of
capsules and average number of contained seeds), the crossed plants,
compared with the self-fertilised, yielded seeds in the ratio of 100 to
64.
These crossed plants produced, as already stated, fifty-six
spontaneously self-fertilised capsules, and the self-fertilised plants
produced twenty-nine such capsules. The former contained on an average,
in comparison with the latter, seeds in the proportion of 100 to 99.
In Pot 3, on the opposite sides of which a large number of crossed and
self-fertilised seeds had been sown and the seedlings allowed to
struggle together, the crossed plants had at first no great advantage.
At one time the tallest crossed was 25 1/8 inches high, and the tallest
self-fertilised plants 21 3/8. But the difference afterwards became much
greater. The plants on both sides, from being so crowded, were poor
specimens. The flowers were allowed to fertilise themselves
spontaneously under a net; the crossed plants produced thirty-seven
capsules, the self-fertilised plants only eighteen, or as 100 to 47. The
former contained on an average 3.62 seeds per capsule; and the latter
3.38 seeds, or as 100 to 93. Combining these data (i.e., number of
capsules and average number of seeds), the crowded crossed plants
produced seeds compared with the self-fertilised as 100 to 45. These
latter seeds, however, were decidedly heavier, a hundred weighing 41.64
grains, than those from the capsules on the crossed plants, of which a
hundred weighed 36.79 grains; and this probably was due to the fewer
capsules borne by the self-fertilised plants having been better
nourished. We thus see that the crossed plants in this the first
generation, when grown under favourable conditions, and when grown under
unfavourable conditions from being much crowded, greatly exceeded in
height, and in the number of capsules produced, and slightly in the
number of seeds per capsule, the self-fertilised plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Flowers on the crossed plants of the last generation (Table 2/1) were
crossed by pollen from distinct plants of the same generation; and
flowers on the self-fertilised plants were fertilised by pollen from the
same flower. The seeds thus produced were treated in every respect as
before, and we have in Table 2/2 the result.
TABLE 2/2. Ipomoea purpurea (Second Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 87 : 67 4/8.
Pot 1 : 83 : 68 4/8.
Pot 1 : 83 : 80 4/8.
Pot 2 : 85 4/8 : 61 4/8.
Pot 2 : 89 : 79.
Pot 2 : 77 4/8 : 41.
Total : 505 : 398.
Here again every single crossed plant is taller than its antagonist. The
self-fertilised plant in Pot 1, which ultimately reached the unusual
height of 80 4/8 inches, was for a long time taller than the opposed
crossed plant, though at last beaten by it. The average height of the
six crossed plants is 84.16 inches, whilst that of the six
self-fertilised plants is 66.33 inches, or as 100 to 79.
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
Seeds from the crossed plants of the last generation (Table 2/2) again
crossed, and from the self-fertilised plants again self-fertilised, were
treated in all respects exactly as before, with the following result:--
TABLE 2/3. Ipomoea purpurea (Third Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 74 : 56 4/8.
Pot 1 : 72 : 51 4/8.
Pot 1 : 73 4/8 : 54.
Pot 2 : 82 : 59.
Pot 2 : 81 : 30.
Pot 2 : 82 : 66.
Total : 464.5 : 317.
Again all the crossed plants are higher than their antagonists: their
average height is 77.41 inches, whereas that of the self-fertilised is
52.83 inches, or as 100 to 68.
I attended closely to the fertility of the plants of this third
generation. Thirty flowers on the crossed plants were crossed with
pollen from other crossed plants of the same generation, and the
twenty-six capsules thus produced contained, on an average, 4.73 seeds;
whilst thirty flowers on the self-fertilised plants, fertilised with the
pollen from the same flower, produced twenty-three capsules, each
containing 4.43 seeds. Thus the average number of seeds in the crossed
capsules was to that in the self-fertilised capsules as 100 to 94. A
hundred of the crossed seeds weighed 43.27 grains, whilst a hundred of
the self-fertilised seeds weighed only 37.63 grains. Many of these
lighter self-fertilised seeds placed on damp sand germinated before the
crossed; thus thirty-six of the former germinated whilst only thirteen
of the latter or crossed seeds germinated. In Pot 1 the three crossed
plants produced spontaneously under the net (besides the twenty-six
artificially cross-fertilised capsules) seventy-seven self-fertilised
capsules containing on an average 4.41 seeds; whilst the three
self-fertilised plants produced spontaneously (besides the twenty-three
artificially self-fertilised capsules) only twenty-nine self-fertilised
capsules, containing on an average 4.14 seeds. Therefore the average
number of seeds in the two lots of spontaneously self-fertilised
capsules was as 100 to 94. Taking into consideration the number of
capsules together with the average number of seeds, the crossed plants
(spontaneously self-fertilised) produced seeds in comparison with the
self-fertilised plants (spontaneously self-fertilised) in the proportion
of 100 to 35. By whatever method the fertility of these plants is
compared, the crossed are more fertile than the self-fertilised plants.
I tried in several ways the comparative vigour and powers of growth of
the crossed and self-fertilised plants of this third generation. Thus,
four self-fertilised seeds which had just germinated were planted on one
side of a pot, and after an interval of forty-eight hours, four crossed
seeds in the same state of germination were planted on the opposite
side; and the pot was kept in the hothouse. I thought that the advantage
thus given to the self-fertilised seedlings would have been so great
that they would never have been beaten by the crossed ones. They were
not beaten until all had grown to a height of 18 inches; and the degree
to which they were finally beaten is shown in Table 2/4. We here see
that the average height of the four crossed plants is 76.62, and of the
four self-fertilised plants 65.87 inches, or as 100 to 86; therefore
less than when both sides started fair.
TABLE 2/4. Ipomoea purpurea (Third Generation, the self-fertilised
plants having had a start of forty-eight hours).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 3 : 78 4/8 : 73 4/8.
Pot 3 : 77 4/8 : 53.
Pot 3 : 73 : 61 4/8.
Pot 3 : 77 4/8 : 75 4/8.
Total : 306.5 : 263.5.
Crossed and self-fertilised seeds of the third generation were also sown
out of doors late in the summer, and therefore under unfavourable
conditions, and a single stick was given to each lot of plants to twine
up. The two lots were sufficiently separate so as not to interfere with
each other’s growth, and the ground was clear of weeds. As soon as they
were killed by the first frost (and there was no difference in their
hardiness), the two tallest crossed plants were found to be 24.5 and
22.5 inches, whilst the two tallest self-fertilised plants were only 15
and 12.5 inches in height, or as 100 to 59.
I likewise sowed at the same time two lots of the same seeds in a part
of the garden which was shady and covered with weeds. The crossed
seedlings from the first looked the most healthy, but they twined up a
stick only to a height of 7 1/4 inches; whilst the self-fertilised were
not able to twine at all; and the tallest of them was only 3 1/2 inches
in height.
