Professor of Botany in the University of Munich.
There is scarcely any subject to which Darwin devoted so much time and
work as to his researches into the biology of flowers, or, in other
words, to the consideration of the question to what extent the
structural and physiological characters of flowers are correlated with
their function of producing fruits and seeds. We know from his own words
what fascination these studies possessed for him. We repeatedly find,
for example, in his letters expressions such as this:--"Nothing in my
life has ever interested me more than the fertilisation of such plants
as Primula and Lythrum, or again Anacamptis or Listera." ("More Letters
of Charles Darwin", Vol. II. page 419.)
Expressions of this kind coming from a man whose theories exerted an
epoch-making influence, would be unintelligible if his researches into
the biology of flowers had been concerned only with records of isolated
facts, however interesting these might be. We may at once take it
for granted that the investigations were undertaken with the view of
following up important problems of general interest, problems which are
briefly dealt with in this essay.
Darwin published the results of his researches in several papers and in
three larger works, (i) "On the various contrivances by which British
and Foreign Orchids are fertilised by insects" (First edition, London,
1862; second edition, 1877; popular edition, 1904.) (ii) "The effects of
Cross and Self fertilisation in the vegetable kingdom" (First edition,
1876; second edition, 1878). (iii) "The different forms of Flowers on
plants of the same species" (First edition, 1877; second edition, 1880).
Although the influence of his work is considered later, we may here
point out that it was almost without a parallel; not only does it
include a mass of purely scientific observations, but it awakened
interest in very wide circles, as is shown by the fact that we find the
results of Darwin's investigations in floral biology universally quoted
in school books; they are even willingly accepted by those who, as
regards other questions, are opposed to Darwin's views.
The works which we have mentioned are, however, not only of special
interest because of the facts they contribute, but because of the MANNER
in which the facts are expressed. A superficial reader seeking merely
for catch-words will, for instance, probably find the book on cross and
self-fertilisation rather dry because of the numerous details which
it contains: it is, indeed, not easy to compress into a few words the
general conclusions of this volume. But on closer examination, we cannot
be sufficiently grateful to the author for the exactness and objectivity
with which he enables us to participate in the scheme of his researches.
He never tries to persuade us, but only to convince us that his
conclusions are based on facts; he always gives prominence to such facts
as appear to be in opposition to his opinions,--a feature of his work in
accordance with a maxim which he laid down:--"It is a golden rule,
which I try to follow, to put every fact which is opposed to one's
preconceived opinion in the strongest light." ("More Letters", Vol. II.
page 324.)
The result of this method of presentation is that the works mentioned
above represent a collection of most valuable documents even for those
who feel impelled to draw from the data other conclusions than those of
the author. Each investigation is the outcome of a definite question, a
"preconceived opinion," which is either supported by the facts or
must be abandoned. "How odd it is that anyone should not see that all
observation must be for or against some view if it is to be of any
service!" (Ibid. Vol. I. page 195.)
The points of view which Darwin had before him were principally the
following. In the first place the proof that a large number of the
peculiarities in the structure of flowers are not useless, but of the
greatest significance in pollination must be of considerable importance
for the interpretation of adaptations; "The use of each trifling detail
of structure is far from a barren search to those who believe in natural
selection." ("Fertilisation of Orchids" (1st edition), page 351; (2nd
edition 1904) page 286.) Further, if these structural relations are
shown to be useful, they may have been acquired because from the many
variations which have occurred along different lines, those have been
preserved by natural selection "which are beneficial to the organism
under the complex and ever-varying conditions of life." (Ibid. page
351.) But in the case of flowers there is not only the question of
adaptation to fertilisation to be considered. Darwin, indeed,
soon formed the opinion which he has expressed in the following
sentence,--"From my own observations on plants, guided to a certain
extent by the experience of the breeders of animals, I became convinced
many years ago that it is a general law of nature that flowers are
adapted to be crossed, at least occasionally, by pollen from a distinct
plant." ("Cross and Self fertilisation" (1st edition), page 6.)
The experience of animal breeders pointed to the conclusion that
continual in-breeding is injurious. If this is correct, it raises the
question whether the same conclusion holds for plants. As most flowers
are hermaphrodite, plants afford much more favourable material than
animals for an experimental solution of the question, what results
follow from the union of nearly related sexual cells as compared with
those obtained by the introduction of new blood. The answer to this
question must, moreover, possess the greatest significance for the
correct understanding of sexual reproduction in general.
