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Chapter VIII
DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL
VIGOUR AND IN OTHER RESPECTS.
Greater constitutional vigour of crossed plants.
The effects of great crowding.
Competition with other kinds of plants.
Self-fertilised plants more liable to premature death.
Crossed plants generally flower before the self-fertilised.
Negative effects of intercrossing flowers on the same plant.
Cases described.
Transmission of the good effects of a cross to later generations.
Effects of crossing plants of closely related parentage.
Uniform colour of the flowers on plants self-fertilised during several
generations and cultivated under similar conditions.
GREATER CONSTITUTIONAL VIGOUR OF CROSSED PLANTS.
As in almost all my experiments an equal number of crossed and
self-fertilised seeds, or more commonly seedlings just beginning to
sprout, were planted on the opposite sides of the same pots, they had to
compete with one another; and the greater height, weight, and fertility
of the crossed plants may be attributed to their possessing greater
innate constitutional vigour. Generally the plants of the two lots
whilst very young were of equal height; but afterwards the crossed
gained insensibly on their opponents, and this shows that they possessed
some inherent superiority, though not displayed at a very early period
in life. There were, however, some conspicuous exceptions to the rule of
the two lots being at first equal in height; thus the crossed seedlings
of the broom (Sarothamnus scoparius) when under three inches in height
were more than twice as tall as the self-fertilised plants.
After the crossed or the self-fertilised plants had once grown decidedly
taller than their opponents, a still increasing advantage would tend to
follow from the stronger plants robbing the weaker ones of nourishment
and overshadowing them. This was evidently the case with the crossed
plants of Viola tricolor, which ultimately quite overwhelmed the
self-fertilised. But that the crossed plants have an inherent
superiority, independently of competition, was sometimes well shown when
both lots were planted separately, not far distant from one another, in
good soil in the open ground. This was likewise shown in several cases,
even with plants growing in close competition with one another, by one
of the self-fertilised plants exceeding for a time its crossed opponent,
which had been injured by some accident or was at first sickly, but
being ultimately conquered by it. The plants of the eighth generation of
Ipomoea were raised from small seeds produced by unhealthy parents, and
the self-fertilised plants grew at first very rapidly, so that when the
plants of both lots were about three feet in height, the mean height of
the crossed to that of the self-fertilised was as 100 to 122; when they
were about six feet high the two lots were very nearly equal, but
ultimately when between eight and nine feet in height, the crossed
plants asserted their usually superiority, and were to the
self-fertilised in height as 100 to 85.
The constitutional superiority of the crossed over the self-fertilised
plants was proved in another way in the third generation of Mimulus, by
self-fertilised seeds being sown on one side of a pot, and after a
certain interval of time crossed seeds on the opposite side. The
self-fertilised seedlings thus had (for I ascertained that the seeds
germinated simultaneously) a clear advantage over the crossed in the
start for the race. Nevertheless they were easily beaten (as may be seen
under the head of Mimulus) when the crossed seeds were sown two whole
days after the self-fertilised. But when the interval was four days, the
two lots were nearly equal throughout life. Even in this latter case the
crossed plants still possessed an inherent advantage, for after both
lots had grown to their full height they were cut down, and without
being disturbed were transferred to a larger pot, and when in the
ensuing year they had again grown to their full height they were
measured; and now the tallest crossed plants were to the tallest
self-fertilised plants in height as 100 to 75, and in fertility (i.e.,
by weight of seeds produced by an equal number of capsules from both
lots) as 100 to 34.
My usual method of proceeding, namely, to plant several pairs of crossed
and self-fertilised seeds in an equal state of germination on the
opposite sides of the same pots, so that the plants were subjected to
moderately severe mutual competition, was I think the best that could
have been followed, and was a fair test of what occurs in a state of
nature. For plants sown by nature generally come up crowded, and are
almost always exposed to very severe competition with one another and
with other kinds of plants. This latter consideration led me to make
some trials, chiefly but not exclusively with Ipomoea and Mimulus, by
sowing crossed and self-fertilised seeds on the opposite sides of large
pots in which other plants had long been growing, or in the midst of
other plants out of doors. The seedlings were thus subjected to very
severe competition with plants of other kinds; and in all such cases,
the crossed seedlings exhibited a great superiority in their power of
growth over the self-fertilised.
After the germinating seedlings had been planted in pairs on the
opposite sides of several pots, the remaining seeds, whether or not in a
state of germination, were in most cases sown very thickly on the two
sides of an additional large pot; so that the seedlings came up
extremely crowded, and were subjected to extremely severe competition
and unfavourable conditions. In such cases the crossed plants almost
invariably showed a greater superiority over the self-fertilised, than
did the plants which grew in pairs in the pots.
Sometimes crossed and self-fertilised seeds were sown in separate rows
in the open ground, which was kept clear of weeds; so that the seedlings
were not subjected to any competition with other kinds of plants. Those
however in each row had to struggle with the adjoining ones in the same
row. When fully grown, several of the tallest plants in each row were
selected, measured, and compared. The result was in several cases (but
not so invariably as might have been expected) that the crossed plants
did not exceed in height the self-fertilised in nearly so great a degree
as when grown in pairs in the pots. Thus with the plants of Digitalis,
which competed together in pots, the crossed were to the self-fertilised
in height as 100 to 70; whilst those which were grown separately were
only as 100 to 85. Nearly the same result was observed with Brassica.