Lastly, two lots of the same seeds were sown in the midst of a bed of
candy-tuft (Iberis) growing vigorously. The seedlings came up, but all
the self-fertilised ones soon died excepting one, which never twined and
grew to a height of only 4 inches. Many of the crossed seedlings, on the
other hand, survived; and some twined up the stems of the Iberis to the
height of 11 inches. These cases prove that the crossed seedlings have
an immense advantage over the self-fertilised, both when growing
isolated under very unfavourable conditions, and when put into
competition with each other or with other plants, as would happen in a
state of nature.
CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.
Seedlings raised as before from the crossed and self-fertilised plants
of the third generation in Table 2/3, gave results as follows:--
TABLE 2/5. Ipomoea purpurea (Fourth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 84 : 80.
Pot 1 : 47 : 44 1/2.
Pot 2 : 83 : 73 1/2.
Pot 2 : 59 : 51 1/2.
Pot 3 : 82 : 56 1/2.
Pot 3 : 65 1/2 : 63.
Pot 3 : 68 : 52.
Total : 488.5 : 421.0.
Here the average height of the seven crossed plants is 69.78 inches, and
that of the seven self-fertilised plants 60.14; or as 100 to 86. This
smaller difference relatively to that in the former generations, may be
attributed to the plants having been raised during the depth of winter,
and consequently to their not having grown vigorously, as was shown by
their general appearance and from several of them never reaching the
summits of the rods. In Pot 2, one of the self-fertilised plants was for
a long time taller by two inches than its opponent, but was ultimately
beaten by it, so that all the crossed plants exceeded their opponents in
height. Of twenty-eight capsules produced by the crossed plants
fertilised by pollen from a distinct plant, each contained on an average
4.75 seeds; of twenty-seven self-fertilised capsules on the
self-fertilised plants, each contained on an average 4.47 seeds; so that
the proportion of seeds in the crossed and self-fertilised capsules was
as 100 to 94.
Some of the same seeds, from which the plants in Table 2/5 had been
raised, were planted, after they had germinated on damp sand, in a
square tub, in which a large Brugmansia had long been growing. The soil
was extremely poor and full of roots; six crossed seeds were planted in
one corner, and six self-fertilised seeds in the opposite corner. All
the seedlings from the latter soon died excepting one, and this grew to
the height of only 1 1/2 inches. Of the crossed plants three survived,
and they grew to the height of 2 1/2 inches, but were not able to twine
round a stick; nevertheless, to my surprise, they produced some small
miserable flowers. The crossed plants thus had a decided advantage over
the self-fertilised plants under this extremity of bad conditions.
CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.
These were raised in the same manner as before, and when measured gave
the following results:--
TABLE 2/6. Ipomoea purpurea (Fifth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 96 : 73.
Pot 1 : 86 : 78.
Pot 1 : 69 : 29.
Pot 2 : 84 : 51.
Pot 2 : 84 : 84.
Pot 2 : 76 1/4 : 59.
Total : 495.25 : 374.00.
The average height of the six crossed plants is 82.54 inches, and that
of the six self-fertilised plants 62.33 inches, or as 100 to 75. Every
crossed plant exceeded its antagonist in height. In Pot 1 the middle
plant on the crossed side was slightly injured whilst young by a blow,
and was for a time beaten by its opponent, but ultimately recovered the
usual superiority. The crossed plants produced spontaneously a vast
number more capsules than did the self-fertilised plants; and the
capsules of the former contained on an average 3.37 seeds, whilst those
of the latter contained only 3.0 per capsule, or as 100 to 89. But
looking only to the artificially fertilised capsules, those on the
crossed plants again crossed contained on an average 4.46 seeds, whilst
those on the self-fertilised plants again self-fertilised contained 4.77
seeds; so that the self-fertilised capsules were the more fertile of the
two, and of this unusual fact I can offer no explanation.
CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.
These were raised in the usual manner, with the following result. I
should state that there were originally eight plants on each side; but
as two of the self-fertilised became extremely unhealthy and never grew
to near their full height, these as well as their opponents have been
struck out of the list. If they had been retained, they would have made
the average height of the crossed plants unfairly greater than that of
the self-fertilised. I have acted in the same manner in a few other
instances, when one of a pair plainly became very unhealthy.
TABLE 2/7. Ipomoea purpurea (Sixth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 93 : 50 1/2.
Pot 1 : 91 : 65.
Pot 2 : 79 : 50.
Pot 2 : 86 1/2 : 87.
Pot 2 : 88 : 62.
Pot 3 : 87 1/2 : 64 1/2.
Total : 525 : 379.
The average height of the six crossed plants is here 87.5, and of the
six self-fertilised plants 63.16, or as 100 to 72. This large difference
was chiefly due to most of the plants, especially the self-fertilised
ones, having become unhealthy towards the close of their growth, and
they were severely attacked by aphides. From this cause nothing can be
inferred with respect to their relative fertility. In this generation we
have the first instance of a self-fertilised plant in Pot 2 exceeding
(though only by half an inch) its crossed opponent. This victory was
fairly won after a long struggle. At first the self-fertilised plant was
several inches taller than its opponent, but when the latter was 4 1/2
feet high it had grown equal; it then grew a little taller than the
self-fertilised plant, but was ultimately beaten by it to the extent of
half an inch, as shown in Table 2/7. I was so much surprised at this
case that I saved the self-fertilised seeds of this plant, which I will
call the “Hero,” and experimented on its descendants, as will hereafter
be described.
Besides the plants included in Table 2/7, nine crossed and nine
self-fertilised plants of the same lot were raised in two other pots, 4
and 5. These pots had been kept in the hothouse, but from want of room
were, whilst the plants were young, suddenly moved during very cold
weather into the coldest part of the greenhouse. They all suffered
greatly, and never quite recovered. After a fortnight only two of the
nine self-fertilised seedlings were alive, whilst seven of the crossed
survived. The tallest of these latter plants when measured was 47 inches
in height, whilst the tallest of the two surviving self-fertilised
plants was only 32 inches. Here again we see how much more vigorous the
crossed plants are than the self-fertilised.
CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.
These were raised as heretofore with the following result:--
TABLE 2/8. Ipomoea purpurea (Seventh Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 84 4/8 : 74 6/8.
Pot 1 : 84 6/8 : 84.
Pot 1 : 76 2/8 : 55 4/8.
Pot 2 : 84 4/8 : 65.
Pot 2 : 90 : 51 2/8.
Pot 2 : 82 2/8 : 80 4/8.
Pot 3 : 83 : 67 6/8.