We see, therefore, that the problems which Darwin had before him in his
researches into the biology of flowers were of the greatest importance,
and at the same time that the point of view from which he attacked the
problems was essentially a teleological one.
We may next inquire in what condition he found the biology of flowers at
the time of his first researches, which were undertaken about the year
1838. In his autobiography he writes,--"During the summer of 1839,
and, I believe, during the previous summer, I was led to attend to the
cross-fertilisation of flowers by the aid of insects, from having come
to the conclusion in my speculations on the origin of species, that
crossing played an important part in keeping specific forms constant."
("The Life and Letters of Charles Darwin", Vol. I. page 90, London,
1888.) In 1841 he became acquainted with Sprengel's work: his researches
into the biology of flowers were thus continued for about forty years.
It is obvious that there could only be a biology of flowers after it had
been demonstrated that the formation of seeds and fruit in the flower
is dependent on pollination and subsequent fertilisation. This proof
was supplied at the end of the seventeenth century by R.J. Camerarius
(1665-1721). He showed that normally seeds and fruits are developed only
when the pollen reaches the stigma. The manner in which this happens was
first thoroughly investigated by J.G. Kolreuter (1733-1806 (Kolreuter,
"Vorlaufige Nachricht von einigen das Geschlecht der Planzen
betreffenden Versuchen und Beobachtungen", Leipzig, 1761; with three
supplements, 1763-66. Also, "Mem. de l'acad. St Petersbourg", Vol. XV.
1809.)), the same observer to whom we owe the earliest experiments in
hybridisation of real scientific interest. Kolreuter mentioned that
pollen may be carried from one flower to another partly by wind and
partly by insects. But he held the view, and that was, indeed, the
natural assumption, that self-fertilisation usually occurs in a flower,
in other words that the pollen of a flower reaches the stigma of
the same flower. He demonstrated, however, certain cases in which
cross-pollination occurs, that is in which the pollen of another flower
of the same species is conveyed to the stigma. He was familiar with the
phenomenon, exhibited by numerous flowers, to which Sprengel afterwards
applied the term Dichogamy, expressing the fact that the anthers and
stigmas of a flower often ripen at different times, a peculiarity
which is now recognised as one of the commonest means of ensuring
cross-pollination.
With far greater thoroughness and with astonishing power of observation
C.K. Sprengel (1750-1816) investigated the conditions of pollination of
flowers. Darwin was introduced by that eminent botanist Robert Brown to
Sprengel's then but little appreciated work,--"Das entdeckte Geheimniss
der Natur im Bau und in der Befruchtung der Blumen" (Berlin, 1793); this
is by no means the least service to Botany rendered by Robert Brown.
Sprengel proceeded from a naive teleological point of view. He firmly
believed "that the wise Author of nature had not created a single hair
without a definite purpose." He succeeded in demonstrating a number of
beautiful adaptations in flowers for ensuring pollination; but his work
exercised but little influence on his contemporaries and indeed for a
long time after his death. It was through Darwin that Sprengel's work
first achieved a well deserved though belated fame. Even such botanists
as concerned themselves with researches into the biology of flowers
appear to have formerly attached much less value to Sprengel's work
than it has received since Darwin's time. In illustration of this we may
quote C.F. Gartner whose name is rightly held in the highest esteem as
that of one of the most eminent hybridologists. In his work "Versuche
und Beobachtungen uder die Befruchtungsorgane der vollkommeneren
Gewachse und uber die naturliche und kunstliche Befruchtung durch den
eigenen Pollen" he also deals with flower-pollination. He recognised the
action of the wind, but he believed, in spite of the fact that he
both knew and quoted Kolreuter and Sprengel, that while insects assist
pollination, they do so only occasionally, and he held that insects are
responsible for the conveyance of pollen; thorough investigations
would show "that a very small proportion of the plants included in this
category require this assistance in their native habitat." (Gartner,
"Versucher und Beobachtungen... ", page 335, Stuttgart, 1844.) In the
majority of plants self-pollination occurs.
Seeing that even investigators who had worked for several decades at
fertilisation-phenomena had not advanced the biology of flowers beyond
the initial stage, we cannot be surprised that other botanists followed
to even a less extent the lines laid down by Kolreuter and Sprengel.
This was in part the result of Sprengel's supernatural teleology and in
part due to the fact that his book appeared at a time when other lines
of inquiry exerted a dominating influence.