With Nicotiana the crossed were to the self-fertilised plants in height,
when grown extremely crowded together in pots, as 100 to 54; when grown
much less crowded in pots as 100 to 66, and when grow in the open
ground, so as to be subjected to but little competition, as 100 to 72.
On the other hand with Zea, there was a greater difference in height
between the crossed and self-fertilised plants growing out of doors,
than between the pairs which grew in pots in the hothouse; but this may
be attributed to the self-fertilised plants being more tender, so that
they suffered more than the crossed, when both lots were exposed to a
cold and wet summer. Lastly, with one out of two series of Reseda
odorata, grown out of doors in rows, as well as with Beta vulgaris, the
crossed plants did not at all exceed the self-fertilised in height, or
exceeded them by a mere trifle.
The innate power of the crossed plants to resist unfavourable conditions
far better than did the self-fertilised plants, was shown on two
occasions in a curious manner, namely, with Iberis and in the third
generation of Petunia, by the great superiority in height of the crossed
over the self-fertilised seedlings, when both sets were grown under
extremely unfavourable conditions; whereas owing to special
circumstances exactly the reverse occurred with the plants raised from
the same seeds and grown in pairs in pots. A nearly analogous case was
observed on two other occasions with plants of the first generation of
Nicotiana.
The crossed plants always withstood the injurious effects of being
suddenly removed into the open air after having been kept in the
greenhouse better than did the self-fertilised. On several occasions
they also resisted much better cold and intemperate weather. This was
manifestly the case with some crossed and self-fertilised plants of
Ipomoea, which were suddenly moved from the hothouse to the coldest part
of a cool greenhouse. The offspring of plants of the eighth
self-fertilised generation of Mimulus crossed by a fresh stock, survived
a frost which killed every single self-fertilised and intercrossed plant
of the same old stock. Nearly the same result followed with some crossed
and self-fertilised plants of Viola tricolor. Even the tips of the
shoots of the crossed plants of Sarothamnus scoparius were not touched
by a very severe winter; whereas all the self-fertilised plants were
killed halfway down to the ground, so that they were not able to flower
during the next summer. Young crossed seedlings of Nicotiana withstood a
cold and wet summer much better than the self-fertilised seedlings. I
have met with only one exception to the rule of crossed plants being
hardier than the self-fertilised: three long rows of Eschscholtzia
plants, consisting of crossed seedlings from a fresh stock, of
intercrossed seedlings of the same stock, and of self-fertilised ones,
were left unprotected during a severe winter, and all perished except
two of the self-fertilised. But this case is not so anomalous as it at
first appears, for it should be remembered that the self-fertilised
plants of Eschscholtzia always grow taller and are heavier than the
crossed; the whole benefit of a cross with this species being confined
to increased fertility.
Independently of any external cause which could be detected, the
self-fertilised plants were more liable to premature death than were the
crossed; and this seems to me a curious fact. Whilst the seedlings were
very young, if one died its antagonist was pulled up and thrown away,
and I believe that many more of the self-fertilised died at this early
age than of the crossed; but I neglected to keep any record. With Beta
vulgaris, however, it is certain that a large number of the
self-fertilised seeds perished after germinating beneath the ground,
whereas the crossed seeds sown at the same time did not thus suffer.
When a plant died at a somewhat more advanced age the fact was recorded;
and I find in my notes that out of several hundred plants, only seven of
the crossed died, whilst of the self-fertilised at least twenty-nine
were thus lost, that is more than four times as many. Mr. Galton, after
examining some of my tables, remarks: "It is very evident that the
columns with the self-fertilised plants include the larger number of
exceptionally small plants;" and the frequent presence of such puny
plants no doubt stands in close relation with their liability to
premature death. The self-fertilised plants of Petunia completed their
growth and began to wither sooner than did the intercrossed plants; and
these latter considerably before the offspring from a cross with a fresh
stock.
PERIOD OF FLOWERING.
In some cases, as with Digitalis, Dianthus, and Reseda, a larger number
of the crossed than of the self-fertilised plants threw up flower-stems;
but this probably was merely the result of their greater power of
growth; for in the first generation of Lobelia fulgens, in which the
self-fertilised plants greatly exceeded in height the crossed plants,
some of the latter failed to throw up flower-stems. With a large number
of species, the crossed plants exhibited a well-marked tendency to
flower before the self-fertilised ones growing in the same pots. It
should however be remarked that no record was kept of the flowering of
many of the species; and when a record was kept, the flowering of the
first plant in each pot was alone observed, although two or more pairs
grew in the same pot. I will now give three lists,--one of the species
in which the first plant that flowered was a crossed one,--a second in
which the first that flowered was a self-fertilised plant,--and a third
of those which flowered at the same time.
[SPECIES, OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF CROSSED
PARENTAGE.
Ipomoea purpurea.
I record in my notes that in all ten generations many of the crossed
plants flowered before the self-fertilised; but no details were kept.
Mimulus luteus (First Generation).
Ten flowers on the crossed plants were fully expanded before one on the
self-fertilised.
Mimulus luteus (Second and Third Generation).