Pot 3 : 86 : 60 2/8.
Pot 4 : 84 2/8 : 75 2/8.
Total : 755.50 : 614.25.
Each of these nine crossed plants is higher than its opponent, though in
one case only by three-quarters of an inch. Their average height is
83.94 inches, and that of the self-fertilised plants 68.25, or as 100 to
81. These plants, after growing to their full height, became very
unhealthy and infested with aphides, just when the seeds were setting,
so that many of the capsules failed, and nothing can be said on their
relative fertility.
CROSSED AND SELF-FERTILISED PLANTS OF THE EIGHTH GENERATION.
As just stated, the plants of the last generation, from which the
present ones were raised, were very unhealthy and their seeds of
unusually small size; and this probably accounts for the two lots
behaving differently to what they did in any of the previous or
succeeding generations. Many of the self-fertilised seeds germinated
before the crossed ones, and these were of course rejected. When the
crossed seedlings in Table 2/9 had grown to a height of between 1 and 2
feet, they were all, or almost all, shorter than their self-fertilised
opponents, but were not then measured. When they had acquired an average
height of 32.28 inches, that of the self-fertilised plants was 40.68, or
as 100 to 122. Moreover, every one of the self-fertilised plants, with a
single exception, exceeded its crossed opponent. When, however, the
crossed plants had grown to an average height of 77.56 inches, they just
exceeded (namely, by .7 of an inch) the average height of the
self-fertilised plants; but two of the latter were still taller than
their crossed opponents. I was so much astonished at this whole case,
that I tied string to the summits of the rods; the plants being thus
allowed to continue climbing upwards. When their growth was complete
they were untwined, stretched straight, and measured. The crossed plants
had now almost regained their accustomed superiority, as may be seen in
Table 2/9.
The average height of the eight crossed plants is here 113.25 inches,
and that of the self-fertilised plants 96.65, or as 100 to 85.
Nevertheless two of the self-fertilised plants, as may be seen in Table
2/9, were still higher than their crossed opponents. The latter
manifestly had much thicker stems and many more lateral branches, and
looked altogether more vigorous than the self-fertilised plants, and
generally flowered before them. The earlier flowers produced by these
self-fertilised plants did not set any capsules, and their anthers
contained only a small amount of pollen; but to this subject I shall
return. Nevertheless capsules produced by two other self-fertilised
plants of the same lot, not included in Table 2/9, which had been highly
favoured by being grown in separate pots, contained the large average
number of 5.1 seeds per capsule.
TABLE 2/9. Ipomoea purpurea (Eighth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 111 6/8 : 96.
Pot 1 : 127 : 54.
Pot 1 : 130 6/8 : 93 4/8.
Pot 2 : 97 2/8 : 94.
Pot 2 : 89 4/8 : 125 6/8.
Pot 3 : 103 6/8 : 115 4/8.
Pot 3 : 100 6/8 : 84 6/8.
Pot 3 : 147 4/8 : 109 6/8.
Total : 908.25 : 773.25.
CROSSED AND SELF-FERTILISED PLANTS OF THE NINTH GENERATION.
The plants of this generation were raised in the same manner as before,
with the result shown in Table 2/10.
The fourteen crossed plants average in height 81.39 inches and the
fourteen self-fertilised plants 64.07, or as 100 to 79. One
self-fertilised plant in Pot 3 exceeded, and one in Pot 4 equalled in
height, its opponent. The self-fertilised plants showed no sign of
inheriting the precocious growth of their parents; this having been due,
as it would appear, to the abnormal state of the seeds from the
unhealthiness of their parents. The fourteen self-fertilised plants
yielded only forty spontaneously self-fertilised capsules, to which must
be added seven, the product of ten flowers artificially self-fertilised.
On the other hand, the fourteen crossed plants yielded 152 spontaneously
self-fertilised capsules; but thirty-six flowers on these plants were
crossed (yielding thirty-three capsules), and these flowers would
probably have produced about thirty spontaneously self-fertilised
capsules. Therefore an equal number of the crossed and self-fertilised
plants would have produced capsules in the proportion of about 182 to
47, or as 100 to 26. Another phenomenon was well pronounced in this
generation, but I believe had occurred previously to a slight extent;
namely, that most of the flowers on the self-fertilised plants were
somewhat monstrous. The monstrosity consisted in the corolla being
irregularly split so that it did not open properly, with one or two of
the stamens slightly foliaceous, coloured, and firmly coherent to the
corolla. I observed this monstrosity in only one flower on the crossed
plants. The self-fertilised plants, if well nourished, would almost
certainly, in a few more generations, have produced double flowers, for
they had already become in some degree sterile. (2/1. See on this
subject ‘Variation of Animals and Plants under Domestication’ chapter 18
2nd edition volume 2 page 152.)
TABLE 2/10. Ipomoea purpurea (Ninth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 83 4/8 : 57.
Pot 1 : 85 4/8 : 71.
Pot 1 : 83 4/8 : 48 3/8.
Pot 2 : 83 2/8 : 45.
Pot 2 : 64 2/8 : 43 6/8.
Pot 2 : 64 3/8 : 38 4/8.
Pot 3 : 79 : 63.
Pot 3 : 88 1/8 : 71.
Pot 3 : 61 : 89 4/8.
Pot 4 : 82 4/8 : 82 4/8.
Pot 4 : 90 : 76 1/8.
Pot 5 : 89 4/8 : 67.
Pot 5 : 92 4/8 : 74 2/8.
Pot 5 : 92 4/8 : 70.
Crowded plants.
Total : 1139.5 : 897.0.
CROSSED AND SELF-FERTILISED PLANTS OF THE TENTH GENERATION.
Six plants were raised in the usual manner from the crossed plants of
the last generation (Table 2/10) again intercrossed, and from the
self-fertilised again self-fertilised. As one of the crossed plants in
Pot 1 in Table 2/11 became much diseased, having crumpled leaves, and
producing hardly any capsules, it and its opponent have been struck out
of the table.
TABLE 2/11. Ipomoea purpurea (Tenth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 92 3/8 : 47 2/8.
Pot 1 : 94 4/8 : 34 6/8.
Pot 2 : 87 : 54 4/8.
Pot 2 : 89 5/8 : 49 2/8.
Pot 2 : 105 : 66 2/8.
Total : 468.5 : 252.0.
