CHAPTER 2.XII.
'DIFFERENT FORMS OF FLOWERS ON PLANTS OF THE SAME SPECIES.'
1877.
[The volume bearing the above title was published in 1877, and was
dedicated by the author to Professor Asa Gray, "as a small tribute of
respect and affection." It consists of certain earlier papers re-edited,
with the addition of a quantity of new matter. The subjects treated in the
book are:--
1. Heterostyled Plants.
2. Polygamous, Dioecious, and Gynodioecious Plants.
3. Cleistogamic Flowers.
The nature of heterostyled plants may be illustrated in the primrose, one
of the best known examples of the class. If a number of primroses be
gathered, it will be found that some plants yield nothing but "pin-eyed"
flowers, in which the style (or organ for the transmission of the pollen to
the ovule) is long, while the others yield only "thrum-eyed" flowers with
short styles. Thus primroses are divided into two sets or castes differing
structurally from each other. My father showed that they also differ
sexually, and that in fact the bond between the two castes more nearly
resembles that between separate sexes than any other known relationship.
Thus for example a long-styled primrose, though it can be fertilised by its
own pollen, is not FULLY fertile unless it is impregnated by the pollen of
a short-styled flower. Heterostyled plants are comparable to hermaphrodite
animals, such as snails, which require the concourse of two individuals,
although each possesses both the sexual elements. The difference is that
in the case of the primrose it is PERFECT FERTILITY, and not simply
FERTILITY, that depends on the mutual action of the two sets of
individuals.
The work on heterostyled plants has a special bearing, to which the author
attached much importance, on the problem of origin of species. (See
'Autobiography,' volume i.)
He found that a wonderfully close parallelism exists between hybridisation
and certain forms of fertilisation among heterostyled plants. So that it
is hardly an exaggeration to say that the "illegitimately" reared seedlings
are hybrids, although both their parents belong to identically the same
species. In a letter to Professor Huxley, my father writes as if his
researches on heterostyled plants tended to make him believe that sterility
is a selected or acquired quality. But in his later publications, e.g. in
the sixth edition of the 'Origin,' he adheres to the belief that sterility
is an incidental rather than a selected quality. The result of his work on
heterostyled plants is of importance as showing that sterility is no test
of specific distinctness, and that it depends on differentiation of the
sexual elements which is independent of any racial difference. I imagine
that it was his instinctive love of making out a difficulty which to a
great extent kept him at work so patiently on the heterostyled plants. But
it was the fact that general conclusions of the above character could be
drawn from his results which made him think his results worthy of
publication. (See 'Forms of Flowers,' page 243.)
The papers which on this subject preceded and contributed to 'Forms of
Flowers' were the following:--
"On the two Forms or Dimorphic Condition in the Species of Primula, and on
their remarkable Sexual Relations." Linn. Soc. Journal, 1862.)
"On the Existence of Two Forms, and on their Reciprocal Sexual Relations,
in several Species of the Genus Linum." Linn. Soc. Journal, 1863.
"On the Sexual Relations of the Three Forms of Lythrum salicaria," Ibid.
1864.
"On the Character and Hybrid-like Nature of the Offspring from the
Illegitimate Unions of Dimorphic and Trimorphic Plants." Ibid. 1869.
"On the Specific Differences between Primula veris, Brit. Fl. (var.
Officinalis, Linn.), P. vulgaris, Brit. Fl. (var. acaulis, Linn.) and P.
elatior, Jacq.; and on the Hybrid Nature of the Common Oxlip. With
Supplementary Remarks on Naturally Produced Hybrids in the Genus
Verbascum." Ibid. 1869.
The following letter shows that he began the work on heterostyled plants
with an erroneous view as to the meaning of the facts.]
CHARLES DARWIN TO J.D. HOOKER.
Down, May 7 [1860].