In both these generations a crossed plant flowered before one of the
self-fertilised in all three pots.
Mimulus luteus (Fifth Generation).
In all three pots a crossed plant flowered first; yet the
self-fertilised plants, which belonged to the new tall variety, were in
height to the crossed as 126 to 100.
Mimulus luteus.
Plants derived from a cross with a fresh stock as well as the
intercrossed plants of the old stock, flowered before the
self-fertilised plants in nine out of the ten pots.
Salvia coccinea.
A crossed plant flowered before any one of the self-fertilised in all
three pots.
Origanum vulgare.
During two successive seasons several crossed plants flowered before the
self-fertilised.
Brassica oleracea (First Generation).
All the crossed plants growing in pots and in the open ground flowered
first.
Brassica oleracea (Second Generation).
A crossed plant in three out of the four pots flowered before any one of
the self-fertilised.
Iberis umbellata.
In both pots a crossed plant flowered first.
Eschscholtzia californica.
Plants derived from the Brazilian stock crossed by the English stock
flowered in five out of the nine pots first; in four of them a
self-fertilised plant flowered first; and not in one pot did an
intercrossed plant of the old stock flower first.
Viola tricolor.
A crossed plant in five out of the six pots flowered before any one of
the self-fertilised.
Dianthus caryophyllus (First Generation).
In two large beds of plants, four of the crossed plants flowered before
any one of the self-fertilised.
Dianthus caryophyllus (Second Generation).
In both pots a crossed plant flowered first.
Dianthus caryophyllus (Third Generation).
In three out of the four pots a crossed plant flowered first; yet the
crossed were to the self-fertilised in height only as 100 to 99, but in
weight as 100 to 49.
Dianthus caryophyllus.
Plants derived from a cross with a fresh stock, and the intercrossed
plants of the old stock, both flowered before the self-fertilised in
nine out of the ten pots.
Hibiscus africanus.
In three out of the four pots a crossed plant flowered before any one of
the self-fertilised; yet the latter were to the crossed in height as 109
to 100.
Tropaeolum minus.
A crossed plant flowered before any one of the self-fertilised in three
out of the four pots, and simultaneously in the fourth pot.
Limnanthes douglasii.
A crossed plant flowered before any one of the self-fertilised in four
out of the five pots.
Phaseolus multiflorus.
In both pots a crossed plant flowered first.
Specularia speculum.
In all four pots a crossed plant flowered first.
Lobelia ramosa (First Generation).
In all four pots a crossed plant flowered before any one of the
self-fertilised.
Lobelia ramosa (Second Generation).
In all four pots a crossed plant flowered some days before any one of
the self-fertilised.
Nemophila insignis.
In four out of the five pots a crossed plant flowered first.
Borago officinalis.
In both pots a crossed plant flowered first.
Petunia violacea (Second Generation).
In all three pots a crossed plant flowered first.
Nicotiana tabacum.
A plant derived from a cross with a fresh stock flowered before any one
of the self-fertilised plants of the fourth generation, in fifteen out
of the sixteen pots.
Cyclamen persicum.
During two successive seasons a crossed plant flowered some weeks before
any one of the self-fertilised in all four pots.
Primula veris (equal-styled var.)
In all three pots a crossed plant flowered first.
Primula sinensis.
In all four pots plants derived from an illegitimate cross between
distinct plants flowered before any one of the self-fertilised plants.
Primula sinensis.
A legitimately crossed plant flowered before any one of the
self-fertilised plants in seven out of the eight pots.
Fagopyrum esculentum.
A legitimately crossed plant flowered from one to two days before any
one of the self-fertilised plants in all three pots.
Zea mays.
In all four pots a crossed plant flowered first.
Phalaris canariensis.
The crossed plants flowered before the self-fertilised in the open
ground, but simultaneously in the pots.
SPECIES OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF SELF-FERTILISED
PARENTAGE.
Eschscholtzia californica (First Generation).
The crossed plants were at first taller than the self-fertilised, but on
their second growth during the following year the self-fertilised
exceeded the crossed in height, and now they flowered first in three out
of the four pots.
Lupinus luteus.
Although the crossed plants were to the self-fertilised in height as 100
to 82; yet in all three pots the self-fertilised plants flowered first.
Clarkia elegans.
Although the crossed plants were, as in the last case, to the
self-fertilised in height as 100 to 82, yet in the two pots the
self-fertilised flowered first.
Lobelia fulgens (First Generation).
The crossed plants were to the self-fertilised in height only as 100 to
127, and the latter flowered much before the crossed.
Petunia violacea (Third Generation).
The crossed plants were to the self-fertilised in height as 100 to 131,
and in three out of the four pots a self-fertilised plant flowered
first; in the fourth pot simultaneously.
Petunia violacea (Fourth generation).
Although the crossed plants were to the self-fertilised in height as 100
to 69, yet in three out of the five pots a self-fertilised plant
flowered first; in the fourth pot simultaneously, and only in the fifth
did a crossed plant flower first.
Nicotiana tabacum (First Generation).
The crossed plants were to the self-fertilised in height only as 100 to
178, and a self-fertilised plant flowered first in all four pots.
Nicotiana tabacum (Third Generation).
The crossed plants were to the self-fertilised in height as 100 to 101,
and in four out of the five pots a self-fertilised plant flowered first.