The five crossed plants average 93.7 inches, and the five
self-fertilised only 50.4, or as 100 to 54. This difference, however, is
so great that it must be looked at as in part accidental. The six
crossed plants (the diseased one here included) yielded spontaneously
101 capsules, and the six self-fertilised plants 88, the latter being
chiefly produced by one of the plants. But as the diseased plant, which
yielded hardly any seed, is here included, the ratio of 101 to 88 does
not fairly give the relative fertility of the two lots. The stems of the
six crossed plants looked so much finer than those of the six
self-fertilised plants, that after the capsules had been gathered and
most of the leaves had fallen off, they were weighed. Those of the
crossed plants weighed 2,693 grains, whilst those of the self-fertilised
plants weighed only 1,173 grains, or as 100 to 44; but as the diseased
and dwarfed crossed plant is here included, the superiority of the
former in weight was really greater.]
THE EFFECTS ON THE OFFSPRING OF CROSSING DIFFERENT FLOWERS ON THE SAME
PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.
In all the foregoing experiments, seedlings from flowers crossed by
pollen from a distinct plant (though in the later generations more or
less closely related) were put into competition with, and almost
invariably proved markedly superior in height to the offspring from
self-fertilised flowers. I wished, therefore, to ascertain whether a
cross between two flowers on the same plant would give to the offspring
any superiority over the offspring from flowers fertilised with their
own pollen. I procured some fresh seed and raised two plants, which were
covered with a net; and several of their flowers were crossed with
pollen from a distinct flower on the same plant. Twenty-nine capsules
thus produced contained on an average 4.86 seeds per capsule; and 100 of
these seeds weighed 36.77 grains. Several other flowers were fertilised
with their own pollen, and twenty-six capsules thus produced contained
on an average 4.42 seeds per capsule; 100 of which weighed 42.61 grains.
So that a cross of this kind appears to have increased slightly the
number of seeds per capsule, in the ratio of 100 to 91; but these
crossed seeds were lighter than the self-fertilised in the ratio of 86
to 100. I doubt, however, from other observations, whether these results
are fully trustworthy. The two lots of seeds, after germinating on sand,
were planted in pairs on the opposite sides of nine pots, and were
treated in every respect like the plants in the previous experiments.
The remaining seeds, some in a state of germination and some not so,
were sown on the opposite sides of a large pot (Number 10); and the four
tallest plants on each side of this pot were measured. The result is
shown in Table 2/12.
TABLE 2/12. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 82 : 77 4/8.
Pot 1 : 75 : 87.
Pot 1 : 65 : 64.
Pot 1 : 76 : 87 2/8.
Pot 2 : 78 4/8 : 84.
Pot 2 : 43 : 86 4/8.
Pot 2 : 65 4/8 : 90 4/8.
Pot 3 : 61 2/8 : 86.
Pot 3 : 85 : 69 4/8.
Pot 3 : 89 : 87 4/8.
Pot 4 : 83 : 80 4/8.
Pot 4 : 73 4/8 : 88 4/8.
Pot 4 : 67 : 84 4/8.
Pot 5 : 78 : 66 4/8.
Pot 5 : 76 6/8 : 77 4/8.
Pot 5 : 57 : 81 4/8.
Pot 6 : 70 4/8 : 80.
Pot 6 : 79 : 82 4/8.
Pot 6 : 79 6/8 : 55 4/8.
Pot 7 : 76 : 77.
Pot 7 : 84 4/8 : 83 4/8.
Pot 7 : 79 : 73 4/8.
Pot 8 : 73 : 76 4/8.
Pot 8 : 67 : 82.
Pot 8 : 83 : 80 4/8.
Pot 9 : 73 2/8 : 78 4/8.
Pot 9 : 78 : 67 4/8.
Pot 10 : 34 : 82 4/8.
Pot 10 : 82 : 36 6/8.
Pot 10 : 84 6/8 : 69 4/8.
Pot 10 : 71 : 75 2/8.
Crowded plants.
Total : 2270.25 : 2399.75.
The average height of the thirty-one crossed plants is 73.23 inches, and
that of the thirty-one self-fertilised plants 77.41 inches; or as 100 to
106. Looking to each pair, it may be seen that only thirteen of the
crossed plants, whilst eighteen of the self-fertilised plants exceed
their opponents. A record was kept with respect to the plant which
flowered first in each pot; and only two of the crossed flowered before
one of the self-fertilised in the same pot; whilst eight of the
self-fertilised flowered first. It thus appears that the crossed plants
are slightly inferior in height and in earliness of flowering to the
self-fertilised. But the inferiority in height is so small, namely as
100 to 106, that I should have felt very doubtful on this head, had I
not cut down all the plants (except those in the crowded pot Number 10)
close to the ground and weighed them. The twenty-seven crossed plants
weighed 16 1/2 ounces, and the twenty-seven self-fertilised plants 20
1/2 ounces; and this gives a ratio of 100 to 124.
A self-fertilised plant of the same parentage as those in Table 2/12 had
been raised in a separate pot for a distinct purpose; and it proved
partially sterile, the anthers containing very little pollen. Several
flowers on this plant were crossed with the little pollen which could be
obtained from the other flowers on the same plant; and other flowers
were self-fertilised. From the seeds thus produced four crossed and four
self-fertilised plants were raised, which were planted in the usual
manner on the opposite sides of two pots. All these four crossed plants
were inferior in height to their opponents; they averaged 78.18 inches,
whilst the four self-fertilised plants averaged 84.8 inches; or as 100
to 108. (2/2. From one of these self-fertilised plants, spontaneously
self-fertilised, I gathered twenty-four capsules, and they contained on
an average only 3.2 seeds per capsule; so that this plant had apparently
inherited some of the sterility of its parent.) This case, therefore,
confirms the last. Taking all the evidence together, we must conclude
that these strictly self-fertilised plants grew a little taller, were
heavier, and generally flowered before those derived from a cross
between two flowers on the same plant. These latter plants thus present
a wonderful contrast with those derived from a cross between two
distinct individuals.
THE EFFECTS ON THE OFFSPRING OF A CROSS WITH A DISTINCT OR FRESH STOCK
BELONGING TO THE SAME VARIETY.
From the two foregoing series of experiments we see, firstly, the good
effects during several successive generations of a cross between
distinct plants, although these were in some degree inter-related and
had been grown under nearly the same conditions; and, secondly, the
absence of all such good effects from a cross between flowers on the
same plant; the comparison in both cases being made with the offspring
of flowers fertilised with their own pollen. The experiments now to be
given show how powerfully and beneficially plants, which have been
intercrossed during many successive generations, having been kept all
the time under nearly uniform conditions, are affected by a cross with
another plant belonging to the same variety, but to a distinct family or
stock, which had grown under different conditions.