At the hands of Linnaeus systematic botany reached a vigorous
development, and at the beginning of the nineteenth century the anatomy
and physiology of plants grew from small beginnings to a flourishing
branch of science. Those who concerned themselves with flowers
endeavoured to investigate their development and structure or the most
minute phenomena connected with fertilisation and the formation of the
embryo. No room was left for the extension of the biology of flowers on
the lines marked out by Kolreuter and Sprengel. Darwin was the first to
give new life and a deeper significance to this subject, chiefly
because he took as his starting-point the above-mentioned problems, the
importance of which is at once admitted by all naturalists.
The further development of floral biology by Darwin is in the first
place closely connected with the book on the fertilisation of Orchids.
It is noteworthy that the title includes the sentence,--"and on the good
effects of intercrossing."
The purpose of the book is clearly stated in the introduction:--"The
object of the following work is to show that the contrivances by which
Orchids are fertilised, are as varied and almost as perfect as any of
the most beautiful adaptations in the animal kingdom; and, secondly,
to show that these contrivances have for their main object the
fertilisation of each flower by the pollen of another flower."
("Fertilisation of Orchids", page 1.) Orchids constituted a particularly
suitable family for such researches. Their flowers exhibit a striking
wealth of forms; the question, therefore, whether the great variety
in floral structure bears any relation to fertilisation (In the older
botanical literature the word fertilisation is usually employed in cases
where POLLINATION is really in question: as Darwin used it in this sense
it is so used here.) must in this case possess special interest.
Darwin succeeded in showing that in most of the orchids examined
self-fertilisation is either an impossibility, or, under natural
conditions, occurs only exceptionally. On the other hand these plants
present a series of extraordinarily beautiful and remarkable adaptations
which ensure the transference of pollen by insects from one flower to
another. It is impossible to describe adequately in a few words the
wealth of facts contained in the Orchid book. A few examples may,
however, be quoted in illustration of the delicacy of the observations
and of the perspicuity employed in interpreting the facts.
The majority of orchids differ from other seed plants (with the
exception of the Asclepiads) in having no dust-like pollen. The pollen,
or more correctly, the pollen-tetrads, remain fastened together as
club-shaped pollinia usually borne on a slender pedicel. At the base of
the pedicel is a small viscid disc by which the pollinium is attached
to the head or proboscis of one of the insects which visit the flower.
Darwin demonstrated that in Orchis and other flowers the pedicel of
the pollinium, after its removal from the anther, undergoes a curving
movement. If the pollinium was originally vertical, after a time it
assumed a horizontal position. In the latter position, if the insect
visited another flower, the pollinium would exactly hit the sticky
stigmatic surface and thus effect fertilisation. The relation between
the behaviour of the viscid disc and the secretion of nectar by the
flower is especially remarkable. The flowers possess a spur which in
some species (e.g. Gymnadenia conopsea, Platanthera bifolia, etc.)
contains honey (nectar), which serves as an attractive bait for insects,
but in others (e.g. our native species of Orchis) the spur is empty.
Darwin held the opinion, confirmed by later investigations, that in the
case of flowers without honey the insects must penetrate the wall of the
nectarless spurs in order to obtain a nectar-like substance. The glands
behave differently in the nectar-bearing and in the nectarless flowers.
In the former they are so sticky that they at once adhere to the body of
the insect; in the nectarless flowers firm adherence only occurs after
the viscid disc has hardened. It is, therefore, adaptively of value
that the insects should be detained longer in the nectarless flowers (by
having to bore into the spur),--than in flowers in which the nectar is
freely exposed. "If this relation, on the one hand, between the viscid
matter requiring some little time to set hard, and the nectar being so
lodged that moths are delayed in getting it; and, on the other hand,
between the viscid matter being at first as viscid as ever it will
become, and the nectar lying all ready for rapid suction, be accidental,
it is a fortunate accident for the plant. If not accidental, and I
cannot believe it to be accidental, what a singular case of adaptation!"
("Fertilisation of Orchids" (1st edition), page 53.)