Canna warscewiczi.
In the three generations taken together the crossed were to the
self-fertilised in height as 100 to 101; in the first generation the
self-fertilised plants showed some tendency to flower first, and in the
third generation they flowered first in nine out of the twelve pots.
SPECIES IN WHICH THE CROSSED AND SELF-FERTILISED PLANTS FLOWERED ALMOST
SIMULTANEOUSLY.
Mimulus luteus (Sixth Generation).
The crossed plants were inferior in height and vigour to the
self-fertilised plants, which all belonged to the new white-flowered
tall variety, yet in only half the pots did the self-fertilised plants
flower first, and in the other half the crossed plants.
Viscaria oculata.
The crossed plants were only a little taller than the self-fertilised
(namely, as 100 to 97), but considerably more fertile, yet both lots
flowered almost simultaneously.
Lathyrus odoratus (Second Generation).
Although the crossed plants were to the self-fertilised in height as 100
to 88, yet there was no marked difference in their period of flowering.
Lobelia fulgens (Second Generation).
Although the crossed plants were to the self-fertilised in height as 100
to 91, yet they flowered simultaneously.
Nicotiana tabacum (Third Generation).
Although the crossed plants were to the self-fertilised in height as 100
to 83, yet in half the pots a self-fertilised plant flowered first, and
in the other half a crossed plant.]
These three lists include fifty-eight cases, in which the period of
flowering of the crossed and self-fertilised plants was recorded. In
forty-four of them a crossed plant flowered first either in a majority
of the pots or in all; in nine instances a self-fertilised plant
flowered first, and in five the two lots flowered simultaneously. One of
the most striking cases is that of Cyclamen, in which the crossed plants
flowered some weeks before the self-fertilised in all four pots during
two seasons. In the second generation of Lobelia ramosa, a crossed plant
flowered in all four pots some days before any one of the
self-fertilised. Plants derived from a cross with a fresh stock
generally showed a very strongly marked tendency to flower before the
self-fertilised and the intercrossed plants of the old stock; all three
lots growing in the same pots. Thus with Mimulus and Dianthus, in only
one pot out of ten, and in Nicotiana in only one pot out of sixteen, did
a self-fertilised plant flower before the plants of the two crossed
kinds,--these latter flowering almost simultaneously.
A consideration of the two first lists, especially of the second one,
shows that a tendency to flower first is generally connected with
greater power of growth, that is, with greater height. But there are
some remarkable exceptions to this rule, proving that some other cause
comes into play. Thus the crossed plants both of Lupinus luteus and
Clarkia elegans were to the self-fertilised plants in height as 100 to
82, and yet the latter flowered first. In the third generation of
Nicotiana, and in all three generations of Canna, the crossed and
self-fertilised plants were of nearly equal height, yet the
self-fertilised tended to flower first. On the other hand, with Primula
sinensis, plants raised from a cross between two distinct individuals,
whether these were legitimately or illegitimately crossed, flowered
before the illegitimately self-fertilised plants, although all the
plants were of nearly equal height in both cases. So it was with respect
to height and flowering with Phaseolus, Specularia, and Borago. The
crossed plants of Hibiscus were inferior in height to the
self-fertilised, in the ratio of 100 to 109, and yet they flowered
before the self-fertilised in three out of the four pots. On the whole,
there can be no doubt that the crossed plants exhibit a tendency to
flower before the self-fertilised, almost though not quite so strongly
marked as to grow to a greater height, to weigh more, and to be more
fertile.
A few other cases not included in the above three lists deserve notice.
In all three pots of Viola tricolor, naturally crossed plants the
offspring of crossed plants flowered before naturally crossed plants the
offspring of self-fertilised plants. Flowers on two plants, both of
self-fertilised parentage, of the sixth generation of Mimulus luteus
were intercrossed, and other flowers on the same plants were fertilised
with their own pollen; intercrossed seedlings and seedlings of the
seventh self-fertilised generation were thus raised, and the latter
flowered before the intercrossed in three out of the five pots. Flowers
on a plant both of Mimulus luteus and of Ipomoea purpurea were crossed
with pollen from other flowers on the same plant, and other flowers were
fertilised with their own pollen; intercrossed seedlings of this
peculiar kind, and others strictly self-fertilised being thus raised. In
the case of the Mimulus the self-fertilised plants flowered first in
seven out of the eight pots, and in the case of the Ipomoea in eight out
of the ten pots; so that an intercross between the flowers on the same
plant was very far from giving to the offspring thus raised, any
advantage over the strictly self-fertilised plants in their period of
flowering.
EFFECTS OF CROSSING FLOWERS ON THE SAME PLANT.