[Several flowers on the crossed plants of the ninth generation in Table
2/10, were crossed with pollen from another crossed plant of the same
lot. The seedlings thus raised formed the tenth intercrossed generation,
and I will call them the “INTERCROSSED PLANTS.” Several other flowers on
the same crossed plants of the ninth generation were fertilised (not
having been castrated) with pollen taken from plants of the same
variety, but belonging to a distinct family, which had been grown in a
distant garden at Colchester, and therefore under somewhat different
conditions. The capsules produced by this cross contained, to my
surprise, fewer and lighter seeds than did the capsules of the
intercrossed plants; but this, I think, must have been accidental. The
seedlings raised from them I will call the “COLCHESTER-CROSSED.” The two
lots of seeds, after germinating on sand, were planted in the usual
manner on the opposite sides of five pots, and the remaining seeds,
whether or not in a state of germination, were thickly sown on the
opposite sides of a very large pot, Number 6 in Table 2/13. In three of
the six pots, after the young plants had twined a short way up their
sticks, one of the Colchester-crossed plants was much taller than any
one of the intercrossed plants on the opposite side of the same pot; and
in the three other pots somewhat taller. I should state that two of the
Colchester-crossed plants in Pot 4, when about two-thirds grown, became
much diseased, and were, together with their intercrossed opponents,
rejected. The remaining nineteen plants, when almost fully grown, were
measured, with the following result:
TABLE 2/13. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Colchester-Crossed Plants.
Column 3: Intercrossed Plants of the Tenth Generation.
Pot 1 : 87 : 78.
Pot 1 : 87 4/8 : 68 4/8.
Pot 1 : 85 1/8 : 94 4/8.
Pot 2 : 93 6/8 : 60.
Pot 2 : 85 4/8 : 87 2/8.
Pot 2 : 90 5/8 : 45 4/8.
Pot 3 : 84 2/8 : 70 1/8.
Pot 3 : 92 4/8 : 81 6/8.
Pot 3 : 85 : 86 2/8.
Pot 4 : 95 6/8 : 65 1/8.
Pot 5 : 90 4/8 : 85 6/8.
Pot 5 : 86 6/8 : 63.
Pot 5 : 84 : 62 6/8.
Pot 6 : 90 4/8 : 43 4/8.
Pot 6 : 75 : 39 6/8.
Pot 6 : 71 : 30 2/8.
Pot 6 : 83 6/8 : 86.
Pot 6 : 63 : 53.
Pot 6 : 65 : 48 6/8.
Crowded plants in a very large pot.
Total : 1596.50 : 1249.75.
In sixteen out of these nineteen pairs, the Colchester-crossed plant
exceeded in height its intercrossed opponent. The average height of the
Colchester-crossed is 84.03 inches, and that of the intercrossed 65.78
inches; or as 100 to 78. With respect to the fertility of the two lots,
it was too troublesome to collect and count the capsules on all the
plants; so I selected two of the best pots, 5 and 6, and in these the
Colchester-crossed produced 269 mature and half-mature capsules, whilst
an equal number of the intercrossed plants produced only 154 capsules;
or as 100 to 57. By weight the capsules from the Colchester-crossed
plants were to those from the intercrossed plants as 100 to 51; so that
the former probably contained a somewhat larger average number of
seeds.]
We learn from this important experiment that plants in some degree
related, which had been intercrossed during the nine previous
generations, when they were fertilised with pollen from a fresh stock,
yielded seedlings as superior to the seedlings of the tenth intercrossed
generation, as these latter were to the self-fertilised plants of the
corresponding generation. For if we look to the plants of the ninth
generation in Table 2/10 (and these offer in most respects the fairest
standard of comparison) we find that the intercrossed plants were in
height to the self-fertilised as 100 to 79, and in fertility as 100 to
26; whilst the Colchester-crossed plants are in height to the
intercrossed as 100 to 78, and in fertility as 100 to 51.
[THE DESCENDANTS OF THE SELF-FERTILISED PLANT, NAMED HERO, WHICH
APPEARED IN THE SIXTH SELF-FERTILISED GENERATION.
In the five generations before the sixth, the crossed plant of each pair
was taller than its self-fertilised opponent; but in the sixth
generation (Table 2/7, Pot 2) the Hero appeared, which after a long and
dubious struggle conquered its crossed opponent, though by only half an
inch. I was so much surprised at this fact, that I resolved to ascertain
whether this plant would transmit its powers of growth to its seedlings.
Several flowers on Hero were therefore fertilised with their own pollen,
and the seedlings thus raised were put into competition with
self-fertilised and intercrossed plants of the corresponding generation.
The three lots of seedlings thus all belong to the seventh generation.
Their relative heights are shown in Tables 2/14 and 2/15.
TABLE 2/14. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants of the Seventh Generation, Children of
Hero.
Column 3: Self-fertilised Plants of the Seventh Generation.
Pot 1 : 74 : 89 4/8.
Pot 1 : 60 : 61.
Pot 1 : 55 2/8 : 49.
Pot 2 : 92 : 82.
Pot 2 : 91 6/8 : 56.
Pot 2 : 74 2/8 : 38.
Total : 447.25 : 375.50.
The average height of the six self-fertilised children of Hero is 74.54
inches, whilst that of the ordinary self-fertilised plants of the
corresponding generation is only 62.58 inches, or as 100 to 84.
TABLE 2/15. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants of the Seventh Generation, Children of
Hero.
Column 3: Intercrossed Plants of the Seventh Generation.
Pot 3 : 92 : 76 6/8.
Pot 4 : 87 : 89.
Pot 4 : 87 6/8 : 86 6/8.
Total : 266.75 : 252.50.
Here the average height of the three self-fertilised children of Hero is
88.91 inches, whilst that of the intercrossed plants is 84.16; or as 100
to 95. We thus see that the self-fertilised children of Hero certainly
inherit the powers of growth of their parents; for they greatly exceed
in height the self-fertilised offspring of the other self-fertilised
plants, and even exceed by a trifle the intercrossed plants,--all of the
corresponding generation.
Several flowers on the self-fertilised children of Hero in Table 2/14
were fertilised with pollen from the same flower; and from the seeds
thus produced, self-fertilised plants of the eighth generation
(grandchildren of Hero) were raised. Several other flowers on the same
plants were crossed with pollen from the other children of Hero. The
seedlings raised from this cross may be considered as the offspring of
the union of brothers and sisters. The result of the competition between
these two sets of seedlings (namely self-fertilised and the offspring of
brothers and sisters) is given in Table 2/16.
TABLE 2/16. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Grandchildren of Hero, from the
Self-fertilised Children. Eighth Generation.
Column 3: Grandchildren from a cross between the self-fertilised
children of Hero. Eighth Generation.
Pot 1 : 86 6/8 : 95 6/8.
Pot 1 : 90 3/8 : 95 3/8.
Pot 2 : 96 : 85.
Pot 2 : 77 2/8 : 93.
Pot 3 : 73 : 86 2/8.