Among exotic orchids Catasetum is particularly remarkable. One and the
same species bears different forms of flowers. The species known as
Catasetum tridentatum has pollinia with very large viscid discs;
on touching one of the two filaments (antennae) which occur on the
gynostemium of the flower the pollinia are shot out to a fairly long
distance (as far as 1 metre) and in such manner that they alight on the
back of the insect, where they are held. The antennae have, moreover,
acquired an importance, from the point of view of the physiology of
stimulation, as stimulus-perceiving organs. Darwin had shown that it is
only a touch on the antennae that causes the explosion, while contact,
blows, wounding, etc. on other places produce no effect. This form of
flower proved to be the male. The second form, formerly regarded as
a distinct species and named Monachanthus viridis, is shown to be the
female flower. The anthers have only rudimentary pollinia and do not
open; there are no antennae, but on the other hand numerous seeds
are produced. Another type of flower, known as Myanthus barbatus, was
regarded by Darwin as a third form: this was afterwards recognised
by Rolfe (Rolfe, R.A. "On the sexual forms of Catasetum with special
reference to the researches of Darwin and others," "Journ. Linn. Soc."
Vol. XXVII. (Botany), 1891, pages 206-225.) as the male flower
of another species, Catasetum barbatum Link, an identification in
accordance with the discovery made by Cruger in Trinidad that it always
remains sterile.
Darwin had noticed that the flowers of Catasetum do not secrete nectar,
and he conjectured that in place of it the insects gnaw a tissue in
the cavity of the labellum which has a "slightly sweet, pleasant and
nutritious taste." This conjecture as well as other conclusions drawn by
Darwin from Catasetum have been confirmed by Cruger--assuredly the best
proof of the acumen with which the wonderful floral structure of this
"most remarkable of the Orchids" was interpretated far from its native
habitat.
As is shown by what we have said about Catasetum, other problems in
addition to those concerned with fertilisation are dealt with in the
Orchid book. This is especially the case in regard to flower morphology.
The scope of flower morphology cannot be more clearly and better
expressed than by these words: "He will see how curiously a flower may
be moulded out of many separate organs--how perfect the cohesion of
primordially distinct parts may become,--how organs may be used for
purposes widely different from their proper function,--how other organs
may be entirely suppressed, or leave mere useless emblems of their
former existence." ("Fertilisation of Orchids", page 289.)
In attempting, from this point of view, to refer the floral structure
of orchids to their original form, Darwin employed a much more thorough
method than that of Robert Brown and others. The result of this was the
production of a considerable literature, especially in France, along
the lines suggested by Darwin's work. This is the so-called anatomical
method, which seeks to draw conclusions as to the morphology of the
flower from the course of the vascular bundles in the several parts. (He
wrote in one of his letters, "... the destiny of the whole human race is
as nothing to the course of vessels of orchids" ("More Letters", Vol.
II. page 275.) Although the interpretation of the orchid flower given
by Darwin has not proved satisfactory in one particular point--the
composition of the labellum--the general results have received universal
assent, namely "that all Orchids owe what they have in common to descent
from some monocotyledonous plant, which, like so many other plants of
the same division, possessed fifteen organs arranged alternately three
within three in five whorls." ("Fertilisation of Orchids" (1st edition),
page 307.) The alterations which their original form has undergone have
persisted so far as they were found to be of use.
We see also that the remarkable adaptations of which we have given some
examples are directed towards cross-fertilisation. In only a few of
the orchids investigated by Darwin--other similar cases have since been
described--was self-fertilisation found to occur regularly or usually.
The former is the case in the Bee Ophrys (Ophrys apifera), the mechanism
of which greatly surprised Darwin. He once remarked to a friend that one
of the things that made him wish to live a few thousand years was his
desire to see the extinction of the Bee Ophrys, an end to which he
believed its self-fertilising habit was leading. ("Life and Letters",
Vol. III. page 276 (footnote).) But, he wrote, "the safest conclusion,
as it seems to me, is, that under certain unknown circumstances, and
perhaps at very long intervals of time, one individual of the Bee Ophrys
is crossed by another." ("Fertilisation of Orchids" page 71.)
If, on the one hand, we remember how much more sure self-fertilisation
would be than cross-fertilisation, and, on the other hand, if we call to
mind the numerous contrivances for cross-fertilisation, the
conclusion is naturally reached that "it is an astonishing fact that
self-fertilisation should not have been an habitual occurrence. It
apparently demonstrates to us that there must be something injurious in
the process. Nature thus tells us, in the most emphatic manner, that she
abhors perpetual self-fertilisation... For may we not further infer as
probable, in accordance with the belief of the vast majority of the
breeders of our domestic productions, that marriage between near
relations is likewise in some way injurious, that some unknown great
good is derived from the union of individuals which have been kept
distinct for many generations?" (Ibid., page 359.)
