THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE
PRODUCTION OF SEEDS.
Fertility of plants of crossed and self-fertilised parentage, both lots
being fertilised in the same manner.
Fertility of the parent-plants when first crossed and self-fertilised,
and of their crossed and self-fertilised offspring when again crossed
and self-fertilised.
Comparison of the fertility of flowers fertilised with their own pollen
and with that from other flowers on the same plant.
Self-sterile plants.
Causes of self-sterility.
The appearance of highly self-fertile varieties.
Self-fertilisation apparently in some respects beneficial, independently
of the assured production of seeds.
Relative weights and rates of germination of seeds from crossed and
self-fertilised flowers.
The present chapter is devoted to the Fertility of plants, as influenced
by cross-fertilisation and self-fertilisation. The subject consists of
two distinct branches; firstly, the relative productiveness or fertility
of flowers crossed with pollen from a distinct plant and with their own
pollen, as shown by the proportional number of capsules which they
produce, together with the number of the contained seeds. Secondly, the
degree of innate fertility or sterility of the seedlings raised from
crossed and self-fertilised seeds; such seedlings being of the same age,
grown under the same conditions, and fertilised in the same manner.
These two branches of the subject correspond with the two which have to
be considered by any one treating of hybrid plants; namely, in the first
place the comparative productiveness of a species when fertilised with
pollen from a distinct species and with its own pollen; and in the
second place, the fertility of its hybrid offspring. These two classes
of cases do not always run parallel; thus some plants, as Gartner has
shown, can be crossed with great ease, but yield excessively sterile
hybrids; while others are crossed with extreme difficulty, but yield
fairly fertile hybrids.
The natural order to follow in this chapter would have been first to
consider the effects on the fertility of the parent-plants of crossing
them, and of fertilising them with their own pollen; but as we have
discussed in the two last chapters the relative height, weight, and
constitutional vigour of crossed and self-fertilised plants--that is, of
plants raised from crossed and self-fertilised seeds--it will be
convenient here first to consider their relative fertility. The cases
observed by me are given in Table 9/D, in which plants of crossed and
self-fertilised parentage were left to fertilise themselves, being
either crossed by insects or spontaneously self-fertilised. It should be
observed that the results cannot be considered as fully trustworthy, for
the fertility of a plant is a most variable element, depending on its
age, health, nature of the soil, amount of water given, and temperature
to which it is exposed. The number of the capsules produced and the
number of the contained seeds, ought to have been ascertained on a large
number of crossed and self-fertilised plants of the same age and treated
in every respect alike. In these two latter respects my observations may
be trusted, but a sufficient number of capsules were counted only in a
few instances. The fertility, or as it may perhaps better be called the
productiveness, of a plant depends on the number of capsules produced,
and on the number of seeds which these contain. But from various causes,
chiefly from the want of time, I was often compelled to rely on the
number of the capsules alone. Nevertheless, in the more interesting
cases, the seeds were also counted or weighed. The average number of
seeds per capsule is a more valuable criterion of fertility than the
number of capsules produced. This latter circumstance depends partly on
the size of the plant; and we know that crossed plants are generally
taller and heavier than the self-fertilised; but the difference in this
respect is rarely sufficient to account for the difference in the number
of the capsules produced. It need hardly be added that in Table 9/D the
same number of crossed and self-fertilised plants are always compared.
Subject to the foregoing sources of doubt I will now give the table, in
which the parentage of the plants experimented on, and the manner of
determining their fertility are explained. Fuller details may be found
in the previous part of this work, under the head of each species.
TABLE 9/D.--RELATIVE FERTILITY OF PLANTS OF CROSSED AND SELF-FERTILISED
PARENTAGE, BOTH SETS BEING FERTILISED IN THE SAME MANNER. FERTILITY
JUDGED OF BY VARIOUS STANDARDS. THAT OF THE CROSSED PLANTS TAKEN AS 100.
Column 1: Name of plant and feature observed.
Column 2: x, in the expression, as 100 to x.
Ipomoea purpurea--first generation: seeds per capsule on crossed and
self-fertilised plants, not growing much crowded, spontaneously
self-fertilised under a net, in number: 99.
Ipomoea purpurea--seeds per capsule on crossed and self-fertilised
plants from the same parents as in the last case, but growing much
crowded, spontaneously self-fertilised under a net, in number: 93.
Ipomoea purpurea--productiveness of the same plants, as judged by the
number of capsules produced, and average number of seeds per capsule:
45.
Ipomoea purpurea--third generation: seeds per capsule on crossed and
self-fertilised plants, spontaneously self-fertilised under a net, in
number: 94.
Ipomoea purpurea--productiveness of the same plants, as judged by the
number of capsules produced, and the average number of seeds per
capsule: 35.
Ipomoea purpurea--fifth generation: seeds per capsule on crossed and
self-fertilised plants, left uncovered in the hothouse, and
spontaneously fertilised: 89.
Ipomoea purpurea--ninth generation: number of capsules on crossed plants
to those on self-fertilised plants, spontaneously self-fertilised under
a net: 26.
Mimulus luteus--an equal number of capsules on plants descended from
self-fertilised plants of the 8th generation crossed by a fresh stock,
and on plants of the 9th self-fertilised generation, both sets having
been left uncovered and spontaneously fertilised, contained seeds, by
weight: 30.
Mimulus luteus--productiveness of the same plants, as judged by the
number of capsules produced, and the average weight of seeds per
capsule: 3.
Vandellia nummularifolia--seeds per capsule from cleistogene flowers on
the crossed and self-fertilised plants, in number: 106.
Salvia coccinea--crossed plants, compared with self-fertilised plants,
produced flowers, in number: 57.
Iberis umbellata--plants left uncovered in greenhouse; intercrossed
plants of the 3rd generation, compared with self-fertilised plants of
the 3rd generation, yielded seeds, in number: 75.
Iberis umbellata--plants from a cross between two varieties, compared
with self-fertilised plants of the 3rd generation, yielded seeds, by
weight : 75.
Papaver vagum--crossed and self-fertilised plants, left uncovered,
produced capsules, in number: 99.
Eschscholtzia californica--Brazilian stock; plants left uncovered and
cross-fertilised by bees; capsules on intercrossed plants of the 2nd
generation, compared with capsules on self-fertilised plants of 2nd
generation, contained seeds, in number: 78.
Eschscholtzia californica--productiveness of the same plants, as judged
by the number of capsules produced, and the average number of seeds per
capsule: 89.
Eschscholtzia californica--plants left uncovered and cross-fertilised by
bees; capsules on plants derived from intercrossed plants of the 2nd
generation of the Brazilian stock crossed by English stock, compared
with capsules on self-fertilised plants of 2nd generation, contained
seeds, in number: 63.
Eschscholtzia californica--productiveness of the same plants, as judged
by the number of capsules produced, and the average number of seeds per
capsule: 40.
Reseda odorata--crossed and self-fertilised plants, left uncovered and
cross-fertilised by bees; produced capsules in number (about): 100.
Viola tricolor--crossed and self-fertilised plants, left uncovered and
cross-fertilised by bees, produced capsules in number: 10.
Delphinium consolida--crossed and self-fertilised plants, left uncovered
in the greenhouse, produced capsules in number: 56.
Viscaria oculata--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced capsules in number: 77.
Dianthus caryophyllus--plants spontaneously self-fertilised under a net;
capsules on intercrossed and self-fertilised plants of the 3rd
generation contained seeds in number: 125.
Dianthus caryophyllus--plants left uncovered and cross-fertilised by
insects: offspring from plants self-fertilised for three generations and
then crossed by an intercrossed plant of the same stock, compared with
plants of the 4th self-fertilised generation, produced seeds by weight:
73.
Dianthus caryophyllus--plants left uncovered and cross-fertilised by
insects: offspring from plants self-fertilised for three generations and
then crossed by a fresh stock, compared with plants of the 4th
self-fertilised generation, produced seeds by weight: 33.
Tropaeolum minus--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced seeds in number: 64.
Limnanthes douglasii--crossed and self-fertilised plants, left uncovered
in the greenhouse, produced capsules in number (about): 100.
Lupinus luteus--crossed and self-fertilised plants of the 2nd
generation, left uncovered in the greenhouse, produced seeds in number
(judged from only a few pods): 88.
Phaseolus multiflorus--crossed and self-fertilised plants, left
uncovered in the greenhouse, produced seeds in number (about): 100.
Lathyrus odoratus--crossed and self-fertilised plants of the 2nd
generation, left uncovered in the greenhouse, but certainly
self-fertilised, produced pods in number: 91.
Clarkia elegans--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced capsules in number: 60.