In the discussion on the results of a cross with a fresh stock, given
under Table 7/C in the last chapter, it was shown that the mere act of
crossing by itself does no good; but that the advantages thus derived
depend on the plants which are crossed, either consisting of distinct
varieties which will almost certainly differ somewhat in constitution,
or on the progenitors of the plants which are crossed, though identical
in every external character, having been subjected to somewhat different
conditions and having thus acquired some slight difference in
constitution. All the flowers produced by the same plant have been
developed from the same seed; those which expand at the same time have
been exposed to exactly the same climatic influences; and the stems have
all been nourished by the same roots. Therefore in accordance with the
conclusion just referred to, no good ought to result from crossing
flowers on the same plant. (8/1. It is, however, possible that the
stamens which differ in length or construction in the same flower may
produce pollen differing in nature, and in this manner a cross might be
made effective between the several flowers on the same plant. Mr. Macnab
states in a communication to M. Verlot 'La Production des Varietes' 1865
page 42, that seedlings raised from the shorter and longer stamens of
rhododendron differ in character; but the shorter stamens apparently are
becoming rudimentary, and the seedlings are dwarfs, so that the result
may be simply due to a want of fertilising power in the pollen, as in
the case of the dwarfed plants of Mirabilis raised by Naudin by the use
of too few pollen-grains. Analogous statements have been made with
respect to the stamens of Pelargonium. With some of the Melastomaceae,
seedlings raised by me from flowers fertilised by pollen from the
shorter stamens, certainly differed in appearance from those raised from
the longer stamens, with differently coloured anthers; but here, again,
there is some reason for believing that the shorter stamens are tending
towards abortion. In the very different case of trimorphic heterostyled
plants, the two sets of stamens in the same flower have widely different
fertilising powers.) In opposition to this conclusion is the fact that a
bud is in one sense a distinct individual, and is capable of
occasionally or even not rarely assuming new external characters, as
well as new constitutional peculiarities. Plants raised from buds which
have thus varied may be propagated for a great length of time by grafts,
cuttings, etc., and sometimes even by seminal generation. (8/2. I have
given numerous cases of such bud-variations in my 'Variation of Animals
and Plants under Domestication' chapter 11 2nd edition volume 1 page
448.) There exist also numerous species in which the flowers on the same
plant differ from one another,--as in the sexual organs of monoecious
and polygamous plants,--in the structure of the circumferential flowers
in many Compositae, Umbelliferae, etc.,--in the structure of the central
flower in some plants,--in the two kinds of flowers produced by
cleistogene species,--and in several other such cases. These instances
clearly prove that the flowers on the same plant have often varied
independently of one another in many important respects, such variations
having been fixed, like those on distinct plants during the development
of species.
It was therefore necessary to ascertain by experiment what would be the
effect of intercrossing flowers on the same plant, in comparison with
fertilising them with their own pollen or crossing them with pollen from
a distinct plant. Trials were carefully made on five genera belonging to
four families; and in only one case, namely, Digitalis, did the
offspring from a cross between the flowers on the same plant receive any
benefit, and the benefit here was small compared with that derived from
a cross between distinct plants. In the chapter on Fertility, when we
consider the effects of cross-fertilisation and self-fertilisation on
the productiveness of the parent-plants we shall arrive at nearly the
same result, namely, that a cross between the flowers on the same plant
does not at all increase the number of the seeds, or only occasionally
and to a slight degree. I will now give an abstract of the results of
the five trials which were made.
1. Digitalis purpurea.
Seedlings raised from intercrossed flowers on the same plant, and others
from flowers fertilised with their own pollen, were grown in the usual
manner in competition with one another on the opposite sides of ten
pots. In this and the four following cases, the details may be found
under the head of each species. In eight pots, in which the plants did
not grow much crowded, the flower-stems on sixteen intercrossed plants
were in height to those on sixteen self-fertilised plants, as 100 to 94.
In the two other pots on which the plants grew much crowded, the
flower-stems on nine intercrossed plants were in height to those on nine
self-fertilised plants, as 100 to 90. That the intercrossed plants in
these two latter pots had a real advantage over their self-fertilised
opponents, was well shown by their relative weights when cut down, which
was as 100 to 78. The mean height of the flower-stems on the twenty-five
intercrossed plants in the ten pots taken together, was to that of the
flower-stems on the twenty-five self-fertilised plants, as 100 to 92.
Thus the intercrossed plants were certainly superior to the
self-fertilised in some degree; but their superiority was small compared
with that of the offspring from a cross between distinct plants over the
self-fertilised, this being in the ratio of 100 to 70 in height. Nor
does this latter ratio show at all fairly the great superiority of the
plants derived from a cross between distinct individuals over the
self-fertilised, as the former produced more than twice as many
flower-stems as the latter, and were much less liable to premature
death.
2. Ipomoea purpurea.
Thirty-one intercrossed plants raised from a cross between flowers on
the same plants were grown in ten pots in competition with the same
number of self-fertilised plants, and the former were to the latter in
height as 100 to 105. So that the self-fertilised plants were a little
taller than the intercrossed; and in eight out of the ten pots a
self-fertilised plant flowered before any one of the crossed plants in
the same pots. The plants which were not greatly crowded in nine of the
pots (and these offer the fairest standard of comparison) were cut down
and weighed; and the weight of the twenty-seven intercrossed plants was
to that of the twenty-seven self-fertilised as 100 to 124; so that by
this test the superiority of the self-fertilised was strongly marked. To
this subject of the superiority of the self-fertilised plants in certain
cases, I shall have to recur in a future chapter. If we now turn to the
offspring from a cross between distinct plants when put into competition
with self-fertilised plants, we find that the mean height of
seventy-three such crossed plants, in the course of ten generations, was
to that of the same number of self-fertilised plants as 100 to 77; and
in the case of the plants of the tenth generation in weight as 100 to
44. Thus the contrast between the effects of crossing flowers on the
same plant, and of crossing flowers on distinct plants, is wonderfully
great.