This view was supported by observations on plants of other families,
e.g. Papilionaceae; it could, however, in the absence of experimental
proof, be regarded only as a "working hypothesis."
All adaptations to cross-pollination might also be of use simply because
they made pollination possible when for any reason self-pollination had
become difficult or impossible. Cross-pollination would, therefore, be
of use, not as such, but merely as a means of pollination in general;
it would to some extent serve as a remedy for a method unsuitable in
itself, such as a modification standing in the way of self-pollination,
and on the other hand as a means of increasing the chance of pollination
in the case of flowers in which self-pollination was possible, but which
might, in accidental circumstances, be prevented. It was, therefore,
very important to obtain experimental proof of the conclusion to which
Darwin was led by the belief of the majority of breeders and by the
evidence of the widespread occurrence of cross-pollination and of the
remarkable adaptations thereto.
This was supplied by the researches which are described in the two other
works named above. The researches on which the conclusions rest had, in
part at least, been previously published in separate papers: this is the
case as regards the heterostyled plants. The discoveries which Darwin
made in the course of his investigations of these plants belong to the
most brilliant in biological science.
The case of Primula is now well known. C.K. Sprengel and others were
familiar with the remarkable fact that different individuals of the
European species of Primula bear differently constructed flowers; some
plants possess flowers in which the styles project beyond the stamens
attached to the corolla-tube (long-styled form), while in others the
stamens are inserted above the stigma which is borne on a short style
(short-styled form). It has been shown by Breitenbach that both forms of
flower may occur on the same plant, though this happens very rarely. An
analogous case is occasionally met with in hybrids, which bear flowers
of different colour on the same plant (e.g. Dianthus caryophyllus).
Darwin showed that the external differences are correlated with others
in the structure of the stigma and in the nature of the pollen.
The long-styled flowers have a spherical stigma provided with large
stigmatic papillae; the pollen grains are oblong and smaller than those
of the short-styled flowers. The number of the seeds produced is smaller
and the ovules larger, probably also fewer in number. The short-styled
flowers have a smooth compressed stigma and a corolla of somewhat
different form; they produce a greater number of seeds.
These different forms of flowers were regarded as merely a case of
variation, until Darwin showed "that these heterostyled plants are
adapted for reciprocal fertilisation; so that the two or three forms,
though all are hermaphrodites, are related to one another almost
like the males and females of ordinary unisexual animals." ("Forms
of Flowers" (1st edition), page 2.) We have here an example of
hermaphrodite flowers which are sexually different. There are essential
differences in the manner in which fertilisation occurs. This may
be effected in four different ways; there are two legitimate and two
illegitimate types of fertilisation. The fertilisation is legitimate
if pollen from the long-styled flowers reaches the stigma of the
short-styled form or if pollen of the short-styled flowers is brought
to the stigma of the long-styled flower, that is the organs of the
same length of the two different kinds of flower react on one
another. Illegitimate fertilisation is represented by the two kinds of
self-fertilisation, also by cross-fertilisation, in which the pollen of
the long-styled form reaches the stigma of the same type of flower and,
similarly, by cross-pollination in the case of the short-styled flowers.
The applicability of the terms legitimate and illegitimate depends, on
the one hand, upon the fact that insects which visit the different forms
of flowers pollinate them in the manner suggested; the pollen of the
short-styled flowers adhere to that part of the insect's body which
touches the stigma of the long-styled flower and vice versa. On the
other hand, it is based also on the fact that experiment shows that
artificial pollination produces a very different result according as
this is legitimate or illegitimate; only the legitimate union ensures
complete fertility, the plants thus produced being stronger than those
which are produced illegitimately.