...I have this morning been looking at my experimental cowslips, and I find
some plants have all flowers with long stamens and short pistils, which I
will call "male plants," others with short stamens and long pistils, which
I will call "female plants." This I have somewhere seen noticed, I think
by Henslow; but I find (after looking at my two sets of plants) that the
stigmas of the male and female are of slightly different shape, and
certainly different degree of roughness, and what has astonished me, the
pollen of the so-called female plant, though very abundant, is more
transparent, and each granule is exactly only 2/3 of the size of the pollen
of the so-called male plant. Has this been observed? I cannot help
suspecting [that] the cowslip is in fact dioecious, but it may turn out all
a blunder, but anyhow I will mark with sticks the so-called male and female
plants and watch their seeding. It would be a fine case of gradation
between an hermaphrodite and unisexual condition. Likewise a sort of case
of balancement of long and short pistils and stamens. Likewise perhaps
throws light on oxlips...
I have now examined primroses and find exactly the same difference in the
size of the pollen, correlated with the same difference in the length of
the style and roughness of the stigmas.
CHARLES DARWIN TO ASA GRAY.
June 8 [1860].
...I have been making some little trifling observations which have
interested and perplexed me much. I find with primroses and cowslips, that
about an equal number of plants are thus characterised.
SO-CALLED (by me) MALE plant. Pistil much shorter than stamens; stigma
rather smooth,--POLLEN GRAINS LARGE, throat of corolla short.
SO-CALLED FEMALE plant. Pistil much longer than stamens, stigma rougher,
POLLEN-GRAINS SMALLER,--throat of corolla long.
I have marked a lot of plants, and expected to find the so-called male
plant barren; but judging from the feel of the capsules, this is not the
case, and I am very much surprised at the difference in the size of the
pollen...If it should prove that the so-called male plants produce less
seed than the so-called females, what a beautiful case of gradation from
hermaphrodite to unisexual condition it will be! If they produce about
equal number of seed, how perplexing it will be.
CHARLES DARWIN TO J.D. HOOKER.
Down, December 17 [1860?].
...I have just been ordering a photograph of myself for a friend; and have
ordered one for you, and for heaven's sake oblige me, and burn that now
hanging up in your room.--It makes me look atrociously wicked.
...In the spring I must get you to look for long pistils and short pistils
in the rarer species of Primula and in some allied Genera. It holds with
P. Sinensis. You remember all the fuss I made on this subject last spring;
well, the other day at last I had time to weigh the seeds, and by Jove the
plants of primroses and cowslip with short pistils and large grained pollen
(Thus the plants which he imagined to be tending towards a male condition
were more productive than the supposed females.) are rather more fertile
than those with long pistils, and small-grained pollen. I find that they
require the action of insects to set them, and I never will believe that
these differences are without some meaning.
Some of my experiments lead me to suspect that the large-grained pollen
suits the long pistils and the small-grained pollen suits the short
pistils; but I am determined to see if I cannot make out the mystery next
spring.
How does your book on plants brew in your mind? Have you begun it?...
Remember me most kindly to Oliver. He must be astonished at not having a
string of questions, I fear he will get out of practice!
[The Primula-work was finished in the autumn of 1861, and on November 8th
he wrote to Sir J.D. Hooker:--
"I have sent my paper on dimorphism in Primula to the Linn. Soc. I shall
go up and read it whenever it comes on; I hope you may be able to attend,
for I do not suppose many will care a penny for the subject."
With regard to the reading of the paper (on November 21st), he wrote to the
same friend:--
"I by no means thought that I produced a "tremendous effect" in the Linn.
Soc., but by Jove the Linn. Soc. produced a tremendous effect on me, for I
could not get out of bed till late next evening, so that I just crawled
home. I fear I must give up trying to read any paper or speak; it is a
horrid bore, I can do nothing like other people."
To Dr. Gray he wrote, (December 1861):--
"You may rely on it, I will send you a copy of my Primula paper as soon as
I can get one; but I believe it will not be printed till April 1st, and
therefore after my Orchid Book. I care more for your and Hooker's opinion
than for that of all the rest of the world, and for Lyell's on geological
points. Bentham and Hooker thought well of my paper when read; but no one
can judge of evidence by merely hearing a paper."