Nemophila insignis--crossed and self-fertilised plants, covered by a net
and spontaneously self-fertilised in the greenhouse, produced capsules
in number: 29.
Petunia violacea--left uncovered and cross-fertilised by insects: plants
of the 5th intercrossed and self-fertilised generations produced seeds,
as judged by the weight of an equal number of capsules: 86.
Petunia violacea--left uncovered as above: offspring of plants
self-fertilised for four generations and then crossed by a fresh stock,
compared with plants of the 5th self-fertilised generation, produced
seeds, as judged by the weight of an equal number of capsules: 46.
Cyclamen persicum--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced capsules in number: 12.
Anagallis collina--crossed and self-fertilised plants, left uncovered in
the greenhouse, produced capsules in number: 8.
Primula veris--left uncovered in open ground and cross-fertilised by
insects: offspring from plants of the 3rd illegitimate generation
crossed by a fresh stock, compared with plants of the 4th illegitimate
and self-fertilised generation, produced capsules in number: 5.
Same plants in the following year: 3.5.
Primula veris--(equal-styled variety): left uncovered in open ground and
cross-fertilised by insects: offspring from plants self-fertilised for
two generations and then crossed by another variety, compared with
plants of the 3rd self-fertilised generation, produced capsules in
number: 15.
Primula veris--(equal-styled variety) same plants; average number of
seeds per capsule: 71.
Primula veris--(equal-styled variety) productiveness of the same plants,
as judged by the number of capsules produced and the average number of
seeds per capsule: 11.
This table includes thirty-three cases relating to twenty-three species,
and shows the degree of innate fertility of plants of crossed parentage
in comparison with those of self-fertilised parentage; both lots being
fertilised in the same manner. With several of the species, as with
Eschscholtzia, Reseda, Viola, Dianthus, Petunia, and Primula, both lots
were certainly cross-fertilised by insects, and so it probably was with
several of the others; but in some of the species, as with Nemophila,
and in some of the trials with Ipomoea and Dianthus, the plants were
covered up, and both lots were spontaneously self-fertilised. This also
was necessarily the case with the capsules produced by the cleistogene
flowers of Vandellia.
The fertility of the crossed plants is represented in Table 9/D by 100,
and that of the self-fertilised by the other figures. There are five
cases in which the fertility of the self-fertilised plants is
approximately equal to that of the crossed; nevertheless, in four of
these cases the crossed plants were plainly taller, and in the fifth
somewhat taller than the self-fertilised. But I should state that in
some of these five cases the fertility of the two lots was not strictly
ascertained, as the capsules were not actually counted, from appearing
equal in number and from all apparently containing a full complement of
seeds. In only two instances in the table, namely, with Vandellia and in
the third generation of Dianthus, the capsules on the self-fertilised
plants contained more seed than those on the crossed plants. With
Dianthus the ratio between the number of seeds contained in the
self-fertilised and crossed capsules was as 125 to 100; both sets of
plants were left to fertilise themselves under a net; and it is almost
certain that the greater fertility of the self-fertilised plants was
here due merely to their having varied and become less strictly
dichogamous, so as to mature their anthers and stigmas more nearly at
the same time than is proper to the species. Excluding the seven cases
now referred to, there remain twenty-six in which the crossed plants
were manifestly much more fertile, sometimes to an extraordinary degree,
than the self-fertilised with which they grew in competition. The most
striking instances are those in which plants derived from a cross with a
fresh stock are compared with plants of one of the later self-fertilised
generations; yet there are some striking cases, as that of Viola,
between the intercrossed plants of the same stock and the
self-fertilised, even in the first generation. The results most to be
trusted are those in which the productiveness of the plants was
ascertained by the number of capsules produced by an equal number of
plants, together with the actual or average number of seeds in each
capsule. Of such cases there are twelve in the table, and the mean of
their mean fertility is as 100 for the crossed plants, to 59 for the
self-fertilised plants. The Primulaceae seem eminently liable to suffer
in fertility from self-fertilisation.
The following short table, Table 9/E, includes four cases which have
already been partly given in the last table.
TABLE 9/E.--INNATE FERTILITY OF PLANTS FROM A CROSS WITH A FRESH STOCK,
COMPARED WITH THAT OF INTERCROSSED PLANTS OF THE SAME STOCK, AND WITH
THAT OF SELF-FERTILISED PLANTS, ALL OF THE CORRESPONDING GENERATION.
FERTILITY JUDGED OF BY THE NUMBER OR WEIGHT OF SEEDS PRODUCED BY AN
EQUAL NUMBER OF PLANTS.
Column 1: Name of plant and feature observed.
Column 2: Plants from a cross with a fresh stock.
Column 3: Intercrossed plants of the same stock.
Column 4: Self-fertilised plants.
Mimulus luteus--the intercrossed plants are derived from a cross between
two plants of the 8th self-fertilised generation. The self-fertilised
plants belong to the 9th generation: 100 : 4 : 3.
Eschscholtzia californica--the intercrossed and self-fertilised plants
belong to the 2nd generation: 100 : 45 : 40.
Dianthus caryophyllus--the intercrossed plants are derived from
self-fertilised of the 3rd generation, crossed by intercrossed plants of
the 3rd generation. The self-fertilised plants belong to the 4th
generation: 100 : 45 : 33.
Petunia violacea--the intercrossed and self-fertilised plants belong to
the 5th generation: 100 : 54 : 46.
NB.--In the above cases, excepting in that of Eschscholtzia, the plants
derived from a cross with a fresh stock belong on the mother-side to the
same stock with the intercrossed and self-fertilised plants, and to the
corresponding generation.
These cases show us how greatly superior in innate fertility the
seedlings from plants self-fertilised or intercrossed for several
generations and then crossed by a fresh stock are, in comparison with
the seedlings from plants of the old stock, either intercrossed or
self-fertilised for the same number of generations. The three lots of
plants in each case were left freely exposed to the visits of insects,
and their flowers without doubt were cross-fertilised by them.
Table 9/E further shows us that in all four cases the intercrossed
plants of the same stock still have a decided though small advantage in
fertility over the self-fertilised plants.
With respect to the state of the reproductive organs in the
self-fertilised plants of Tables 9/D and 9/E, only a few observations
were made. In the seventh and eighth generation of Ipomoea, the anthers
in the flowers of the self-fertilised plants were plainly smaller than
those in the flowers of the intercrossed plants. The tendency to
sterility in these same plants was also shown by the first-formed
flowers, after they had been carefully fertilised, often dropping off,
in the same manner as frequently occurs with hybrids. The flowers
likewise tended to be monstrous. In the fourth generation of Petunia,
the pollen produced by the self-fertilised and intercrossed plants was
compared, and they were far more empty and shrivelled grains in the
former.
RELATIVE FERTILITY OF FLOWERS CROSSED WITH POLLEN FROM A DISTINCT PLANT
AND WITH THEIR OWN POLLEN. THIS HEADING INCLUDES FLOWERS ON THE
PARENT-PLANTS, AND ON THE CROSSED AND SELF-FERTILISED SEEDLINGS OF THE
FIRST OR A SUCCEEDING GENERATION.
I will first treat of the parent-plants, which were raised from seeds
purchased from nursery-gardens, or taken from plants growing in my
garden, or growing wild, and surrounded in every case by many
individuals of the same species. Plants thus circumstanced will commonly
have been intercrossed by insects; so that the seedlings which were
first experimented on will generally have been the product of a cross.
Consequently any difference in the fertility of their flowers, when
crossed and self-fertilised, will have been caused by the nature of the
pollen employed; that is, whether it was taken from a distinct plant or
from the same flower. The degrees of fertility shown in Table 9/F, were
determined in each case by the average number of seeds per capsule,
ascertained either by counting or weighing.
Another element ought properly to have been taken into account, namely,
the proportion of flowers which yielded capsules when they were crossed
and self-fertilised; and as crossed flowers generally produce a larger
proportion of capsules, their superiority in fertility, if this element
had been taken into account, would have been much more strongly marked
than appears in Table 9/F. But had I thus acted, there would have been
greater liability to error, as pollen applied to the stigma at the wrong
time fails to produce any effect, independently of its greater or less
potency. A good illustration of the great difference in the results
which sometimes follows, if the number of capsules produced relatively
to the number of flowers fertilised be included in the calculation, was
afforded by Nolana prostrata. Thirty flowers on some plants of this
species were crossed and produced twenty-seven capsules, each containing
five seeds; thirty-two flowers on the same plants were self-fertilised
and produced only six capsules, each containing five seeds. As the
number of seeds per capsule is here the same, the fertility of the
crossed and self-fertilised flowers is given in Table 9/F as equal, or
as 100 to 100. But if the flowers which failed to produce capsules be
included, the crossed flowers yielded on an average 4.50 seeds, whilst
the self-fertilised flowers yielded only 0.94 seeds, so that their
relative fertility would have been as 100 to 21. I should here state
that it has been found convenient to reserve for separate discussion the
cases of flowers which are usually quite sterile with their own pollen.