Pot 3 : 66 : 82 2/8.
Pot 3 : 84 4/8 : 70 6/8.
Pot 4 : 88 1/8 : 66 3/8.
Pot 4 : 84 : 15 4/8.
Pot 4 : 36 2/8 : 38.
Pot 4 : 74 : 78 3/8.
Pot 5 : 90 1/8 : 82 6/8.
Pot 5 : 90 5/8 : 83 6/8.
Total : 1037.00 : 973.16.
The average height of the thirteen self-fertilised grandchildren of Hero
is 79.76 inches, and that of the grandchildren from a cross between the
self-fertilised children is 74.85; or as 100 to 94. But in Pot 4 one of
the crossed plants grew only to a height of 15 1/2 inches; and if this
plant and its opponent are struck out, as would be the fairest plan, the
average height of the crossed plants exceeds only by a fraction of an
inch that of the self-fertilised plants. It is therefore clear that a
cross between the self-fertilised children of Hero did not produce any
beneficial effect worth notice; and it is very doubtful whether this
negative result can be attributed merely to the fact of brothers and
sisters having been united, for the ordinary intercrossed plants of the
several successive generations must often have been derived from the
union of brothers and sisters (as shown in Chapter 1), and yet all of
them were greatly superior to the self-fertilised plants. We are
therefore driven to the suspicion, which we shall soon see strengthened,
that Hero transmitted to its offspring a peculiar constitution adapted
for self-fertilisation.
It would appear that the self-fertilised descendants of Hero have not
only inherited from Hero a power of growth equal to that of the ordinary
intercrossed plants, but have become more fertile when self-fertilised
than is usual with the plants of the present species. The flowers on the
self-fertilised grandchildren of Hero in Table 2.16 (the eighth
generation of self-fertilised plants) were fertilised with their own
pollen and produced plenty of capsules, ten of which (though this is too
few a number for a safe average) contained 5.2 seeds per capsule,--a
higher average than was observed in any other case with the
self-fertilised plants. The anthers produced by these self-fertilised
grandchildren were also as well developed and contained as much pollen
as those on the intercrossed plants of the corresponding generation;
whereas this was not the case with the ordinary self-fertilised plants
of the later generations. Nevertheless some few of the flowers produced
by the grandchildren of Hero were slightly monstrous, like those of the
ordinary self-fertilised plants of the later generations. In order not
to recur to the subject of fertility, I may add that twenty-one
self-fertilised capsules, spontaneously produced by the
great-grandchildren of Hero (forming the ninth generation of
self-fertilised plants), contained on an average 4.47 seeds; and this is
as high an average as the self-fertilised flowers of any generation
usually yielded.
Several flowers on the self-fertilised grandchildren of Hero in Table
2/16 were fertilised with pollen from the same flower; and the seedlings
raised from them (great-grandchildren of Hero) formed the ninth
self-fertilised generation. Several other flowers were crossed with
pollen from another grandchild, so that they may be considered as the
offspring of brothers and sisters, and the seedlings thus raised may be
called the INTERCROSSED great-grandchildren. And lastly, other flowers
were fertilised with pollen from a distinct stock, and the seedlings
thus raised may be called the COLCHESTER-CROSSED great-grandchildren. In
my anxiety to see what the result would be, I unfortunately planted the
three lots of seeds (after they had germinated on sand) in the hothouse
in the middle of winter, and in consequence of this the seedlings
(twenty in number of each kind) became very unhealthy, some growing only
a few inches in height, and very few to their proper height. The result,
therefore, cannot be fully trusted; and it would be useless to give the
measurements in detail. In order to strike as fair an average as
possible, I first excluded all the plants under 50 inches in height,
thus rejecting all the most unhealthy plants. The six self-fertilised
thus left were on an average 66.86 inches high; the eight intercrossed
plants 63.2 high; and the seven Colchester-crossed 65.37 high; so that
there was not much difference between the three sets, the
self-fertilised plants having a slight advantage. Nor was there any
great difference when only the plants under 36 inches in height were
excluded. Nor again when all the plants, however much dwarfed and
unhealthy, were included. In this latter case the Colchester-crossed
gave the lowest average of all; and if these plants had been in any
marked manner superior to the other two lots, as from my former
experience I fully expected they would have been, I cannot but think
that some vestige of such superiority would have been evident,
notwithstanding the very unhealthy condition of most of the plants. No
advantage, as far as we can judge, was derived from intercrossing two of
the grandchildren of Hero, any more than when two of the children were
crossed. It appears therefore that Hero and its descendants have varied
from the common type, not only in acquiring great power of growth, and
increased fertility when subjected to self-fertilisation, but in not
profiting from a cross with a distinct stock; and this latter fact, if
trustworthy, is a unique case, as far as I have observed in all my
experiments.]
SUMMARY ON THE GROWTH, VIGOUR, AND FERTILITY OF THE SUCCESSIVE
GENERATIONS OF THE CROSSED AND SELF-FERTILISED PLANTS OF Ipomoea
purpurea, TOGETHER WITH SOME MISCELLANEOUS OBSERVATIONS.
In Table 2/17, we see the average or mean heights of the ten successive
generations of the intercrossed and self-fertilised plants, grown in
competition with each other; and in the right hand column we have the
ratios of the one to the other, the height of the intercrossed plants
being taken at 100. In the bottom line the mean height of the
seventy-three intercrossed plants is shown to be 85.84 inches, and that
of the seventy-three self-fertilised plants 66.02 inches, or as 100 to
77.
TABLE 2/17. Ipomoea purpurea. Summary of measurements of the ten
generations.
Heights of Plants in inches:
Column 1: Name of Generation.
Column 2: Number of Crossed Plants.
Column 3: Average Height of Crossed Plants.
Column 4: Number of Self-fertilised Plants.
Column 5: Average Height of Self-fertilised Plants.
Column 6: n in Ratio between Average Heights of Crossed and
Self-fertilised Plants, expressed as 100 to n.
First generation Table 2/1 : 6 : 86.00 : 6 : 65.66 : 76.
Second generation Table 2/2 : 6 : 84.16 : 6 : 66.33 : 79.
Third generation Table 2/3 : 6 : 77.41 : 6 : 52.83 : 68.
Fourth generation Table 2/5 : 7 : 69.78 : 7 : 60.14 : 86.
Fifth generation Table 2/6 : 6 : 82.54 : 6 : 62.33 : 75.
Sixth generation Table 2/7 : 6 : 87.50 : 6 : 63.16 : 72.
Seventh generation Table 2/8 : 9 : 83.94 : 9 : 68.25 : 81.
Eighth generation Table 2/9 : 8 : 113.25 : 8 : 96.65 : 85.