The work on Primula was the means of bringing my father in contact with the
late Mr. John Scott, then working as a gardener in the Botanic Gardens at
Edinburgh,--an employment which he seems to have chosen in order to gratify
his passion for natural history. He wrote one or two excellent botanical
papers, and ultimately obtained a post in India. (While in India he made
some admirable observations on expression for my father.) He died in 1880.
A few phrases may be quoted from letters to Sir J.D. Hooker, showing my
father's estimate of Scott:--
"If you know, do please tell me who is John Scott of the Botanical Gardens
of Edinburgh; I have been corresponding largely with him; he is no common
man."
"If he had leisure he would make a wonderful observer; to my judgment I
have come across no one like him."
"He has interested me strangely, and I have formed a very high opinion of
his intellect. I hope he will accept pecuniary assistance from me; but he
has hitherto refused." (He ultimately succeeded in being allowed to pay
for Mr. Scott's passage to India.)
"I know nothing of him excepting from his letters; these show remarkable
talent, astonishing perseverance, much modesty, and what I admire,
determined difference from me on many points."
So highly did he estimate Scott's abilities that he formed a plan (which
however never went beyond an early stage of discussion) of employing him to
work out certain problems connected with intercrossing.
The following letter refers to my father's investigations on Lythrum (He
was led to this, his first case of trimorphism by Lecoq's 'Geographie
Botanique,' and this must have consoled him for the trick this work played
him in turning out to be so much larger than he expected. He wrote to Sir
J.D. Hooker: "Here is a good joke: I saw an extract from Lecoq,
'Geograph. Bot.,' and ordered it and hoped that it was a good sized
pamphlet, and nine thick volumes have arrived!"), a plant which reveals
even a more wonderful condition of sexual complexity than that of Primula.
For in Lythrum there are not merely two, but three castes, differing
structurally and physiologically from each other:]
CHARLES DARWIN TO ASA GRAY.
Down, August 9 [1862].
My dear Gray,
It is late at night, and I am going to write briefly, and of course to beg
a favour.
The Mitchella very good, but pollen apparently equal-sized. I have just
examined Hottonia, grand difference in pollen. Echium vulgare, a humbug,
merely a case like Thymus. But I am almost stark staring mad over Lythrum
(On another occasion he wrote (to Dr. Gray) with regard to Lythrum: "I
must hold hard, otherwise I shall spend my life over dimorphism."); if I
can prove what I fully believe, it is a grand case of TRIMORPHISM, with
three different pollens and three stigmas; I have castrated and fertilised
above ninety flowers, trying all the eighteen distinct crosses which are
possible within the limits of this one species! I cannot explain, but I
feel sure you would think it a grand case. I have been writing to
Botanists to see if I can possibly get L. hyssopifolia, and it has just
flashed on me that you might have Lythrum in North America, and I have
looked to your Manual. For the love of heaven have a look at some of your
species, and if you can get me seed, do; I want much to try species with
few stamens, if they are dimorphic; Nesaea verticillata I should expect to
be trimorphic. Seed! Seed! Seed! I should rather like seed of
Mitchella. But oh, Lythrum!
Your utterly mad friend,
C. DARWIN.
P.S.--There is reason in my madness, for I can see that to those who
already believe in change of species, these facts will modify to a certain
extent the whole view of Hybridity. (A letter to Dr. Gray (July, 1862)
bears on this point: "A few days ago I made an observation which has
surprised me more than it ought to do--it will have to be repeated several
times, but I have scarcely a doubt of its accuracy. I stated in my Primula
paper that the long-styled form of Linum grandiflorum was utterly sterile
with its own pollen; I have lately been putting the pollen of the two forms
on the stigma of the SAME flower; and it strikes me as truly wonderful,
that the stigma distinguishes the pollen; and is penetrated by the tubes of
the one and not by those of the other; nor are the tubes exserted. Or
(which is the same thing) the stigma of the one form acts on and is acted
on by pollen, which produces not the least effect on the stigma of the
other form. Taking sexual power as the criterion of difference, the two
forms of this one species may be said to be generically distinct.")