TABLE 9/f.--relative fertility of the flowers on the parent-plants used
in my experiments, when fertilised with pollen from a distinct plant and
with their own pollen. Fertility judged of by the average number of
seeds per capsule. Fertility of crossed flowers taken as 100.
Column 1: Name of plant and feature observed.
Column 2: x, in the expression 100 to x.
Ipomoea purpurea--crossed and self-fertilised flowers yielded seeds as
(about): 100.
Mimulus luteus--crossed and self-fertilised flowers yielded seeds as (by
weight): 79.
Linaria vulgaris--crossed and self-fertilised flowers yielded seeds as:
14.
Vandellia nummularifolia--crossed and self-fertilised flowers yielded
seeds as: 67?
Gesneria pendulina--crossed and self-fertilised flowers yielded seeds as
(by weight): 100.
Salvia coccinea--crossed and self-fertilised flowers yielded seeds as
(about): 100.
Brassica oleracea--crossed and self-fertilised flowers yielded seeds as:
25.
Eschscholtzia californica--(English stock) crossed and self-fertilised
flowers yielded seeds as (by weight): 71.
Eschscholtzia californica--(Brazilian stock grown in England) crossed
and self-fertilised flowers yielded seeds (by weight) as (about): 15.
Delphinium consolida--crossed and self-fertilised flowers
(self-fertilised capsules spontaneously produced, but result supported
by other evidence) yielded seeds as: 59.
Viscaria oculata--crossed and self-fertilised flowers yielded seeds as
(by weight): 38.
Viscaria oculata--crossed and self-fertilised flowers (crossed capsules
compared on following year with spontaneously self-fertilised capsules)
yielded seeds as : 58.
Dianthus caryophyllus--crossed and self-fertilised flowers yielded seeds
as: 92.
Tropaeolum minus--crossed and self-fertilised flowers yielded seeds as:
92.
Tropaeolum tricolorum--crossed and self-fertilised flowers yielded seeds
as: 115. (9/1. Tropaeolum tricolorum and Cuphea purpurea have been
introduced into this table, although seedlings were not raised from
them; but of the Cuphea only six crossed and six self-fertilised
capsules, and of the Tropaeolum only six crossed and eleven
self-fertilised capsules, were compared. A larger proportion of the
self-fertilised than of the crossed flowers of the Tropaeolum produced
fruit.)
Limnanthes douglasii--crossed and self-fertilised flowers yielded seeds
as (about): 100.
Sarothamnus scoparius--crossed and self-fertilised flowers yielded seeds
as: 41.
Ononis minutissima--crossed and self-fertilised flowers yielded seeds
as: 65.
Cuphea purpurea--crossed and self-fertilised flowers yielded seeds as:
113.
Passiflora gracilis--crossed and self-fertilised flowers yielded seeds
as: 85.
Specularia speculum--crossed and self-fertilised flowers yielded seeds
as: 72.
Lobelia fulgens--crossed and self-fertilised flowers yielded seeds as
(about): 100.
Nemophila insignis--crossed and self-fertilised flowers yielded seeds as
(by weight): 69.
Borago officinalis--crossed and self-fertilised flowers yielded seeds
as: 60.
Nolana prostrata--crossed and self-fertilised flowers yielded seeds as:
100.
Petunia violacea--crossed and self-fertilised flowers yielded seeds as
(by weight): 67.
Nicotiana tabacum--crossed and self-fertilised flowers yielded seeds as
(by weight): 150.
Cyclamen persicum--crossed and self-fertilised flowers yielded seeds as:
38.
Anagallis collina--crossed and self-fertilised flowers yielded seeds as:
96.
Canna warscewiczi--crossed and self-fertilised flowers (on three
generations of crossed and self-fertilised plants taken all together)
yielded seeds as: 85.
Table 9/G gives the relative fertility of flowers on crossed plants
again cross-fertilised, and of flowers on self-fertilised plants again
self-fertilised, either in the first or in a later generation. Here two
causes combine to diminish the fertility of the self-fertilised flowers;
namely, the lesser efficacy of pollen from the same flower, and the
innate lessened fertility of plants derived from self-fertilised seeds,
which as we have seen in the previous Table 9/D is strongly marked. The
fertility was determined in the same manner as in Table 9/F, that is, by
the average number of seeds per capsule; and the same remarks as before,
with respect to the different proportion of flowers which set capsules
when they are cross-fertilised and self-fertilised, are here likewise
applicable.
TABLE 9/G.--RELATIVE FERTILITY OF FLOWERS ON CROSSED AND SELF-FERTILISED
PLANTS OF THE FIRST OR SOME SUCCEEDING GENERATION; THE FORMER BEING
AGAIN FERTILISED WITH POLLEN FROM A DISTINCT PLANT, AND THE LATTER AGAIN
WITH THEIR OWN POLLEN. FERTILITY JUDGED OF BY THE AVERAGE NUMBER OF
SEEDS PER CAPSULE. FERTILITY OF CROSSED FLOWERS TAKEN AS 100.
Column 1: Name of plant and feature observed.
Column 2: x, in the expression, 100 to x.
Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the first generation yielded seeds as: 93.
Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 3rd generation yielded seeds as: 94.
Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 4th generation yielded seeds as: 94.
Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 5th generation yielded seeds as: 107.
Mimulus luteus--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 3rd generation yielded seeds as (by
weight): 65.
Mimulus luteus--same plants of the 3rd generation treated in the same
manner on the following year yielded seeds as (by weight): 34.
Mimulus luteus--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 4th generation yielded seeds as (by
weight): 40.
Viola tricolor--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 1st generation yielded seeds as: 69.
Dianthus caryophyllus--crossed and self-fertilised flowers on the
crossed and self-fertilised plants of the 1st generation yielded seeds
as: 65.
Dianthus caryophyllus--flowers on self-fertilised plants of the 3rd
generation crossed by intercrossed plants, and other flowers again
self-fertilised yielded seeds as: 97.
Dianthus caryophyllus--flowers on self-fertilised plants of the 3rd
generation crossed by a fresh stock, and other flowers again
self-fertilised yielded seeds as: 127.
Lathytus odoratus--crossed and self-fertilised flowers on the crossed
and self-fertilised plants of the 1st generation yielded seeds as: 65.
Lobelia ramosa--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 1st generation yielded seeds as (by
weight): 60.
Petunia violacea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 1st generation yielded seeds as (by
weight): 68.
Petunia violacea--crossed and self-fertilised flowers on the crossed and
self-fertilised plants of the 4th generation yielded seeds as (by
weight): 72.
Petunia violacea--flowers on self-fertilised plants of the 4th
generation crossed by a fresh stock, and other flowers again
self-fertilised yielded seeds as (by weight): 48.
Nicotiana tabacum--crossed and self-fertilised flowers on the crossed
and self-fertilised plants of the 1st generation yielded seeds as (by
weight): 97.
Nicotiana tabacum--flowers on self-fertilised plants of the 2nd
generation crossed by intercrossed plants, and other flowers again
self-fertilised yielded seeds as (by estimation): 110.
Nicotiana tabacum--flowers on self-fertilised plants of the 3rd
generation crossed by a fresh stock, and other flowers again
self-fertilised yielded seeds as (by estimation): 110.
Anagallis collina--flowers on red variety crossed by a blue variety, and
other flowers on the red variety self-fertilised yielded seeds as: 48.
Canna warscewiczi--crossed and self-fertilised flowers on the crossed
and self-fertilised plants of three generations taken together yielded
seeds as: 85.
As both these tables relate to the fertility of flowers fertilised by
pollen from another plant and by their own pollen, they may be
considered together. The difference between them consists in the
self-fertilised flowers in Table 9/G, being produced by self-fertilised
parents, and the crossed flowers by crossed parents, which in the later
generations had become somewhat closely inter-related, and had been
subjected all the time to nearly the same conditions. These two tables
include fifty cases relating to thirty-two species. The flowers on many
other species were crossed and self-fertilised, but as only a few were
thus treated, the results cannot be trusted, as far as fertility is
concerned, and are not here given. Some other cases have been rejected,
as the plants were in an unhealthy condition. If we look to the figures
in the two tables expressing the ratios between the mean relative
fertility of the crossed and self-fertilised flowers, we see that in a
majority of cases (i.e., in thirty-five out of fifty) flowers fertilised
by pollen from a distinct plant yield more, sometimes many more, seeds
than flowers fertilised with their own pollen; and they commonly set a
larger proportion of capsules. The degree of infertility of the
self-fertilised flowers differs extremely in the different species, and
even, as we shall see in the section on self-sterile plants, in the
individuals of the same species, as well as under slightly changed
conditions of life. Their fertility ranges from zero to fertility
equalling that of the crossed flowers; and of this fact no explanation
can be offered. There are fifteen cases in the two tables in which the
number of seeds per capsule produced by the self-fertilised flowers
equals or even exceeds that yielded by the crossed flowers. Some few of
these cases are, I believe, accidental; that is, would not recur on a
second trial. This was apparently the case with the plants of the fifth
generation of Ipomoea, and in one of the experiments with Dianthus.