If we take 100 as the number of flowers which produce seeds as the
result of legitimate fertilisation, we obtain the following numbers from
illegitimate fertilisation:
Primula officinalis (P. veris) (Cowslip)... 69 Primula elatior
(Oxlip).................... 27 Primula acaulis (P. vulgaris)
(Primrose)... 60
Further, the plants produced by the illegitimate method of fertilisation
showed, e.g. in P. officinalis, a decrease in fertility in later
generations, sterile pollen and in the open a feebler growth. (Under
very favourable conditions (in a greenhouse) the fertility of the plants
of the fourth generation increases--a point, which in view of various
theoretical questions, deserves further investigation.) They behave in
fact precisely in the same way as hybrids between species of different
genera. This result is important, "for we thus learn that the difficulty
in sexually uniting two organic forms and the sterility of their
offspring, afford no sure criterion of so-called specific distinctness"
("Forms of Flowers", page 242): the relative or absolute sterility
of the illegitimate unions and that of their illegitimate descendants
depend exclusively on the nature of the sexual elements and on their
inability to combine in a particular manner. This functional difference
of sexual cells is characteristic of the behaviour of hybrids as of the
illegitimate unions of heterostyled plants. The agreement becomes
even closer if we regard the Primula plants bearing different forms of
flowers not as belonging to a systematic entity or "species," but as
including several elementary species. The legitimately produced plants
are thus true hybrids (When Darwin wrote in reference to the different
forms of heterostyled plants, "which all belong to the same species
as certainly as do the two sexes of the same species" ("Cross and
Self fertilisation", page 466), he adopted the term species in a
comprehensive sense. The recent researches of Bateson and Gregory ("On
the inheritance of Heterostylism in Primula"; "Proc. Roy. Soc." Ser. B,
Vol. LXXVI. 1905, page 581) appear to me also to support the view that
the results of illegitimate crossing of heterostyled Primulas correspond
with those of hybridisation. The fact that legitimate pollen effects
fertilisation, even if illegitimate pollen reaches the stigma a short
time previously, also points to this conclusion. Self-pollination in the
case of the short-styled form, for example, is not excluded. In spite
of this, the numerical proportion of the two forms obtained in the open
remains approximately the same as when the pollination was exclusively
legitimate, presumably because legitimate pollen is prepotent.), with
which their behaviour in other respects, as Darwin showed, presents so
close an agreement. This view receives support also from the fact that
descendants of a flower fertilised illegitimately by pollen from another
plant with the same form of flower belong, with few exceptions, to the
same type as that of their parents. The two forms of flower, however,
behave differently in this respect. Among 162 seedlings of the
long-styled illegitimately pollinated plants of Primula officinalis,
including five generations, there were 156 long-styled and only six
short-styled forms, while as the result of legitimate fertilisation
nearly half of the offspring were long-styled and half short-styled. The
short-styled illegitimately pollinated form gave five long-styled
and nine short-styled; the cause of this difference requires further
explanation. The significance of heterostyly, whether or not we now
regard it as an arrangement for the normal production of hybrids, is
comprehensively expressed by Darwin: "We may feel sure that plants have
been rendered heterostyled to ensure cross-fertilisation, for we now
know that a cross between the distinct individuals of the same species
is highly important for the vigour and fertility of the offspring."
("Forms of Flowers", page 258.) If we remember how important the
interpretation of heterostyly has become in all general problems as,
for example, those connected with the conditions of the formation of
hybrids, a fact which was formerly overlooked, we can appreciate how
Darwin was able to say in his autobiography: "I do not think anything in
my scientific life has given me so much satisfaction as making out the
meaning of the structure of these plants." ("Life and Letters", Vol. I.
page 91.)
The remarkable conditions represented in plants with three kinds of
flowers, such as Lythrum and Oxalis, agree in essentials with those
in Primula. These cannot be considered in detail here; it need only be
noted that the investigation of these cases was still more laborious.
In order to establish the relative fertility of the different unions in
Lythrum salicaria 223 different fertilisations were made, each flower
being deprived of its male organs and then dusted with the appropriate
pollen.
In the book containing the account of heterostyled plants other
species are dealt with which, in addition to flowers opening normally
(chasmogamous), also possess flowers which remain closed but are capable
of producing fruit. These cleistogamous flowers afford a striking
example of habitual self-pollination, and H. von Mohl drew special
attention to them as such shortly after the appearance of Darwin's
Orchid book. If it were only a question of producing seed in the
simplest way, cleistogamous flowers would be the most conveniently
constructed. The corolla and frequently other parts of the flower are
reduced; the development of the seed may, therefore, be accomplished
with a smaller expenditure of building material than in chasmogamous
flowers; there is also no loss of pollen, and thus a smaller amount
suffices for fertilisation.
Almost all these plants, as Darwin pointed out, have also chasmogamous
flowers which render cross-fertilisation possible. His view that
cleistogamous flowers are derived from originally chasmogamous flowers
has been confirmed by more recent researches. Conditions of nutrition in
the broader sense are the factors which determine whether chasmogamous
or cleistogamous flowers are produced, assuming, of course, that the
plants in question have the power of developing both forms of flower.
