More Letters of Charles Darwin Volume II

C >> Charles Darwin >> More Letters of Charles Darwin Volume II

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49

P.S.--Do you happen to know, when there are only four stamens, whether it
is the petal or sepal-facers which are preserved? and whether in the four-
stamened forms the pistil is rectangularly bent or is straight?

LETTER 621. TO ASA GRAY.
Down, February 16th [1862?].

I have been trying a few experiments on Melastomads; and they seem to
indicate that the pollen of the two curious sets of anthers (i.e. the
petal-facers and the sepal-facers) have very different powers; and it does
not seem that the difference is connected with any tendency to abortion in
the one set. Now I think I can understand the structure of the flower and
means of fertilisation, if there be two forms,--one with the pistil bent
rectangularly out of the flower, and the other with it nearly straight.

Our hot-house and green-house plants have probably all descended by
cuttings from a single plant of each species; so I can make out nothing
from them. I applied in vain to Bentham and Hooker; but Oliver picked out
some sentences from Naudin, which seem to indicate differences in the
position of the pistil.

I see that Rhexia grows in Massachusetts; and I suppose has two different
sets of stamens. Now, if in your power, would you observe the position of
the pistil in different plants, in lately opened flowers of the same age?
(I specify this because in Monochaetum I find great changes of position in
the pistils and stamens, as flower gets old). Supposing that my prophecy
should turn out right, please observe whether in both forms the passage
into the flower is not [on] the upper side of the pistil, owing to the
basal part of the pistil lying close to the ring of filaments on the under
side of the flower. Also I should like to know the colour of the two sets
of anthers. This would take you only a few minutes, and is the only way I
see that I can find out whether these plants are dimorphic in this peculiar
way--i.e., only in the position of the pistil (621/1. In Exacum and in
Saintpaulia the flowers are dimorphic in this sense: the style projects to
either the right or the left side of the corolla, from which it follows
that a right-handed flower would fertilise a left-handed one, and vice
versa. See Willis, "Flowering Plants and Ferns," 1897, Volume I., page
73.) and in its relation to the two kinds of pollen. I am anxious about
this, because if it should prove so, it will show that all plants with
longer and shorter or otherwise different anthers will have to be examined
for dimorphism.

LETTER 622. TO ASA GRAY.
March 15th [1862].

...I wrote some little time ago about Rhexia; since then I have been
carefully watching and experimenting on another genus, Monochaetum; and I
find that the pistil is first bent rectangularly (as in the sketch sent),
and then in a few days becomes straight: the stamens also move. If there
be not two forms of Rhexia, will you compare the position of the part in
young and old flowers? I have a suspicion (perhaps it will be proved wrong
when the seed-capsules are ripe) that one set of anthers are adapted to the
pistil in early state, and the other set for it in its later state. If
bees visit the Rhexia, for Heaven's sake watch exactly how the anther and
stigma strike them, both in old and young flowers, and give me a sketch.

Again I say, do not hate me.

LETTER 623. TO J.D. HOOKER.
Leith Hill Place, Dorking, Thursday, 15th [May 1862].

You stated at the Linnean Society that different sets of seedling Cinchona
(623/1. Cinchona is apparently heterostyled: see "Forms of Flowers,"
Edition II., page 134.) grew at very different rate, and from my Primula
case you attributed it probably to two sorts of pollen. I confess I
thought you rash, but I now believe you were quite right. I find the
yellow and crimson anthers of the same flower in the Melastomatous
Heterocentron roseum have different powers; the yellow producing on the
same plant thrice as many seeds as the crimson anthers. I got my
neighbour's most skilful gardener to sow both kinds of seeds, and yesterday
he came to me and said it is a most extraordinary thing that though both
lots have been treated exactly alike, one lot all remain dwarfs and the
other lot are all rising high up. The dwarfs were produced by the pollen
of the crimson anthers. In Monochaetum ensiferum the facts are more
complex and still more strange; as the age and position of the pistils
comes into play, in relation to the two kinds of pollen. These facts seem
to me so curious that I do not scruple to ask you to see whether you can
lend me any Melastomad just before flowering, with a not very small flower,
and which will endure for a short time a greenhouse or sitting-room; when
fertilised and watered I could send it to Mr. Turnbull's to a cool stove to
mature seed. I fully believe the case is worth investigation.

P.S.--You will not have time at present to read my orchid book; I never
before felt half so doubtful about anything which I published. When you
read it, do not fear "punishing" me if I deserve it.

Adios. I am come here to rest, which I much want.