Nicotiana offers the most anomalous case of any, as the self-fertilised
flowers on the parent-plants, and on their descendants of the second and
third generations, produced more seeds than did the crossed flowers; but
we shall recur to this case when we treat of highly self-fertile
varieties.
It might have been expected that the difference in fertility between the
crossed and self-fertilised flowers would have been more strongly marked
in Table 9/G, in which the plants of one set were derived from
self-fertilised parents, than in Table 9/F, in which flowers on the
parent-plants were self-fertilised for the first time. But this is not
the case, as far as my scanty materials allow of any judgment. There is
therefore no evidence at present, that the fertility of plants goes on
diminishing in successive self-fertilised generations, although there is
some rather weak evidence that this does occur with respect to their
height or growth. But we should bear in mind that in the later
generations the crossed plants had become more or less closely
inter-related, and had been subjected all the time to nearly uniform
conditions.
It is remarkable that there is no close correspondence, either in the
parent-plants or in the successive generations, between the relative
number of seeds produced by the crossed and self-fertilised flowers, and
the relative powers of growth of the seedlings raised from such seeds.
Thus, the crossed and self-fertilised flowers on the parent-plants of
Ipomoea, Gesneria, Salvia, Limnanthes, Lobelia fulgens, and Nolana
produced a nearly equal number of seeds, yet the plants raised from the
crossed seeds exceeded considerably in height those raised from the
self-fertilised seeds. The crossed flowers of Linaria and Viscaria
yielded far more seeds than the self-fertilised flowers; and although
the plants raised from the former were taller than those from the
latter, they were not so in any corresponding degree. With Nicotiana the
flowers fertilised with their own pollen were more productive than those
crossed with pollen from a slightly different variety; yet the plants
raised from the latter seeds were much taller, heavier, and more hardy
than those raised from the self-fertilised seeds. On the other hand, the
crossed seedlings of Eschscholtzia were neither taller nor heavier than
the self-fertilised, although the crossed flowers were far more
productive than the self-fertilised. But the best evidence of a want of
correspondence between the number of seeds produced by crossed and
self-fertilised flowers, and the vigour of the offspring raised from
them, is afforded by the plants of the Brazilian and European stocks of
Eschscholtzia, and likewise by certain individual plants of Reseda
odorata; for it might have been expected that the seedlings from plants,
the flowers of which were excessively self-sterile, would have profited
in a greater degree by a cross, than the seedlings from plants which
were moderately or fully self-fertile, and therefore apparently had no
need to be crossed. But no such result followed in either case: for
instance, the crossed and self-fertilised offspring from a highly
self-fertile plant of Reseda odorata were in average height to each
other as 100 to 82; whereas the similar offspring from an excessively
self-sterile plant were as 100 to 92 in average height.
With respect to the innate fertility of the plants of crossed and
self-fertilised parentage, given in the previous Table 9/D--that is, the
number of seeds produced by both lots when their flowers were fertilised
in the same manner,--nearly the same remarks are applicable, in
reference to the absence of any close correspondence between their
fertility and powers of growth, as in the case of the plants in the
Tables 9/F and 9/G, just considered. Thus the crossed and
self-fertilised plants of Ipomoea, Papaver, Reseda odorata, and
Limnanthes were almost equally fertile, yet the former exceeded
considerably in height the self-fertilised plants. On the other hand,
the crossed and self-fertilised plants of Mimulus and Primula differed
to an extreme degree in innate fertility, but by no means to a
corresponding degree in height or vigour.
In all the cases of self-fertilised flowers included in Tables 9/E, 9/F,
and 9/G, these were fertilised with their own pollen; but there is
another form of self-fertilisation, namely, by pollen from other flowers
on the same plant; but this latter method made no difference in
comparison with the former in the number of seeds produced, or only a
slight difference. Neither with Digitalis nor Dianthus were more seeds
produced by the one method than by the other, to any trustworthy degree.
With Ipomoea rather more seeds, in the proportion of 100 to 91, were
produced from a crossed between flowers on the same plant than from
strictly self-fertilised flowers; but I have reason to suspect that the
result was accidental. With Origanum vulgare, however, a cross between
flowers on plants propagated by stolons from the same stock certainly
increased slightly their fertility. This likewise occurred, as we shall
see in the next section, with Eschscholtzia, perhaps with Corydalis cava
and Oncidium; but not so with Bignonia, Abutilon, Tabernaemontana,
Senecio, and apparently Reseda odorata.
SELF-STERILE PLANTS.
The cases here to be described might have been introduced in Table 9/F,
which gives the relative fertility of flowers fertilised with their own
pollen, and with that from a distinct plant, but it has been found more
convenient to keep them for separate discussion. The present cases must
not be confounded with those to be given in the next chapter relatively
to flowers which are sterile when insects are excluded; for such
sterility depends not merely on the flowers being incapable of
fertilisation with their own pollen, but on mechanical causes, by which
their pollen is prevented from reaching the stigma, or on the pollen and
stigma of the same flower being matured at different periods.
In the seventeenth chapter of my ‘Variation of Animals and Plants under
Domestication’ I had occasion to enter fully on the present subject; and
I will therefore here give only a brief abstract of the cases there
described, but others must be added, as they have an important bearing
on the present work. Kolreuter long ago described plants of Verbascum
phoeniceum which during two years were sterile with their own pollen,
but were easily fertilised by that of four other species; these plants
however afterwards became more or less self-fertile in a strangely
fluctuating manner. Mr. Scott also found that this species, as well as
two of its varieties, were self-sterile, as did Gartner in the case of
Verbascum nigrum. So it was, according to this latter author, with two
plants of Lobelia fulgens, though the pollen and ovules of both were in
an efficient state in relation to other species. Five species of
Passiflora and certain individuals of a sixth species have been found
sterile with their own pollen; but slight changes in their conditions,
such as being grafted on another stock or a change of temperature,
rendered them self-fertile. Flowers on a completely self-impotent plant
of Passiflora alata fertilised with pollen from its own self-impotent
seedlings were quite fertile. Mr. Scott, and afterwards Mr. Munro, found
that some species of Oncidium and of Maxillaria cultivated in a hothouse
in Edinburgh were quite sterile with their own pollen; and Fritz Muller
found this to be the case with a large number of Orchidaceous genera
growing in their native home of South Brazil. (9/2. ‘Botanische Zeitung’
1868 page 114.) He also discovered that the pollen-masses of some
orchids acted on their own stigmas like a poison; and it appears that
Gartner formerly observed indications of this extraordinary fact in the
case of some other plants.
Fritz Muller also states that a species of Bignonia and Tabernaemontana
echinata are both sterile with their own pollen in their native country
of Brazil. (9/3. Ibid 1868 page 626 and 1870 page 274.) Several
Amaryllidaceous and Liliaceous plants are in the same predicament.
Hildebrand observed with care Corydalis cava, and found it completely
self-sterile (9/4. ‘Report of the International Horticultural Congress’
1866.); but according to Caspary a few self-fertilised seeds are
occasionally produced: Corydalis halleri is only slightly self-sterile,
and C. intermedia not at all so. (9/5. ‘Botanische Zeitung’ June 27,
1873.) In another Fumariaceous genus, Hypecoum, Hildebrand observed that
H. grandiflorum was highly self-sterile, whilst H. procumbens was fairly
self-fertile. (9/6. ‘Jahrb. fur wiss. Botanik’ B. 7 page 464.)
Thunbergia alata kept by me in a warm greenhouse was self-sterile early
in the season, but at a later period produced many spontaneously
self-fertilised fruits. So it was with Papaver vagum: another species,
P. alpinum, was found by Professor H. Hoffmann to be quite self-sterile
excepting on one occasion (9/7. ‘Zur Speciesfrage’ 1875 page 47.);
whilst P. somniferum has been with me always completely self-sterile.
Eschscholtzia californica.