[On the same subject he wrote to Sir Joseph Hooker in August 1862:--
"Is Oliver at Kew? When I am established at Bournemouth I am completely
mad to examine any fresh flowers of any Lythraceous plant, and I would
write and ask him if any are in bloom."
Again he wrote to the same friend in October:--
"If you ask Oliver, I think he will tell you I have got a real odd case in
Lythrum, it interests me extremely, and seems to me the strangest case of
propagation recorded amongst plants or animals, viz. a necessary triple
alliance between three hermaphrodites. I feel sure I can now prove the
truth of the case from a multitude of crosses made this summer."
In an article, 'Dimorphism in the Genitalia of Plants' ('Silliman's
Journal,' 1862, volume xxxiv. page 419), Dr. Gray pointed out that the
structural difference between the two forms of Primula had already been
defined in the 'Flora of North America,' as DIOECIO-DIMORPHISM. The use of
this term called forth the following remarks from my father. The letter
also alludes to a review of the 'Fertilisation of Orchids' in the same
volume of 'Silliman's Journal.']
CHARLES DARWIN TO ASA GRAY.
Down, November 26 [1862].
My dear Gray,
The very day after my last letter, yours of November 10th, and the review
in 'Silliman,' which I feared might have been lost, reached me. We were
all very much interested by the political part of your letter; and in some
odd way one never feels that information and opinions painted in a
newspaper come from a living source; they seem dead, whereas all that you
write is full of life. The reviews interested me profoundly; you rashly
ask for my opinion, and you must consequently endure a long letter. First
for Dimorphism; I do not AT PRESENT like the term "Dioecio-dimorphism;" for
I think it gives quite a false notion, that the phenomena are connected
with a separation of the sexes. Certainly in Primula there is unequal
fertility in the two forms, and I suspect this is the case with Linum; and,
therefore I felt bound in the Primula paper to state that it might be a
step towards a dioecious condition; though I believe there are no dioecious
forms in Primulaceae or Linaceae. But the three forms in Lythrum convince
me that the phenomenon is in no way necessarily connected with any tendency
to separation of sexes. The case seems to me in result or function to be
almost identical with what old C.K. Sprengel called "dichogamy," and which
is so frequent in truly hermaphrodite groups; namely, the pollen and stigma
of each flower being mature at different periods. If I am right, it is
very advisable not to use the term "dioecious," as this at once brings
notions of separation of sexes.
...I was much perplexed by Oliver's remarks in the 'Natural History Review'
on the Primula case, on the lower plants having sexes more often separated
than in the higher plants,--so exactly the reverse of what takes place in
animals. Hooker in his review of the 'Orchids' repeats this remark. There
seems to be much truth in what you say ("Forms which are low in the scale
as respects morphological completeness may be high in the scale of rank
founded on specialisation of structure and function."--Dr. Gray, in
'Silliman's Journal.'), and it did not occur to me, about no improbability
of specialisation in CERTAIN lines in lowly organised beings. I could
hardly doubt that the hermaphrodite state is the aboriginal one. But how
is it in the conjugation of Confervae--is not one of the two individuals
here in fact male, and the other female? I have been much puzzled by this
contrast in sexual arrangements between plants and animals. Can there be
anything in the following consideration: By ROUGHEST calculation about
one-third of the British GENERA of aquatic plants belong to the Linnean
classes of Mono and Dioecia; whilst of terrestrial plants (the aquatic
genera being subtracted) only one-thirteenth of the genera belong to these
two classes. Is there any truth in this fact generally? Can aquatic
plants, being confined to a small area or small community of individuals,
require more free crossing, and therefore have separate sexes? But to
return to our point, does not Alph. de Candolle say that aquatic plants
taken as a whole are lowly organised, compared with terrestrial; and may
not Oliver's remark on the separation of the sexes in lowly organised
plants stand in some relation to their being frequently aquatic? Or is
this all rubbish?