The former may fail to appear for some time, but are eventually
developed under favourable conditions of nourishment. The belief of many
authors that there are plants with only cleistogamous flowers cannot
therefore be accepted as authoritative without thorough experimental
proof, as we are concerned with extra-european plants for which it is
often difficult to provide appropriate conditions in cultivation.
Darwin sees in cleistogamous flowers an adaptation to a good supply of
seeds with a small expenditure of material, while chasmogamous flowers
of the same species are usually cross-fertilised and "their offspring
will thus be invigorated, as we may infer from a wide-spread analogy."
("Forms of Flowers" (1st edition), page 341.) Direct proof in support of
this has hitherto been supplied in a few cases only; we shall often
find that the example set by Darwin in solving such problems as these by
laborious experiment has unfortunately been little imitated.
Another chapter of this book treats of the distribution of the sexes in
polygamous, dioecious, and gyno-dioecious plants (the last term, now in
common use, we owe to Darwin). It contains a number of important facts
and discussions and has inspired the experimental researches of Correns
and others.
The most important of Darwin's work on floral biology is, however, that
on cross and self-fertilisation, chiefly because it states the results
of experimental investigations extending over many years. Only
such experiments, as we have pointed out, could determine whether
cross-fertilisation is in itself beneficial, and self-fertilisation
on the other hand injurious; a conclusion which a merely comparative
examination of pollination-mechanisms renders in the highest degree
probable. Later floral biologists have unfortunately almost entirely
confined themselves to observations on floral mechanisms. But there is
little more to be gained by this kind of work than an assumption
long ago made by C.K. Sprengel that "very many flowers have the sexes
separate and probably at least as many hermaphrodite flowers are
dichogamous; it would thus appear that Nature was unwilling that any
flower should be fertilised by its own pollen."
It was an accidental observation which inspired Darwin's experiments on
the effect of cross and self-fertilisation. Plants of Linaria vulgaris
were grown in two adjacent beds; in the one were plants produced by
cross-fertilisation, that is, from seeds obtained after fertilisation
by pollen of another plant of the same species; in the other grew plants
produced by self-fertilisation, that is from seed produced as the result
of pollination of the same flower. The first were obviously superior to
the latter.
Darwin was surprised by this observation, as he had expected a
prejudicial influence of self-fertilisation to manifest itself after
a series of generations: "I always supposed until lately that no
evil effects would be visible until after several generations of
self-fertilisation, but now I see that one generation sometimes suffices
and the existence of dimorphic plants and all the wonderful contrivances
of orchids are quite intelligible to me." ("More Letters", Vol. II. page
373.)
The observations on Linaria and the investigations of the results of
legitimate and illegitimate fertilisation in heterostyled plants were
apparently the beginning of a long series of experiments. These were
concerned with plants of different families and led to results which are
of fundamental importance for a true explanation of sexual reproduction.
The experiments were so arranged that plants were shielded from
insect-visits by a net. Some flowers were then pollinated with their own
pollen, others with pollen from another plant of the same species. The
seeds were germinated on moist sand; two seedlings of the same age, one
from a cross and the other from a self-fertilised flower, were selected
and planted on opposite sides of the same pot. They grew therefore under
identical external conditions; it was thus possible to compare their
peculiarities such as height, weight, fruiting capacity, etc. In other
cases the seedlings were placed near to one another in the open and in
this way their capacity of resisting unfavourable external conditions
was tested. The experiments were in some cases continued to the tenth
generation and the flowers were crossed in different ways. We see,
therefore, that this book also represents an enormous amount of most
careful and patient original work.
The general result obtained is that plants produced as the result of
cross-fertilisation are superior, in the majority of cases, to those
produced as the result of self-fertilisation, in height, resistance to
external injurious influences, and in seed-production.
Ipomoea purpurea may be quoted as an example. If we express the result
of cross-fertilisation by 100, we obtain the following numbers for the
fertilised plants.
Generation. Height. Number of seeds.