This species deserves a fuller consideration. A plant cultivated by
Fritz Muller in South Brazil happened to flower a month before any of
the others, and it did not produce a single capsule. This led him to
make further observations during the next six generations, and he found
that all his plants were completely sterile, unless they were crossed by
insects or were artificially fertilised with pollen from a distinct
plant, in which case they were completely fertile. (9/8. ‘Botanische
Zeitung’ 1868 page 115 and 1869 page 223.) I was much surprised at this
fact, as I had found that English plants, when covered by a net, set a
considerable number of capsules; and that these contained seeds by
weight, compared with those on plants intercrossed by the bees, as 71 to
100. Professor Hildebrand, however, found this species much more
self-sterile in Germany than it was with me in England, for the capsules
produced by self-fertilised flowers, compared with those from
intercrossed flowers, contained seeds in the ratio of only 11 to 100. At
my request Fritz Muller sent me from Brazil seeds of his self-sterile
plants, from which I raised seedlings. Two of these were covered with a
net, and one produced spontaneously only a single capsule containing no
good seeds, but yet, when artificially fertilised with its own pollen,
produced a few capsules. The other plant produced spontaneously under
the net eight capsules, one of which contained no less than thirty
seeds, and on an average about ten seeds per capsule. Eight flowers on
these two plants were artificially self-fertilised, and produced seven
capsules, containing on an average twelve seeds; eight other flowers
were fertilised with pollen from a distinct plant of the Brazilian
stock, and produced eight capsules, containing on an average about
eighty seeds: this gives a ratio of 15 seeds for the self-fertilised
capsules to 100 for the crossed capsules. Later in the season twelve
other flowers on these two plants were artificially self-fertilised; but
they yielded only two capsules, containing three and six seeds. It
appears therefore that a lower temperature than that of Brazil favours
the self-fertility of this plant, whilst a still lower temperature
lessens it. As soon as the two plants which had been covered by the net
were uncovered, they were visited by many bees,and it was interesting to
observe how quickly they became, even the more sterile plant of the two,
covered with young capsules. On the following year eight flowers on
plants of the Brazilian stock of self-fertilised parentage (i.e.,
grandchildren of the plants which grew in Brazil) were again
self-fertilised, and produced five capsules, containing on an average
27.4 seeds, with a maximum in one of forty-two seeds; so that their
self-fertility had evidently increased greatly by being reared for two
generations in England. On the whole we may conclude that plants of the
Brazilian stock are much more self-fertile in this country than in
Brazil, and less so than plants of the English stock in England; so that
the plants of Brazilian parentage retained by inheritance some of their
former sexual constitution. Conversely, seeds from English plants sent
by me to Fritz Muller and grown in Brazil, were much more self-fertile
than his plants which had been cultivated there for several generations;
but he informs me that one of the plants of English parentage which did
not flower the first year, and was thus exposed for two seasons to the
climate of Brazil, proved quite self-sterile, like a Brazilian plant,
showing how quickly the climate had acted on its sexual constitution.
Abutilon darwinii.
Seeds of this plant were sent me by Fritz Muller, who found it, as well
as some other species of the same genus, quite sterile in its native
home of South Brazil, unless fertilised with pollen from a distinct
plant, either artificially or naturally by humming-birds. (9/9.
‘Jenaische Zeitschr. fur Naturwiss’ B. 7 1872 page 22 and 1873 page
441.) Several plants were raised from these seeds and kept in the
hothouse. They produced flowers very early in the spring, and twenty of
them were fertilised, some with pollen from the same flower, and some
with pollen from other flowers on the same plants; but not a single
capsule was thus produced, yet the stigmas twenty-seven hours after the
application of the pollen were penetrated by the pollen-tubes. At the
same time nineteen flowers were crossed with pollen from a distinct
plant, and these produced thirteen capsules, all abounding with fine
seeds. A greater number of capsules would have been produced by the
cross, had not some of the nineteen flowers been on a plant which was
afterwards proved to be from some unknown cause completely sterile with
pollen of any kind. Thus far these plants behaved exactly like those in
Brazil; but later in the season, in the latter part of May and in June,
they began to produce under a net a few spontaneously self-fertilised
capsules. As soon as this occurred, sixteen flowers were fertilised with
their own pollen, and these produced five capsules, containing on an
average 3.4 seeds. At the same time I selected by chance four capsules
from the uncovered plants growing close by, the flowers of which I had
seen visited by humble-bees, and these contained on an average 21.5
seeds; so that the seeds in the naturally intercrossed capsules to those
in the self-fertilised capsules were as 100 to 16. The interesting point
in this case is that these plants, which were unnaturally treated by
being grown in pots in a hothouse, under another hemisphere, with a
complete reversal of the seasons, were thus rendered slightly
self-fertile, whereas they seem always to be completely self-sterile in
their native home.
Senecio cruentus (greenhouse varieties, commonly called Cinerarias,
probably derived from several fruticose or herbaceous species much
intercrossed (9/10. I am much obliged to Mr. Moore and to Mr. Thiselton
Dyer for giving me information with respect to the varieties on which I
experimented. Mr. Moore believes that Senecio cruentas, tussilaginis,
and perhaps heritieri, maderensis and populifolius have all been more or
less blended together in our Cinerarias.))
Two purple-flowered varieties were placed under a net in the greenhouse,
and four corymbs on each were repeatedly brushed with flowers from the
other plant, so that their stigmas were well covered with each other’s
pollen. Two of the eight corymbs thus treated produced very few seeds,
but the other six produced on an average 41.3 seeds per corymb, and
these germinated well. The stigmas on four other corymbs on both plants
were well smeared with pollen from the flowers on their own corymbs;
these eight corymbs produced altogether ten extremely poor seeds, which
proved incapable of germinating. I examined many flowers on both plants,
and found the stigmas spontaneously covered with pollen; but they
produced not a single seed. These plants were afterwards left uncovered
in the same house where many other Cinerarias were in flower; and the
flowers were frequently visited by bees. They then produced plenty of
seed, but one of the two plants less than the other, as this species
shows some tendency to be dioecious.
The trial was repeated on another variety with white petals tipped with
red. Many stigmas on two corymbs were covered with pollen from the
foregoing purple variety, and these produced eleven and twenty-two
seeds, which germinated well. A large number of the stigmas on several
of the other corymbs were repeatedly smeared with pollen from their own
corymb; but they yielded only five very poor seeds, which were incapable
of germination. Therefore the above three plants belonging to two
varieties, though growing vigorously and fertile with pollen from either
of the other two plants, were utterly sterile with pollen from other
flowers on the same plant.
Reseda odorata.
Having observed that certain individuals were self-sterile, I covered
during the summer of 1868 seven plants under separate nets, and will
call these plants A, B, C, D, E, F, G. They all appeared to be quite
sterile with their own pollen, but fertile with that of any other plant.
Fourteen flowers on A were crossed with pollen from B or C, and produced
thirteen fine capsules. Sixteen flowers were fertilised with pollen from
other flowers on the same plant, but yielded not a single capsule.
Fourteen flowers on B were crossed with pollen from A, C or D, and all
produced capsules; some of these were not very fine, yet they contained
plenty of seeds. Eighteen flowers were fertilised with pollen from other
flowers on the same plant, and produced not one capsule.
Ten flowers on C were crossed with pollen from A, B, D or E, and
produced nine fine capsules. Nineteen flowers were fertilised with
pollen from other flowers on the same plant, and produced no capsules.
Ten flowers on D were crossed with pollen from A, B, C or E, and
produced nine fine capsules. Eighteen flowers were fertilised with
pollen from other flowers on the same plant, and produced no capsules.
Seven flowers on E were crossed with pollen from A, C, or D, and all
produced fine capsules. Eight flowers were fertilised with pollen from
other flowers on the same plant, and produced no capsules.
On the plants F and G no flowers were crossed, but very many (number not
recorded) were fertilised with pollen from other flowers on the same
plants, and these did not produce a single capsule.
We thus see that fifty-five flowers on five of the above plants were
reciprocally crossed in various ways; several flowers on each of these
plants being fertilised with pollen from several of the other plants.
These fifty-five flowers produced fifty-two capsules, almost all of
which were of full size and contained an abundance of seeds. On the
other hand, seventy-nine flowers (besides many others not recorded) were
fertilised with pollen from other flowers on the same plants, and these
did not produce a single capsule. In one case in which I examined the
stigmas of the flowers fertilised with their own pollen, these were
penetrated by the pollen-tubes, although such penetration produced no
effect. Pollen falls generally, and I believe always, from the anthers
on the stigmas of the same flower; yet only three out of the above seven
protected plants produced spontaneously any capsules, and these it might
have been thought must have been self-fertilised. There were altogether
seven such capsules; but as they were all seated close to the
artificially crossed flowers, I can hardly doubt that a few grains of
foreign pollen had accidentally fallen on their stigmas. Besides the
above seven plants, four others were kept covered under the SAME large
net; and some of these produced here and there in the most capricious
manner little groups of capsules; and this makes me believe that a bee,
many of which settled on the outside of the net, being attracted by the
odour, had on some one occasion found an entrance, and had intercrossed
a few of the flowers.