...What a magnificent compliment you end your review with! You and Hooker
seem determined to turn my head with conceit and vanity (if not already
turned) and make me an unbearable wretch.
With most cordial thanks, my good and kind friend,
Farewell,
C. DARWIN.
[The following passage from a letter (July 28, 1863), to Prof. Hildebrand,
contains a reference to the reception of the dimorphic work in France:--
"I am extremely much pleased to hear that you have been looking at the
manner of fertilisation of your native Orchids, and still more pleased to
hear that you have been experimenting on Linum. I much hope that you may
publish the result of these experiments; because I was told that the most
eminent French botanists of Paris said that my paper on Primula was the
work of imagination, and that the case was so improbable they did not
believe in my results."]
CHARLES DARWIN TO ASA GRAY.
April 19 [1864].
...I received a little time ago a paper with a good account of your
Herbarium and Library, and a long time previously your excellent review of
Scott's 'Primulaceae,' and I forwarded it to him in India, as it would much
please him. I was very glad to see in it a new case of Dimorphism (I
forget just now the name of the plant); I shall be grateful to hear of any
other cases, as I still feel an interest in the subject. I should be very
glad to get some seed of your dimorphic Plantagos; for I cannot banish the
suspicion that they must belong to a very different class like that of the
common Thyme. (In this prediction he was right. See 'Forms of Flowers,'
page 307.) How could the wind, which is the agent of fertilisation, with
Plantago, fertilise "reciprocally dimorphic" flowers like Primula? Theory
says this cannot be, and in such cases of one's own theories I follow
Agassiz and declare, "that nature never lies." I should even be very glad
to examine the two dried forms of Plantago. Indeed, any dried dimorphic
plants would be gratefully received...
Did my Lythrum paper interest you? I crawl on at the rate of two hours per
diem, with 'Variation under Domestication.'
CHARLES DARWIN TO J.D. HOOKER.
Down, November 26 [1864].
...You do not know how pleased I am that you have read my Lythrum paper; I
thought you would not have time, and I have for long years looked at you as
my Public, and care more for your opinion than that of all the rest of the
world. I have done nothing which has interested me so much as Lythrum,
since making out the complemental males of Cirripedes. I fear that I have
dragged in too much miscellaneous matter into the paper.
...I get letters occasionally, which show me that Natural Selection is
making GREAT progress in Germany, and some amongst the young in France. I
have just received a pamphlet from Germany, with the complimentary title of
"Darwinische Arten-Enstehung-Humbug"!
Farewell, my best of old friends,
C. DARWIN.
CHARLES DARWIN TO ASA GRAY.
September 10, [1867?].
...The only point which I have made out this summer, which could possibly
interest you, is that the common Oxlip found everywhere, more or less
commonly in England, is certainly a hybrid between the primrose and
cowslip; whilst the P. elatior (Jacq.), found only in the Eastern Counties,
is a perfectly distinct and good species; hardly distinguishable from the
common oxlip, except by the length of the seed-capsule relatively to the
calyx. This seems to me rather a horrid fact for all systematic
botanists...
CHARLES DARWIN TO F. HILDEBRAND.
Down, November 16, 1868.
My dear Sir,
I wrote my last note in such a hurry from London, that I quite forgot what
I chiefly wished to say, namely to thank you for your excellent notices in
the 'Bot. Zeitung' of my paper on the offspring of dimorphic plants. The
subject is so obscure that I did not expect that any one would have noticed
my paper, and I am accordingly very much pleased that you should have
brought the subject before the many excellent naturalists of Germany.
Of all the German authors (but they are not many) whose works I have read,
you write by far the clearest style, but whether this is a compliment to a
German writer I do not know.
[The two following letters refer to the small bud-like "Cleistogamic"
flowers found in the violet and many other plants. They do not open and
are necessarily self-fertilised:]
CHARLES DARWIN TO J.D. HOOKER.
Down, May 30 [1862].