3. Mimulus luteus.
Twenty-two plants raised by crossing flowers on the same plant were
grown in competition with the same number of self-fertilised plants; and
the former were to the latter in height as 100 to 105, and in weight as
100 to 103. Moreover, in seven out of the eight pots a self-fertilised
plant flowered before any of the intercrossed plants. So that here again
the self-fertilised exhibit a slight superiority over the intercrossed
plants. For the sake of comparison, I may add that seedlings raised
during three generations from a cross between distinct plants were to
the self-fertilised plants in height as 100 to 65.
4. Pelargonium zonale.
Two plants growing in separate pots, which had been propagated by
cuttings from the same plant, and therefore formed in fact parts of the
same individual, were intercrossed, and other flowers on one of these
plants were self-fertilised; but the seedlings obtained by the two
processes did not differ in height. When, on the other hand, flowers on
one of the above plants were crossed with pollen taken from a distinct
seedling, and other flowers were self-fertilised, the crossed offspring
thus obtained were to the self-fertilised in height as 100 to 74.
5. Origanum vulgare.
A plant which had been long cultivated in my kitchen garden, had spread
by stolons so as to form a large bed or clump. Seedlings raised by
intercrossing flowers on these plants, which strictly consisted of the
same plant, and other seedlings raised from self-fertilised flowers,
were carefully compared from their earliest youth to maturity; and they
did not differ at all in height or in constitutional vigour. Some
flowers on these seedlings were then crossed with pollen taken from a
distinct seedling, and other flowers were self-fertilised; two fresh
lots of seedlings being thus raised, which were the grandchildren of the
plant that had spread by stolons and formed a large clump in my garden.
These differed much in height, the crossed plants being to the
self-fertilised as 100 to 86. They differed, also, to a wonderful degree
in constitutional vigour. The crossed plants flowered first, and
produced exactly twice as many flower-stems; and they afterwards
increased by stolons to such an extent as almost to overwhelm the
self-fertilised plants.
Reviewing these five cases, we see that in four of them, the effect of a
cross between flowers on the same plant (even on offsets of the same
plant growing on separate roots, as with the Pelargonium and Origanum)
does not differ from that of the strictest self-fertilisation. Indeed,
in two of the cases the self-fertilised plants were superior to such
intercrossed plants. With Digitalis a cross between the flowers on the
same plant certainly did do some good, yet very slight compared with
that from a cross between distinct plants. On the whole the results here
arrived at, if we bear in mind that the flower-buds are to a certain
extent distinct individuals and occasionally vary independently of one
another, agree well with our general conclusion, that the advantages of
a cross depend on the progenitors of the crossed plants possessing
somewhat different constitutions, either from having been exposed to
different conditions, or to their having varied from unknown causes in a
manner which we in our ignorance are forced to speak of as spontaneous.
Hereafter I shall have to recur to this subject of the inefficiency of a
cross between the flowers on the same plant, when we consider the part
which insects play in the cross-fertilisation of flowers.
ON THE TRANSMISSION OF THE GOOD EFFECTS FROM A CROSS AND OF THE EVIL
EFFECTS FROM SELF-FERTILISATION.
We have seen that seedlings from a cross between distinct plants almost
always exceed their self-fertilised opponents in height, weight, and
constitutional vigour, and, as will hereafter be shown, often in
fertility. To ascertain whether this superiority would be transmitted
beyond the first generation, seedlings were raised on three occasions
from crossed and self-fertilised plants, both sets being fertilised in
the same manner, and therefore not as in the many cases given in Tables
7/A, 7/B, 7/C, in which the crossed plants were again crossed and the
self-fertilised again self-fertilised.
Firstly, seedlings were raised from self-fertilised seeds produced under
a net by crossed and self-fertilised plants of Nemophila insignis; and
the latter were to the former in height as 133 to 100. But these
seedlings became very unhealthy early in life, and grew so unequally
that some of them in both lots were five times as tall as the others.
Therefore this experiment was quite worthless; but I have felt bound to
give it, as opposed to my general conclusion. I should state that in
this and the two following trials, both sets of plants were grown on the
opposite sides of the same pots, and treated in all respects alike. The
details of the experiments may be found under the head of each species.
Secondly, a crossed and a self-fertilised plant of Heartsease (Viola
tricolor) grew near together in the open ground and near to other plants
of heartsease; and as both produced an abundance of very fine capsules,
the flowers on both were certainly cross-fertilised by insects. Seeds
were collected from both plants, and seedlings raised from them. Those
from the crossed plants flowered in all three pots before those from the
self-fertilised plants; and when fully grown the former were to the
latter in height as 100 to 82. As both sets of plants were the product
of cross-fertilisation, the difference in their growth and period of
flowering was clearly due to their parents having been of crossed and
self-fertilised parentage; and it is equally clear that they transmitted
different constitutional powers to their offspring, the grandchildren of
the plants which were originally crossed and self-fertilised.