In the spring of 1869 four plants raised from fresh seeds were carefully
protected under separate nets; and now the result was widely different
to what it was before. Three of these protected plants became actually
loaded with capsules, especially during the early part of the summer;
and this fact indicates that temperature produces some effect, but the
experiment given in the following paragraph shows that the innate
constitution of the plant is a far more important element. The fourth
plant produced only a few capsules, many of them of small size; yet it
was far more self-fertile than any of the seven plants tried during the
previous year. The flowers on four small branches of this
semi-self-sterile plant were smeared with pollen from one of the other
plants, and they all produced fine capsules.
As I was much surprised at the difference in the results of the trials
made during the two previous years, six fresh plants were protected by
separate nets in the year 1870. Two of these proved almost completely
self-sterile, for on carefully searching them I found only three small
capsules, each containing either one or two seeds of small size, which,
however, germinated. A few flowers on both these plants were
reciprocally fertilised with each other’s pollen, and a few with pollen
from one of the following self-fertile plants, and all these flowers
produced fine capsules. The four other plants whilst still remaining
protected beneath the nets presented a wonderful contrast (though one of
them in a somewhat less degree than the others), for they became
actually covered with spontaneously self-fertilised capsules, as
numerous as, or very nearly so, and as fine as those on the unprotected
plants growing near.
The above three spontaneously self-fertilised capsules produced by the
two almost completely self-sterile plants, contained altogether five
seeds; and from these I raised in the following year (1871) five plants,
which were kept under separate nets. They grew to an extraordinarily
large size, and on August 29th were examined. At first sight they
appeared entirely destitute of capsules; but on carefully searching
their many branches, two or three capsules were found on three of the
plants, half-a-dozen on the fourth, and about eighteen on the fifth
plant. But all these capsules were small, some being empty; the greater
number contained only a single seed, and very rarely more than one.
After this examination the nets were taken off, and the bees immediately
carried pollen from one of these almost self-sterile plants to the
other, for no other plants grew near. After a few weeks the ends of the
branches on all five plants became covered with capsules, presenting a
curious contrast with the lower and naked parts of the same long
branches. These five plants therefore inherited almost exactly the same
sexual constitution as their parents; and without doubt a self-sterile
race of Mignonette could have been easily established.
Reseda lutea.
Plants of this species were raised from seeds gathered from a group of
wild plants growing at no great distance from my garden. After casually
observing that some of these plants were self-sterile, two plants taken
by hazard were protected under separate nets. One of these soon became
covered with spontaneously self-fertilised capsules, as numerous as
those on the surrounding unprotected plants; so that it was evidently
quite self-fertile. The other plant was partially self-sterile,
producing very few capsules, many of which were of small size. When,
however, this plant had grown tall, the uppermost branches became
pressed against the net and grew crooked, and in this position the bees
were able to suck the flowers through the meshes, and brought pollen to
them from the neighbouring plants. These branches then became loaded
with capsules; the other and lower branches remaining almost bare. The
sexual constitution of this species is therefore similar to that of
Reseda odorata.
CONCLUDING REMARKS ON SELF-STERILE PLANTS.
In order to favour as far as possible the self-fertilisation of some of
the foregoing plants, all the flowers on Reseda odorata and some of
those on the Abutilon were fertilised with pollen from other flowers on
the same plant, instead of with their own pollen, and in the case of the
Senecio with pollen from other flowers on the same corymb; but this made
no difference in the result. Fritz Muller tried both kinds of
self-fertilisation in the case of Bignonia, Tabernaemontana and
Abutilon, likewise with no difference in the result. With Eschscholtzia,
however, he found that pollen from other flowers on the same plant was a
little more effective than pollen from the same flower. So did
Hildebrand in Germany; as thirteen out of fourteen flowers of
Eschscholtzia thus fertilised set capsules, these containing on an
average 9.5 seeds; whereas only fourteen flowers out of twenty-one
fertilised with their own pollen set capsules, these containing on an
average 9.0 seeds. (9/11. ‘Pringsheim’s Jahrbuch fur wiss. Botanik’ 7
page 467.) Hildebrand found a trace of a similar difference with
Corydalis cava, as did Fritz Muller with an Oncidium. (9/12. ‘Variation
under Domestication’ chapter 17 2nd edition volume 2 pages 113-115.)
In considering the several cases above given of complete or almost
complete self-sterility, we are first struck with their wide
distribution throughout the vegetable kingdom. Their number is not at
present large, for they can be discovered only by protecting plants from
insects and then fertilising them with pollen from another plant of the
same species and with their own pollen; and the latter must be proved to
be in an efficient state by other trials. Unless all this be done, it is
impossible to know whether their self-sterility may not be due to the
male or female reproductive organs, or to both, having been affected by
changed conditions of life. As in the course of my experiments I have
found three new cases, and as Fritz Muller has observed indications of
several others, it is probable that they will hereafter be proved to be
far from rare. (9/13. Mr. Wilder, the editor of a horticultural journal
in the United States quoted in ‘Gardeners’ Chronicle’ 1868 page 1286,
states that Lilium auratum, Impatiens pallida and fulva, and Forsythia
viridissima, cannot be fertilised with their own pollen.)
As with plants of the same species and parentage, some individuals are
self-sterile and others self-fertile, of which fact Reseda odorata
offers the most striking instances, it is not at all surprising that
species of the same genus differ in this same manner. Thus Verbascum
phoeniceum and nigrum are self-sterile, whilst V. thapsus and lychnitis
are quite self-fertile, as I know by trial. There is the same difference
between some of the species of Papaver, Corydalis, and of other genera.
Nevertheless, the tendency to self-sterility certainly runs to a certain
extent in groups, as we see in the genus Passiflora, and with the
Vandeae amongst Orchids.
Self-sterility differs much in degree in different plants. In those
extraordinary cases in which pollen from the same flower acts on the
stigma like a poison, it is almost certain that the plants would never
yield a single self-fertilised seed. Other plants, like Corydalis cava,
occasionally, though very rarely, produce a few self-fertilised seeds. A
large number of species, as may be seen in Table 9/F, are less fertile
with their own pollen than with that from another plant; and lastly,
some species are perfectly self-fertile. Even with the individuals of
the same species, as just remarked, some are utterly self-sterile,
others moderately so, and some perfectly self-fertile. The cause,
whatever it may be, which renders many plants more or less sterile with
their own pollen, that is, when they are self-fertilised, must be
different, at least to a certain extent, from that which determines the
difference in height, vigour, and fertility of the seedlings raised from
self-fertilised and crossed seeds; for we have already seen that the two
classes of cases do not by any means run parallel. This want of
parallelism would be intelligible, if it could be shown that
self-sterility depended solely on the incapacity of the pollen-tubes to
penetrate the stigma of the same flower deeply enough to reach the
ovules; whilst the greater or less vigorous growth of the seedlings no
doubt depends on the nature of the contents of the pollen-grains and
ovules. Now it is certain that with some plants the stigmatic secretion
does not properly excite the pollen-grains, so that the tubes are not
properly developed, if the pollen is taken from the same flower. This is
the case according to Fritz Muller with Eschscholtzia, for he found that
the pollen-tubes did not penetrate the stigma deeply; and with the
Orchidaceous genus Notylia they failed altogether to penetrate it.
(9/14. ‘Botanische Zeitung’ 1868 pages 114, 115.)
With dimorphic and trimorphic species, an illegitimate union between
plants of the same form presents the closest analogy with
self-fertilisation, whilst a legitimate union closely resembles
cross-fertilisation; and here again the lessened fertility or complete
sterility of an illegitimate union depends, at least in part, on the
incapacity for interaction between the pollen-grains and stigma. Thus
with Linum grandiflorum, as I have elsewhere shown, not more than two or
three out of hundreds of pollen-grains, either of the long-styled or
short-styled form, when placed on the stigma of their own form, emit
their tubes, and these do not penetrate deeply; nor does the stigma
itself change colour, as occurs when it is legitimately fertilised.
(9/15. ‘Journal of the Linnean Society Botany’ volume 7 1863 pages
73-75.)
On the other hand the difference in innate fertility, as well as in
growth between plants raised from crossed and self-fertilised seeds, and
the difference in fertility and growth between the legitimate and
illegitimate offspring of dimorphic and trimorphic plants, must depend
on some incompatibility between the sexual elements contained within the
pollen-grains and ovules, as it is through their union that new
organisms are developed.