...What will become of my book on Variation? I am involved in a
multiplicity of experiments. I have been amusing myself by looking at the
small flowers of Viola. If Oliver (Shortly afterwards he wrote: "Oliver,
the omniscient, has sent me a paper in the 'Bot. Zeitung,' with most
accurate description of all that I saw in Viola.") has had time to study
them, he will have seen the curious case (as it seems to me) which I have
just made clearly out, viz. that in these flowers, the FEW pollen grains
are never shed, or never leave the anther-cells, but emit long pollen
tubes, which penetrate the stigma. To-day I got the anther with the
included pollen grain (now empty) at one end, and a bundle of tubes
penetrating the stigmatic tissue at the other end; I got the whole under a
microscope without breaking the tubes; I wonder whether the stigma pours
some fluid into the anther so as to excite the included grains. It is a
rather odd case of correlation, that in the double sweet violet the small
flowers are double; i.e., have a multitude of minute scales representing
the petals. What queer little flowers they are.
Have you had time to read poor dear Henslow's life? it has interested me
for the man's sake, and, what I did not think possible, has even exalted
his character in my estimation...
[The following is an extract from the letter given in part above, and
refers to Dr. Gray's article on the sexual differences of plants:]
CHARLES DARWIN TO ASA GRAY.
NOVEMBER 26 [1862].
...You will think that I am in the most unpleasant, contradictory,
fractious humour, when I tell you that I do not like your term of
"precocious fertilisation" for your second class of dimorphism [i.e. for
cleistogamic fertilisation]. If I can trust my memory, the state of the
corolla, of the stigma, and the pollen-grains is different from the state
of the parts in the bud; that they are in a condition of special
modification. But upon my life I am ashamed of myself to differ so much
from my betters on this head. The TEMPORARY theory (This view is now
generally accepted.) which I have formed on this class of dimorphism, just
to guide experiment, is that the PERFECT flowers can only be perfectly
fertilised by insects, and are in this case abundantly crossed; but that
the flowers are not always, especially in early spring, visited enough by
insects, and therefore the little imperfect self-fertilising flowers are
developed to ensure a sufficiency of seed for present generations. Viola
canina is sterile, when not visited by insects, but when so visited forms
plenty of seed. I infer from the structure of three or four forms of
Balsamineae, that these require insects; at least there is almost as plain
adaptation to insects as in the Orchids. I have Oxalis acetosella ready in
pots for experiment next spring; and I fear this will upset my little
theory...Campanula carpathica, as I found this summer, is absolutely
sterile if insects are excluded. Specularia speculum is fairly fertile
when enclosed; and this seemed to me to be partially effected by the
frequent closing of the flower; the inward angular folds of the corolla
corresponding with the clefts of the open stigma, and in this action
pushing pollen from the outside of the stigma on to its surface. Now can
you tell me, does S. perfoliata close its flower like S. speculum, with
angular inward folds? if so, I am smashed without some fearful "wriggling."
Are the IMPERFECT flowers of your Specularia the early or the later ones?
very early or very late? It is rather pretty to see the importance of the
closing of flowers of S. speculum.
['Forms of Flowers' was published in July; in June, 1877, he wrote to
Professor Carus with regard to the translation:--
"My new book is not a long one, viz. 350 pages, chiefly of the larger type,
with fifteen simple woodcuts. All the proofs are corrected except the
Index, so that it will soon be published.
"...I do not suppose that I shall publish any more books, though perhaps a
few more papers. I cannot endure being idle, but heaven knows whether I am
capable of any more good work."
The review alluded to in the next letter is at page 445 of the volume of
'Nature' for 1878:]
CHARLES DARWIN TO W. THISELTON DYER.
Down, April 5, 1878.
My dear Dyer,
I have just read in 'Nature' the review of 'Forms of Flowers,' and I am
sure that it is by you. I wish with all my heart that it deserved one
quarter of the praises which you give it. Some of your remarks have
interested me greatly...Hearty thanks for your generous and most kind
sympathy, which does a man real good, when he is as dog-tired as I am at
this minute with working all day, so good-bye.
C. DARWIN.