1 100: 76 100: 64
2 100: 79 -
3 100: 68 100: 94
4 100: 86 100: 94
5 100: 75 100: 89
6 100: 72 -
7 100: 81 -
8 100: 85 -
9 100: 79 100: 26 (Number of capsules)
10 100: 54 -
Taking the average, the ratio as regards growth is 100:77. The
considerable superiority of the crossed plants is apparent in the first
generation and is not increased in the following generations; but there
is some fluctuation about the average ratio. The numbers representing
the fertility of crossed and self-fertilised plants are more difficult
to compare with accuracy; the superiority of the crossed plants is
chiefly explained by the fact that they produce a much larger number
of capsules, not because there are on the average more seeds in each
capsule. The ratio of the capsules was, e.g. in the third generation,
100:38, that of the seeds in the capsules 100:94. It is also especially
noteworthy that in the self-fertilised plants the anthers were smaller
and contained a smaller amount of pollen, and in the eighth generation
the reduced fertility showed itself in a form which is often found in
hybrids, that is the first flowers were sterile. (Complete sterility was
not found in any of the plants investigated by Darwin. Others appear
to be more sensitive; Cluer found Zea Mais "almost sterile" after three
generations of self-fertilisation. (Cf. Fruwirth, "Die Zuchtung der
Landwirtschaftlichen Kulturpflanzen", Berlin, 1904, II. page 6.))
The superiority of crossed individuals is not exhibited in the same
way in all plants. For example in Eschscholzia californica the crossed
seedlings do not exceed the self-fertilised in height and vigour,
but the crossing considerably increases the plant's capacity for
flower-production, and the seedlings from such a mother-plant are more
fertile.
The conception implied by the term crossing requires a closer analysis.
As in the majority of plants, a large number of flowers are in bloom
at the same time on one and the same plant, it follows that insects
visiting the flowers often carry pollen from one flower to another of
the same stock. Has this method, which is spoken of as Geitonogamy, the
same influence as crossing with pollen from another plant? The results
of Darwin's experiments with different plants (Ipomoea purpurea,
Digitalis purpurea, Mimulus luteus, Pelargonium, Origanum) were not in
complete agreement; but on the whole they pointed to the conclusion that
Geitonogamy shows no superiority over self-fertilisation (Autogamy).
(Similarly crossing in the case of flowers of Pelargonium zonale, which
belong to plants raised from cuttings from the same parent, shows no
superiority over self-fertilisation.) Darwin, however, considered it
possible that this may sometimes be the case. "The sexual elements
in the flowers on the same plant can rarely have been differentiated,
though this is possible, as flower-buds are in one sense distinct
individuals, sometimes varying and differing from one another in
structure or constitution." ("Cross and Self fertilisation" (1st
edition), page 444.)
As regards the importance of this question from the point of view of
the significance of cross-fertilisation in general, it may be noted
that later observers have definitely discovered a difference between the
results of autogamy and geitonogamy. Gilley and Fruwirth found that in
Brassica Napus, the length and weight of the fruits as also the total
weight of the seeds in a single fruit were less in the case of autogamy
than in geitonogamy. With Sinapis alba a better crop of seeds was
obtained after geitonogamy, and in the Sugar Beet the average weight
of a fruit in the case of a self-fertilised plant was 0.009 gr., from
geitonogamy 0.012 gr., and on cross-fertilisation 0.013 gr.
On the whole, however, the results of geitonogamy show that the
favourable effects of cross-fertilisation do not depend simply on the
fact that the pollen of one flower is conveyed to the stigma of another.
But the plants which are crossed must in some way be different.
If plants of Ipomoea purpurea (and Mimulus luteus) which have been
self-fertilised for seven generations and grown under the same
conditions of cultivation are crossed together, the plants so crossed
would not be superior to the self-fertilised; on the other hand crossing
with a fresh stock at once proves very advantageous. The favourable
effect of crossing is only apparent, therefore, if the parent plants
are grown under different conditions or if they belong to different
varieties. "It is really wonderful what an effect pollen from a distinct
seedling plant, which has been exposed to different conditions of life,
has on the offspring in comparison with pollen from the same flower or
from a distinct individual, but which has been long subjected to the
same conditions. The subject bears on the very principle of life, which
seems almost to require changes in the conditions." ("More Letters",
Vol. II. page 406.)
The fertility--measured by the number or weight of the seeds produced
by an equal number of plants--noticed under different conditions of
fertilisation may be quoted in illustration.
On crossing On crossing On self-
with a fresh plants of the fertilisation
stock same stock
Mimuleus luteus
(First and ninth generation) 100 4 3
Eschscholzia californica
(second generation) 100 45 40
Dianthus caryophyllus
(third and fourth generation) 100 45 33