If we now turn to the more immediate cause of self-sterility, we clearly
see that in most cases it is determined by the conditions to which the
plants have been subjected. Thus Eschscholtzia is completely
self-sterile in the hot climate of Brazil, but is perfectly fertile
there with the pollen of any other individual. The offspring of
Brazilian plants became in England in a single generation partially
self-fertile, and still more so in the second generation. Conversely,
the offspring of English plants, after growing for two seasons in
Brazil, became in the first generation quite self-sterile. Again,
Abutilon darwinii, which is self-sterile in its native home of Brazil,
became moderately self-fertile in a single generation in an English
hothouse. Some other plants are self-sterile during the early part of
the year, and later in the season become self-fertile. Passiflora alata
lost its self-sterility when grafted on another species. With Reseda,
however, in which some individuals of the same parentage are
self-sterile and others are self-fertile, we are forced in our ignorance
to speak of the cause as due to spontaneous variability; but we should
remember that the progenitors of these plants, either on the male or
female side, may have been exposed to somewhat different conditions. The
power of the environment thus to affect so readily and in so peculiar a
manner the reproductive organs, is a fact which has many important
bearings; and I have therefore thought the foregoing details worth
giving. For instance, the sterility of many animals and plants under
changed conditions of life, such as confinement, evidently comes within
the same general principle of the sexual system being easily affected by
the environment. It has already been proved, that a cross between plants
which have been self-fertilised or intercrossed during several
generations, having been kept all the time under closely similar
conditions, does not benefit the offspring; and on the other hand, that
a cross between plants that have been subjected to different conditions
benefits the offspring to an extraordinary degree. We may therefore
conclude that some degree of differentiation in the sexual system is
necessary for the full fertility of the parent-plants and for the full
vigour of their offspring. It seems also probable that with those plants
which are capable of complete self-fertilisation, the male and female
elements and organs already differ to an extent sufficient to excite
their mutual interaction; but that when such plants are taken to another
country, and become in consequence self-sterile, their sexual elements
and organs are so acted on as to be rendered too uniform for such
interaction, like those of a self-fertilised plant long cultivated under
the same conditions. Conversely, we may further infer that plants which
are self-sterile in their native country, but become self-fertile under
changed conditions, have their sexual elements so acted on, that they
become sufficiently differentiated for mutual interaction.
We know that self-fertilised seedlings are inferior in many respects to
those from a cross; and as with plants in a state of nature pollen from
the same flower can hardly fail to be often left by insects or by the
wind on the stigma, it seems at first sight highly probable that
self-sterility has been gradually acquired through natural selection in
order to prevent self-fertilisation. It is no valid objection to this
belief that the structure of some flowers, and the dichogamous condition
of many others, suffice to prevent the pollen reaching the stigma of the
same flower; for we should remember that with most species many flowers
expand at the same time, and that pollen from the same plant is equally
injurious or nearly so as that from the same flower. Nevertheless, the
belief that self-sterility is a quality which has been gradually
acquired for the special purpose of preventing self-fertilisation must,
I believe, be rejected. In the first place, there is no close
correspondence in degree between the sterility of the parent-plants when
self-fertilised, and the extent to which their offspring suffer in
vigour by this process; and some such correspondence might have been
expected if self-sterility had been acquired on account of the injury
caused by self-fertilisation. The fact of individuals of the same
parentage differing greatly in their degree of self-sterility is
likewise opposed to such a belief; unless, indeed, we suppose that
certain individuals have been rendered self-sterile to favour
intercrossing, whilst other individuals have been rendered self-fertile
to ensure the propagation of the species. The fact of self-sterile
individuals appearing only occasionally, as in the case of Lobelia, does
not countenance this latter view. But the strongest argument against the
belief that self-sterility has been acquired to prevent
self-fertilisation, is the immediate and powerful effect of changed
conditions in either causing or in removing self-sterility. We are not
therefore justified in admitting that this peculiar state of the
reproductive system has been gradually acquired through natural
selection; but we must look at it as an incidental result, dependent on
the conditions to which the plants have been subjected, like the
ordinary sterility caused in the case of animals by confinement, and in
the case of plants by too much manure, heat, etc. I do not, however,
wish to maintain that self-sterility may not sometimes be of service to
a plant in preventing self-fertilisation; but there are so many other
means by which this result might be prevented or rendered difficult,
including as we shall see in the next chapter the prepotency of pollen
from a distinct individual over a plant’s own pollen, that
self-sterility seems an almost superfluous acquirement for this purpose.
Finally, the most interesting point in regard to self-sterile plants is
the evidence which they afford of the advantage, or rather of the
necessity, of some degree or kind of differentiation in the sexual
elements, in order that they should unite and give birth to a new being.
It was ascertained that the five plants of Reseda odorata which were
selected by chance, could be perfectly fertilised by pollen taken from
any one of them, but not by their own pollen; and a few additional
trials were made with some other individuals, which I have not thought
worth recording. So again, Hildebrand and Fritz Muller frequently speak
of self-sterile plants being fertile with the pollen of any other
individual; and if there had been any exceptions to the rule, these
could hardly have escaped their observation and my own. We may therefore
confidently assert that a self-sterile plant can be fertilised by the
pollen of any one out of a thousand or ten thousand individuals of the
same species, but not by its own. Now it is obviously impossible that
the sexual organs and elements of every individual can have been
specialised with respect to every other individual. But there is no
difficulty in believing that the sexual elements of each differ slightly
in the same diversified manner as do their external characters; and it
has often been remarked that no two individuals are absolutely alike.
Therefore we can hardly avoid the conclusion, that differences of an
analogous and indefinite nature in the reproductive system are
sufficient to excite the mutual action of the sexual elements, and that
unless there be such differentiation fertility fails.
THE APPEARANCE OF HIGHLY SELF-FERTILE VARIETIES.
We have just seen that the degree to which flowers are capable of being
fertilised with their own pollen differs much, both with the species of
the same genus, and sometimes with the individuals of the same species.
Some allied cases of the appearance of varieties which, when
self-fertilised, yield more seed and produce offspring growing taller
than their self-fertilised parents, or than the intercrossed plants of
the corresponding generation, will now be considered.
Firstly, in the third and fourth generations of Mimulus luteus, a tall
variety, often alluded to, having large white flowers blotched with
crimson, appeared amongst both the intercrossed and self-fertilised
plants. It prevailed in all the later self-fertilised generations to the
exclusion of every other variety, and transmitted its characters
faithfully, but disappeared from the intercrossed plants, owing no doubt
to their characters being repeatedly blended by crossing. The
self-fertilised plants belonging to this variety were not only taller,
but more fertile than the intercrossed plants; though these latter in
the earlier generations were much taller and more fertile than the
self-fertilised plants. Thus in the fifth generation the self-fertilised
plants were to the intercrossed in height as 126 to 100. In the sixth
generation they were likewise much taller and finer plants, but were not
actually measured; they produced capsules compared with those on the
intercrossed plants, in number, as 147 to 100; and the self-fertilised
capsules contained a greater number of seeds. In the seventh generation
the self-fertilised plants were to the crossed in height as 137 to 100;
and twenty flowers on these self-fertilised plants fertilised with their
own pollen yielded nineteen very fine capsules,--a degree of
self-sterility which I have not seen equalled in any other case. This
variety seems to have become specially adapted to profit in every way by
self-fertilisation, although this process was so injurious to the
parent-plants during the first four generations. It should however be
remembered that seedlings raised from this variety, when crossed by a
fresh stock, were wonderfully superior in height and fertility to the
self-fertilised plants of the corresponding generation.
Secondly, in the sixth self-fertilised generation of Ipomoea a single
plant named the Hero appeared, which exceeded by a little in height its
intercrossed opponent,--a case which had not occurred in any previous
generation. Hero transmitted the peculiar colour of its flowers, as well
as its increased tallness and a high degree of self-fertility, to its
children, grandchildren, and great-grandchildren. The self-fertilised
children of Hero were in height to other self-fertilised plants of the
same stock as 100 to 85. Ten self-fertilised capsules produced by the
grandchildren contained on an average 5.2 seeds; and this is a higher
average than was yielded in any other generation by the capsules of
self-fertilised flowers. The great-grandchildren of Hero derived from a
cross with a fresh stock were so unhealthy, from having been grown at an
unfavourable season, that their average height in comparison with that
of the self-fertilised plants cannot be judged of with any safety; but
it did not appear that they had profited even by a cross of this kind.