Ninth generation Table 2/10 : 14 : 81.39 : 14 : 64.07 : 79.
Tenth generation Table 2/11 : 5 : 93.70 : 5 : 50.40 : 54.
All ten generations together : 73 : 85.84 : 73 : 66.02 : 77.
(DIAGRAM 2/1. Diagram showing the mean heights of the crossed and
self-fertilised plants of Ipomoea purpurea in the ten generations; the
mean height of the crossed plants being taken as 100. On the right hand,
the mean heights of the crossed and self-fertilised plants of all the
generations taken together are shown (as eleven pairs of unequal
vertical lines.))
The mean height of the self-fertilised plants in each of the ten
generations is also shown in the diagram 2/1, that of the intercrossed
plants being taken at 100, and on the right side we see the relative
heights of the seventy-three intercrossed plants, and of the
seventy-three self-fertilised plants. The difference in height between
the crossed and self-fertilised plants will perhaps be best appreciated
by an illustration: If all the men in a country were on an average 6
feet high, and there were some families which had been long and closely
interbred, these would be almost dwarfs, their average height during ten
generations being only 4 feet 8 1/4 inches.
It should be especially observed that the average difference between the
crossed and self-fertilised plants is not due to a few of the former
having grown to an extraordinary height, or to a few of the
self-fertilised being extremely short, but to all the crossed plants
having surpassed their self-fertilised opponents, with the few following
exceptions. The first occurred in the sixth generation, in which the
plant named “Hero” appeared; two in the eighth generation, but the
self-fertilised plants in this generation were in an anomalous
condition, as they grew at first at an unusual rate and conquered for a
time the opposed crossed plants; and two exceptions in the ninth
generation, though one of these plants only equalled its crossed
opponent. Therefore, of the seventy-three crossed plants, sixty-eight
grew to a greater height than the self-fertilised plants, to which they
were opposed.
In the right-hand column of figures, the difference in height between
the crossed and self-fertilised plants in the successive generations is
seen to fluctuate much, as might indeed have been expected from the
small number of plants measured in each generation being insufficient to
give a fair average. It should be remembered that the absolute height of
the plants goes for nothing, as each pair was measured as soon as one of
them had twined up to the summit of its rod. The great difference in the
tenth generation, namely, 100 to 54, no doubt was partly accidental,
though, when these plants were weighed, the difference was even greater,
namely, 100 to 44. The smallest amount of difference occurred in the
fourth and the eighth generations, and this was apparently due to both
the crossed and self-fertilised plants having become unhealthy, which
prevented the former attaining their usual degree of superiority. This
was an unfortunate circumstance, but my experiments were not thus
vitiated, as both lots of plants were exposed to the same conditions,
whether favourable or unfavourable.
There is reason to believe that the flowers of this Ipomoea, when
growing out of doors, are habitually crossed by insects, so that the
first seedlings which I raised from purchased seeds were probably the
offspring of a cross. I infer that this is the case, firstly from
humble-bees often visiting the flowers, and from the quantity of pollen
left by them on the stigmas of such flowers; and, secondly, from the
plants raised from the same lot of seed varying greatly in the colour of
their flowers, for as we shall hereafter see, this indicates much
intercrossing. (2/3. Verlot says ‘Sur la Production des Variétés’ 1865
page 66, that certain varieties of a closely allied plant, the
Convolvulus tricolor, cannot be kept pure unless grown at a distance
from all other varieties.) It is, therefore, remarkable that the plants
raised by me from flowers which were, in all probability,
self-fertilised for the first time after many generations of crossing,
should have been so markedly inferior in height to the intercrossed
plants as they were, namely, as 76 to 100. As the plants which were
self-fertilised in each succeeding generation necessarily became much
more closely interbred in the later than in the earlier generations, it
might have been expected that the difference in height between them and
the crossed plants would have gone on increasing; but, so far is this
from being the case, that the difference between the two sets of plants
in the seventh, eighth, and ninth generations taken together is less
than in the first and second generations together. When, however, we
remember that the self-fertilised and crossed plants are all descended
from the same mother-plant, that many of the crossed plants in each
generation were related, often closely related, and that all were
exposed to the same conditions, which, as we shall hereafter find, is a
very important circumstance, it is not at all surprising that the
difference between them should have somewhat decreased in the later
generations. It is, on the contrary, an astonishing fact, that the
crossed plants should have been victorious, even to a slight degree,
over the self-fertilised plants of the later generations.
The much greater constitutional vigour of the crossed than of the
self-fertilised plants, was proved on five occasions in various ways;
namely, by exposing them, while young, to a low temperature or to a
sudden change of temperature, or by growing them, under very
unfavourable conditions, in competition with full-grown plants of other
kinds.
With respect to the productiveness of the crossed and self-fertilised
plants of the successive generations, my observations unfortunately were
not made on any uniform plan, partly from the want of time, and partly
from not having at first intended to observe more than a single
generation. A summary of the results is here given in a tabulated form,
the fertility of the crossed plants being taken as 100.
TABLE 2/18. Ratio of productiveness of crossed and self-fertilised
plants. Ipomoea purpurea.
FIRST GENERATION OF CROSSED AND SELF-FERTILISED PLANTS GROWING IN
COMPETITION WITH ONE ANOTHER.
Sixty-five capsules produced from flowers on five crossed plants
fertilised by pollen from a distinct plant, and fifty-five capsules
produced from flowers on five self-fertilised plants fertilised by their
own pollen, contained seeds in the proportion of : 100 to 93.
Fifty-six spontaneously self-fertilised capsules on the above five
crossed plants, and twenty-five spontaneously self-fertilised capsules
on the above five self-fertilised plants, yielded seeds in the
proportion of : 100 to 99.
Combining the total number of capsules produced by these plants, and the
average number of seeds in each, the above crossed and self-fertilised
plants yielded seeds in the proportion of : 100 to 64.
Other plants of this first generation grown under unfavourable
conditions and spontaneously self-fertilised, yielded seeds in the
proportion of : 100 to 45.
THIRD GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
Crossed capsules compared with self-fertilised capsules contained seeds
in the ratio of : 100 to 94.
An equal number of crossed and self-fertilised plants, both
spontaneously self-fertilised, produced capsules in the ratio of : 100
to 38.
And these capsules contained seeds in the ratio of : 100 to 94.
Combining these data, the productiveness of the crossed to the
self-fertilised plants, both spontaneously self-fertilised, was as : 100
to 35.
FOURTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
Capsules from flowers on the crossed plants fertilised by pollen from
another plant, and capsules from flowers on the self-fertilised plants
fertilised with their own pollen, contained seeds in the proportion of :
100 to 94.
FIFTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
The crossed plants produced spontaneously a vast number more pods (not
actually counted) than the self-fertilised, and these contained seeds in
the proportion of : 100 to 89.
NINTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS.
Fourteen crossed plants, spontaneously self-fertilised, and fourteen
self-fertilised plants spontaneously self-fertilised, yielded capsules
(the average number of seeds per capsule not having been ascertained) in
the proportion of : 100 to 26.
PLANTS DERIVED FROM A CROSSED WITH A FRESH STOCK COMPARED WITH
INTERCROSSED PLANTS.
The offspring of intercrossed plants of the ninth generation, crossed by
a fresh stock, compared with plants of the same stock intercrossed
during ten generations, both sets of plants left uncovered and naturally
fertilised, produced capsules by weight as : 100 to 51.
We see in this table that the crossed plants are always in some degree
more productive than the self-fertilised plants, by whatever standard
they are compared. The degree differs greatly; but this depends chiefly
on whether an average was taken of the seeds alone, or of the capsules
alone, or of both combined. The relative superiority of the crossed
plants is chiefly due to their producing a much greater number of
capsules, and not to each capsule containing a larger average number of
seeds. For instance, in the third generation the crossed and
self-fertilised plants produced capsules in the ratio of 100 to 38,
whilst the seeds in the capsules on the crossed plants were to those on
the self-fertilised plants only as 100 to 94. In the eighth generation
the capsules on two self-fertilised plants (not included in table 2/18),
grown in separate pots and thus not subjected to any competition,
yielded the large average of 5.1 seeds. The smaller number of capsules
produced by the self-fertilised plants may be in part, but not
altogether, attributed to their lessened size or height; this being
chiefly due to their lessened constitutional vigour, so that they were
not able to compete with the crossed plants growing in the same pots.
The seeds produced by the crossed flowers on the crossed plants were not
always heavier than the self-fertilised seeds on the self-fertilised
plants. The lighter seeds, whether produced from crossed or
self-fertilised flowers, generally germinated before the heavier seeds.
I may add that the crossed plants, with very few exceptions, flowered
before their self-fertilised opponents, as might have been expected from
their greater height and vigour.
The impaired fertility of the self-fertilised plants was shown in
another way, namely, by their anthers being smaller than those in the
flowers on the crossed plants. This was first observed in the seventh
generation, but may have occurred earlier. Several anthers from flowers
on the crossed and self-fertilised plants of the eighth generation were
compared under the microscope; and those from the former were generally
longer and plainly broader than the anthers of the self-fertilised
plants. The quantity of pollen contained in one of the latter was, as
far as could be judged by the eye, about half of that contained in one
from a crossed plant. The impaired fertility of the self-fertilised
plants of the eighth generation was also shown in another manner, which
may often be observed in hybrids--namely, by the first-formed flowers
being sterile. For instance, the fifteen first flowers on a
self-fertilised plant of one of the later generations were carefully
fertilised with their own pollen, and eight of them dropped off; at the
same time fifteen flowers on a crossed plant growing in the same pot
were self-fertilised, and only one dropped off. On two other crossed
plants of the same generation, several of the earliest flowers were
observed to fertilise themselves and to produce capsules. In the plants
of the ninth, and I believe of some previous generations, very many of
the flowers, as already stated, were slightly monstrous; and this
probably was connected with their lessened fertility.
All the self-fertilised plants of the seventh generation, and I believe
of one or two previous generations, produced flowers of exactly the same
tint, namely, of a rich dark purple. So did all the plants, without any
exception, in the three succeeding generations of self-fertilised
plants; and very many were raised on account of other experiments in
progress not here recorded. My attention was first called to this fact
by my gardener remarking that there was no occasion to label the
self-fertilised plants, as they could always be known by their colour.
The flowers were as uniform in tint as those of a wild species growing
in a state of nature; whether the same tint occurred, as is probable, in
the earlier generations, neither my gardener nor self could recollect.
The flowers on the plants which were first raised from purchased seed,
as well as during the first few generations, varied much in the depth of
the purple tint; many were more or less pink, and occasionally a white
variety appeared. The crossed plants continued to the tenth generation
to vary in the same manner as before, but to a much less degree, owing,
probably, to their having become more or less closely inter-related. We
must therefore attribute the extraordinary uniformity of colour in the
flowers on the plants of the seventh and succeeding self-fertilised
generations, to inheritance not having been interfered with by crosses
during several preceding generations, in combination with the conditions
of life having been very uniform.
A plant appeared in the sixth self-fertilised generation, named the
Hero, which exceeded by a little in height its crossed antagonist, and
which transmitted its powers of growth and increased self-fertility to
its children and grandchildren. A cross between the children of Hero did
not give to the grandchildren any advantage over the self-fertilised
grandchildren raised from the self-fertilised children. And as far as my
observations can be trusted, which were made on very unhealthy plants,
the great-grandchildren raised from intercrossing the grandchildren had
no advantage over the seedlings from the grandchildren the product of
continued self-fertilisation; and what is far more remarkable, the
great-grandchildren raised by crossing the grandchildren with a fresh
stock, had no advantage over either the intercrossed or self-fertilised
great-grandchildren. It thus appears that Hero and its descendants
differed in constitution in an extraordinary manner from ordinary plants
of the present species.
Although the plants raised during ten successive generations from
crosses between distinct yet inter-related plants almost invariably
exceeded in height, constitutional vigour, and fertility their
self-fertilised opponents, it has been proved that seedlings raised by
intercrossing flowers on the same plant are by no means superior, on the
contrary are somewhat inferior in height and weight, to seedlings raised
from flowers fertilised with their own pollen. This is a remarkable
fact, which seems to indicate that self-fertilisation is in some manner
more advantageous than crossing, unless the cross brings with it, as is
generally the case, some decided and preponderant advantage; but to this
subject I shall recur in a future chapter.
The benefits which so generally follow from a cross between two plants
apparently depend on the two differing somewhat in constitution or
character. This is shown by the seedlings from the intercrossed plants
of the ninth generation, when crossed with pollen from a fresh stock,
being as superior in height and almost as superior in fertility to the
again intercrossed plants, as these latter were to seedlings from
self-fertilised plants of the corresponding generation. We thus learn
the important fact that the mere act of crossing two distinct plants,
which are in some degree inter-related and which have been long
subjected to nearly the same conditions, does little good as compared
with that from a cross between plants belonging to different stocks or
families, and which have been subjected to somewhat different
conditions. We may attribute the good derived from the crossing of the
intercrossed plants during the ten successive generations to their still
differing somewhat in constitution or character, as was indeed proved by
their flowers still differing somewhat in colour. But the several
conclusions which may be deduced from the experiments on Ipomoea will be
more fully considered in the final chapters, after all my other
observations have been given.