Thirdly, the Sweet Pea (Lathyrus odoratus) habitually fertilises itself
in this country. As I possessed plants, the parents and grandparents of
which had been artificially crossed and other plants descended from the
same parents which had been self-fertilised for many previous
generations, these two lots of plants were allowed to fertilise
themselves under a net, and their self-fertilised seeds saved. The
seedlings thus raised were grown in competition with each other in the
usual manner, and differed in their powers of growth. Those from the
self-fertilised plants which had been crossed during the two previous
generations were to those from the plants self-fertilised during many
previous generations in height as 100 to 90. These two lots of seeds
were likewise tried by being sown under very unfavourable conditions in
poor exhausted soil, and the plants whose grandparents and
great-grandparents had been crossed showed in an unmistakable manner
their superior constitutional vigour. In this case, as in that of the
heartsease, there could be no doubt that the advantage derived from a
cross between two plants was not confined to the offspring of the first
generation. That constitutional vigour due to cross-parentage is
transmitted for many generations may also be inferred as highly
probable, from some of Andrew Knight's varieties of the common pea,
which were raised by crossing distinct varieties, after which time they
no doubt fertilised themselves in each succeeding generation. These
varieties lasted for upwards of sixty years, "but their glory is now
departed." (8/3. See the evidence on this head in my 'Variation under
Domestication' chapter 9 volume 1 2nd edition page 397.) On the other
hand, most of the varieties of the common pea, which there is no reason
to suppose owe their origin to a cross, have had a much shorter
existence. Some also of Mr. Laxton's varieties produced by artificial
crosses have retained their astonishing vigour and luxuriance for a
considerable number of generations; but as Mr. Laxton informs me, his
experience does not extend beyond twelve generations, within which
period he has never perceived any diminution of vigour in his plants.
An allied point may be here noticed. As the force of inheritance is
strong with plants (of which abundant evidence could be given), it is
almost certain that seedlings from the same capsule or from the same
plant would tend to inherit nearly the same constitution; and as the
advantage from a cross depends on the plants which are crossed differing
somewhat in constitution, it may be inferred as probable that under
similar conditions a cross between the nearest relations would not
benefit the offspring so much as one between non-related plants. In
support of this conclusion we have some evidence, as Fritz Muller has
shown by his valuable experiments on hybrid Abutilons, that the union of
brothers and sisters, parents and children, and of other near relations
is highly injurious to the fertility of the offspring. In one case,
moreover, seedlings from such near relations possessed very weak
constitutions. (8/4. 'Jenaische Zeitschrift fur Naturw.' B. 7 pages 22
and 45 1872 and 1873 pages 441-450.) This same observer also found three
plants of a Bignonia growing near together. (8/5. 'Botanische Zeitung'
1868 page 626.) He fertilised twenty-nine flowers on one of them with
their own pollen, and they did not set a single capsule. Thirty flowers
were then fertilised with pollen from a distinct plant, one of the three
growing together, and they yielded only two capsules. Lastly, five
flowers were fertilised with pollen from a fourth plant growing at a
distance, and all five produced capsules. It seems therefore probable,
as Fritz Muller suggests, that the three plants growing near together
were seedlings from the same parent, and that from being closely related
they had little power of fertilising one another. (8/6. Some remarkable
cases are given in my 'Variation under Domestication' chapter 17 2nd
edition volume 2 page 121, of hybrids of Gladiolus and Cistus, any one
of which could be fertilised by pollen from any other, but not by its
own pollen.)
Lastly, the fact of the intercrossed plants in Table 7/A not exceeding
in height the self-fertilised plants in a greater and greater degree in
the later generations, is probably the result of their having become
more and more closely inter-related.
UNIFORM COLOUR OF THE FLOWERS ON PLANTS, SELF-FERTILISED AND GROWN UNDER
SIMILAR CONDITIONS FOR SEVERAL GENERATIONS.
At the commencement of my experiments, the parent-plants of Mimulus
luteus, Ipomoea purpurea, Dianthus caryophyllus, and Petunia violacea,
raised from purchased seeds, varied greatly in the colour of their
flowers. This occurs with many plants which have been long cultivated as
an ornament for the flower-garden, and which have been propagated by
seeds. The colour of the flowers was a point to which I did not at first
in the least attend, and no selection whatever was practised.
Nevertheless, the flowers produced by the self-fertilised plants of the
above four species became absolutely uniform in tint, or very nearly so,
after they had been grown for some generations under closely similar
conditions. The intercrossed plants, which were more or less closely
inter-related in the later generations, and which had been likewise
cultivated all the time under similar conditions, became more uniform in
the colour of their flowers than were the original parent-plants, but
much less so than the self-fertilised plants. When self-fertilised
plants of one of the later generations were crossed with a fresh stock,
and seedlings thus raised, these presented a wonderful contrast in the
diversified tints of their flowers compared with those of the
self-fertilised seedlings. As such cases of flowers becoming uniformly
coloured without any aid from selection seem to me curious, I will give
a full abstract of my observations.
Mimulus luteus.