Thirdly, the plants of Nicotiana on which I experimented appear to come
under the present class of cases; for they varied in their sexual
constitution and were more or less highly self-fertile. They were
probably the offspring of plants which had been spontaneously
self-fertilised under glass for several generations in this country. The
flowers on the parent-plants which were first fertilised by me with
their own pollen yielded half again as many seeds as did those which
were crossed; and the seedlings raised from these self-fertilised seeds
exceeded in height those raised from the crossed seeds to an
extraordinary degree. In the second and third generations, although the
self-fertilised plants did not exceed the crossed in height, yet their
self-fertilised flowers yielded on two occasions considerably more seeds
than the crossed flowers, even than those which were crossed with pollen
from a distinct stock or variety.
Lastly, as certain individual plants of Reseda odorata and lutea are
incomparably more self-fertile than other individuals, the former might
be included under the present heading of the appearance of new and
highly self-fertile varieties. But in this case we should have to look
at these two species as normally self-sterile; and this, judging by my
experience, appears to be the correct view.
We may therefore conclude from the facts now given, that varieties
sometimes arise which when self-fertilised possess an increased power of
producing seeds and of growing to a greater height, than the
intercrossed or self-fertilised plants of the corresponding
generation--all the plants being of course subjected to the same
conditions. The appearance of such varieties is interesting, as it bears
on the existence under nature of plants which regularly fertilise
themselves, such as Ophrys apifera and a few other orchids, or as
Leersia oryzoides, which produces an abundance of cleistogene flowers,
but most rarely flowers capable of cross-fertilisation.
Some observations made on other plants lead me to suspect that
self-fertilisation is in some respects beneficial; although the benefit
thus derived is as a rule very small compared with that from a cross
with a distinct plant. Thus we have seen in the last chapter that
seedlings of Ipomoea and Mimulus raised from flowers fertilised with
their own pollen, which is the strictest possible form of
self-fertilisation, were superior in height, weight, and in early
flowering to the seedlings raised from flowers crossed with pollen from
other flowers on the same plant; and this superiority apparently was too
strongly marked to be accidental. Again, the cultivated varieties of the
common pea are highly self-fertile, although they have been
self-fertilised for many generations; and they exceeded in height
seedlings from a cross between two plants belonging to the same variety
in the ratio of 115 to 100; but then only four pairs of plants were
measured and compared. The self-fertility of Primula veris increased
after several generations of illegitimate fertilisation, which is a
process closely analogous to self-fertilisation, but only as long as the
plants were cultivated under the same favourable conditions. I have also
elsewhere shown that with Primula veris and sinensis, equal-styled
varieties occasionally appear which possess the sexual organs of the two
forms combined in the same flower. (9/16. ‘Journal of the Linnean
Society Botany’ volume 10 1867 pages 417, 419.) Consequently they
fertilise themselves in a legitimate manner and are highly self-fertile;
but the remarkable fact is that they are rather more fertile than
ordinary plants of the same species legitimately fertilised by pollen
from a distinct individual. Formerly it appeared to me probable, that
the increased fertility of these dimorphic plants might be accounted for
by the stigma lying so close to the anthers that it was impregnated at
the most favourable age and time of the day; but this explanation is not
applicable to the above given cases, in which the flowers were
artificially fertilised with their own pollen.
Considering the facts now adduced, including the appearance of those
varieties which are more fertile and taller than their parents and than
the intercrossed plants of the corresponding generation, it is difficult
to avoid the suspicion that self-fertilisation is in some respects
advantageous; though if this be really the case, any such advantage is
as a rule quite insignificant compared with that from a cross with a
distinct plant, and especially with one of a fresh stock. Should this
suspicion be hereafter verified, it would throw light, as we shall see
in the next chapter, on the existence of plants bearing small and
inconspicuous flowers which are rarely visited by insects, and therefore
are rarely intercrossed.
RELATIVE WEIGHT AND PERIOD OF GERMINATION OF SEEDS FROM CROSSED AND
SELF-FERTILISED FLOWERS.
An equal number of seeds from flowers fertilised with pollen from
another plant, and from flowers fertilised with their own pollen, were
weighed, but only in sixteen cases. Their relative weights are given in
the following list; that of the seeds from the crossed flowers being
taken as 100.
Column 1: Name of Plant.
Column 2: x, in the expression, 100 to x.
Ipomoea purpurea (parent plants): 127.
Ipomoea purpurea (third generation): 87.
Salvia coccinea: 100.
Brassica oleracea: 103.
Iberis umbellata (second generation): 136.
Delphinium consolida: 45.
Hibiscus africanus: 105.
Tropaeolum minus: 115.
Lathyrus odoratus (about): 100.
Sarothamnus scoparius: 88.
Specularia speculum: 86.
Nemophila insignis: 105.
Borago officinalis: 111.
Cyclamen persicum (about): 50.
Fagopyrum esculentum: 82.
Canna warscewiczi (3 generations): 102.
It is remarkable that in ten out of these sixteen cases the
self-fertilised seeds were either superior or equal to the crossed in
weight; nevertheless, in six out of the ten cases (namely, with Ipomoea,
Salvia, Brassica, Tropaeolum, Lathyrus, and Nemophila) the plants raised
from these self-fertilised seeds were very inferior in height and in
other respects to those raised from the crossed seeds. The superiority
in weight of the self-fertilised seeds in at least six out of the ten
cases, namely, with Brassica, Hibiscus, Tropaeolum, Nemophila, Borago,
and Canna, may be accounted for in part by the self-fertilised capsules
containing fewer seeds; for when a capsule contains only a few seeds,
these will be apt to be better nourished, so as to be heavier, than when
many are contained in the same capsule. It should, however, be observed
that in some of the above cases, in which the crossed seeds were the
heaviest, as with Sarothamnus and Cyclamen, the crossed capsules
contained a larger number of seeds. Whatever may be the explanation of
the self-fertilised seeds being often the heaviest, it is remarkable in
the case of Brassica, Tropaeolum, Nemophila, and of the first generation
of Ipomoea, that the seedlings raised from them were inferior in height
and in other respects to the seedlings raised from the crossed seeds.
This fact shows how superior in constitutional vigour the crossed
seedlings must have been, for it cannot be doubted that heavy and fine
seeds tend to yield the finest plants. Mr. Galton has shown that this
holds good with Lathyrus odoratus; as has Mr. A.J. Wilson with the
Swedish turnip, Brassica campestris ruta baga. Mr. Wilson separated the
largest and smallest seeds of this latter plant, the ratio between the
weights of the two lots being as 100 to 59, and he found that the
seedlings “from the larger seeds took the lead and maintained their
superiority to the last, both in height and thickness of stem.” (9/17.
‘Gardeners’ Chronicle’ 1867 page 107. Loiseleur-Deslongchamp ‘Les
Cereales’ 1842 pages 208-219, was led by his observations to the
extraordinary conclusion that the smaller grains of cereals produce as
fine plants as the large. This conclusion is, however, contradicted by
Major Hallet’s great success in improving wheat by the selection of the
finest grains. It is possible, however, that man, by long-continued
selection, may have given to the grains of the cereals a greater amount
of starch or other matter, than the seedlings can utilise for their
growth. There can be little doubt, as Humboldt long ago remarked, that
the grains of cereals have been rendered attractive to birds in a degree
which is highly injurious to the species.) Nor can this difference in
the growth of the seedling turnips be attributed to the heavier seeds
having been of crossed, and the lighter of self-fertilised origin, for
it is known that plants belonging to this genus are habitually
intercrossed by insects.
With respect to the relative period of germination of crossed and
self-fertilised seeds, a record was kept in only twenty-one cases; and
the results are very perplexing. Neglecting one case in which the two
lots germinated simultaneously, in ten cases or exactly one-half many of
the self-fertilised seeds germinated before the crossed, and in the
other half many of the crossed before the self-fertilised. In four out
of these twenty cases, seeds derived from a cross with a fresh stock
were compared with self-fertilised seeds from one of the later
self-fertilised generations; and here again in half the cases the
crossed seeds, and in the other half the self-fertilised seeds,
germinated first. Yet the seedlings of Mimulus raised from such
self-fertilised seeds were inferior in all respects to the crossed
seedlings, and in the case of Eschscholtzia they were inferior in
fertility. Unfortunately the relative weight of the two lots of seeds
was ascertained in only a few instances in which their germination was
observed; but with Ipomoea and I believe with some of the other species,
the relative lightness of the self-fertilised seeds apparently
determined their early germination, probably owing to the smaller mass
being favourable to the more rapid completion of the chemical and
morphological changes necessary for germination. On the other hand, Mr.
Galton gave me seeds (no doubt all self-fertilised) of Lathyrus
odoratus, which were divided into two lots of heavier and lighter seeds;
and several of the former germinated first. It is evident that many more
observations are necessary before anything can be decided with respect
to the relative period of germination of crossed and self-fertilised
seeds.