A tall variety, bearing large, almost white flowers blotched with
crimson, appeared amongst the intercrossed and self-fertilised plants of
the third and fourth generations. This variety increased so rapidly,
that in the sixth generation of self-fertilised plants every single one
consisted of it. So it was with all the many plants which were raised,
up to the last or ninth self-fertilised generation. Although this
variety first appeared amongst the intercrossed plants, yet from their
offspring being intercrossed in each succeeding generation, it never
prevailed amongst them; and the flowers on the several intercrossed
plants of the ninth generation differed considerably in colour. On the
other hand, the uniformity in colour of the flowers on the plants of all
the later self-fertilised generations was quite surprising; on a casual
inspection they might have been said to be quite alike, but the crimson
blotches were not of exactly the same shape, or in exactly the same
position. Both my gardener and myself believe that this variety did not
appear amongst the parent-plants, raised from purchased seeds, but from
its appearance amongst both the crossed and self-fertilised plants of
the third and fourth generations; and from what I have seen of the
variation of this species on other occasions, it is probable that it
would occasionally appear under any circumstances. We learn, however,
from the present case that under the peculiar conditions to which my
plants were subjected, this particular variety, remarkable for its
colouring, largeness of the corolla, and increased height of the whole
plant, prevailed in the sixth and all the succeeding self-fertilised
generations to the complete exclusion of every other variety.
Ipomoea purpurea.
My attention was first drawn to the present subject by observing that
the flowers on all the plants of the seventh self-fertilised generation
were of a uniform, remarkably rich, dark purple tint. The many plants
which were raised during the three succeeding generations, up to the
last or tenth, all produced flowers coloured in the same manner. They
were absolutely uniform in tint, like those of a constant species living
in a state of nature; and the self-fertilised plants might have been
distinguished with certainty, as my gardener remarked, without the aid
of labels, from the intercrossed plants of the later generations. These,
however, had more uniformly coloured flowers than those which were first
raised from the purchased seeds. This dark purple variety did not
appear, as far as my gardener and myself could recollect, before the
fifth or sixth self-fertilised generation. However this may have been,
it became, through continued self-fertilisation and the cultivation of
the plants under uniform conditions, perfectly constant, to the
exclusion of every other variety.
Dianthus caryophyllus.
The self-fertilised plants of the third generation all bore flowers of
exactly the same pale rose-colour; and in this respect they differed
quite remarkably from the plants growing in a large bed close by and
raised from seeds purchased from the same nursery garden. In this case
it is not improbable that some of the parent-plants which were first
self-fertilised may have borne flowers thus coloured; but as several
plants were self-fertilised in the first generation, it is extremely
improbable that all bore flowers of exactly the same tint as those of
the self-fertilised plants of the third generation. The intercrossed
plants of the third generation likewise produced flowers almost, though
not quite so uniform in tint as those of the self-fertilised plants.
Petunia violacea.
In this case I happened to record in my notes that the flowers on the
parent-plant which was first self-fertilised were of a "dingy purple
colour." In the fifth self-fertilised generation, every one of the
twenty-one self-fertilised plants growing in pots, and all the many
plants in a long row out of doors, produced flowers of absolutely the
same tint, namely, of a dull, rather peculiar and ugly flesh colour;
therefore, considerably unlike those on the parent-plant. I believe that
this change of colour supervened quite gradually; but I kept no record,
as the point did not interest me until I was struck with the uniform
tint of the flowers on the self-fertilised plants of the fifth
generation. The flowers on the intercrossed plants of the corresponding
generation were mostly of the same dull flesh colour, but not nearly so
uniform as those on the self-fertilised plants, some few being very
pale, almost white. The self-fertilised plants which grew in a long row
in the open ground were also remarkable for their uniformity in height,
as were the intercrossed plants in a less degree, both lots being
compared with a large number of plants raised at the same time under
similar conditions from the self-fertilised plants of the fourth
generation crossed by a fresh stock. I regret that I did not attend to
the uniformity in height of the self-fertilised seedlings in the later
generations of the other species.
These few cases seem to me to possess much interest. We learn from them
that new and slight shades of colour may be quickly and firmly fixed,
independently of any selection, if the conditions are kept as nearly
uniform as is possible, and no intercrossing be permitted. With Mimulus,
not only a grotesque style of colouring, but a larger corolla and
increased height of the whole plant were thus fixed; whereas with most
plants which have been long cultivated for the flower-garden, no
character is more variable than that of colour, excepting perhaps that
of height. From the consideration of these cases we may infer that the
variability of cultivated plants in the above respects is due, firstly,
to their being subjected to somewhat diversified conditions, and,
secondly, to their being often intercrossed, as would follow from the
free access of insects. I do not see how this inference can be avoided,
as when the above plants were cultivated for several generations under
closely similar conditions, and were intercrossed in each generation,
the colour of their flowers tended in some degree to change and to
become uniform. When no intercrossing with other plants of the same
stock was allowed,--that is, when the flowers were fertilised with their
own pollen in each generation--their colour in the later generations
became as uniform as that of plants growing in a state of nature,
accompanied at least in one instance by much uniformity in the height of
the plants. But in saying that the diversified tints of the flowers on
cultivated plants treated in the ordinary manner are due to differences
in the soil, climate, etc., to which they are exposed, I do not wish to
imply that such variations are caused by these agencies in any more
direct manner than that in which the most diversified illnesses, as
colds, inflammation of the lungs or pleura, rheumatism, etc., may be
said to be caused by exposure to cold. In both cases the constitution of
the being which is acted on is of preponderant importance.
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