Whenever you have occasion to write, pray tell me whether you have
Rhododendron Boothii from Bhootan, with a smallish yellow flower, and
pistil bent the wrong way; if so, I would ask Oliver to look for nectary,
for it is an abominable error of Nature that must be corrected. I could
hardly believe my eyes when I saw the pistil.

LETTER 624. TO ASA GRAY.
January 19th [1863].

I have been at those confounded Melastomads again; throwing good money
(i.e. time) after bad. Do you remember telling me you could see no nectar
in your Rhexia? well, I can find none in Monochaetum, and Bates tells me
that the flowers are in the most marked manner neglected by bees and
lepidoptera in Amazonia. Now the curious projections or horns to the
stamens of Monochaetum are full of fluid, and the suspicion occurs to me
that diptera or small hymenoptera may puncture these horns like they
puncture (proved since my orchid book was published) the dry nectaries of
true Orchis. I forget whether Rhexia is common; but I very much wish you
would next summer watch on a warm day a group of flowers, and see whether
they are visited by small insects, and what they do.

LETTER 625. TO I.A. HENRY.
Down, January 20th [1863].

...You must kindly permit me to mention any point on which I want
information. If you are so inclined, I am curious to know from systematic
experiments whether Mr. D. Beaton's statement that the pollen of two
shortest anthers of scarlet Pelargonium produce dwarf plants (625/1. See
"Animals and Plants," Edition II., Volume II., page 150, for a brief
account of Darwin's experiments on this genus. Also loc. cit., page 338
(note), for a suggested experiment.), in comparison with plants produced
from the same mother-plant by the pollen of longer stamens from the same
flower. It would aid me much in some laborious experiments on Melastomads.
I confess I feel a little doubtful; at least, I feel pretty nearly sure
that I know the meaning of short stamens in most plants. This summer (for
another object) I crossed Queen of Scarlet Pelargonium with pollen of long
and short stamens of multiflora alba, and it so turns out that plants from
short stamens are the tallest; but I believe this to have been mere chance.
My few crosses in Pelargonium were made to get seed from the central
peloric or regular flower (I have got one from peloric flower by pollen of
peloric), and this leads me to suggest that it would be very interesting to
test fertility of peloric flowers in three ways,--own peloric pollen on
peloric stigma, common pollen on peloric stigma, peloric pollen on common
stigma of same species. My object is to discover whether with change of
structure of flower there is any change in fertility of pollen or of female
organs. This might also be tested by trying peloric and common pollen on
stigma of a distinct species, and conversely. I believe there is a peloric
and common variety of Tropaeolum, and a peloric or upright and common
variation of some species of Gloxinia, and the medial peloric flowers of
Pelargonium, and probably others unknown to me.

LETTER 626. TO I.A. HENRY.
Hartfield, May 2nd [1863].

In scarlet dwarf Pelargonium, you will find occasionally an additional and
abnormal stamen on opposite and lower side of flower. Now the pollen of
this one occasional short stamen, I think, very likely would produce dwarf
plants. If you experiment on Pelargonium I would suggest your looking out
for this single stamen.

I observed fluctuations in length of pistil in Phloxes, but thought it was
mere variability.

If you could raise a bed of seedling Phloxes of any species except P.
Drummondii, it would be highly desirable to see if two forms are presented,
and I should be very grateful for information and flowers for inspection.
I cannot remember, but I know that I had some reason to look after Phloxes.
(626/1. See "Forms of Flowers," Edition II., page 119, where the
conjecture is hazarded that Phlox subulata shows traces of a former
heterostyled condition.)

I do not know whether you have used microscopes much yet. It adds
immensely to interest of all such work as ours, and is indeed indispensable
for much work. Experience, however, has fully convinced me that the use of
the compound without the simple microscope is absolutely injurious to
progress of N[atural] History (excepting, of course, with Infusoria). I
have, as yet, found no exception to the rule, that when a man has told me
he works with the compound alone his work is valueless.

LETTER 627. TO ASA GRAY.
March 20th [1863].

I wrote to him [Dr. H. Cruger, of Trinidad] to ask him to observe what the
insects did in the flowers of Melastomaceae: he says not proper season
yet, but that on one species a small bee seemed busy about the horn-like
appendages to the anthers. It will be too good luck if my study of the
flowers in the greenhouse has led me to right interpretation of these
appendages.

LETTER 628. TO J.D. HOOKER.
Down, November 28th [1871].

If you had come here on Sunday I should have asked you whether you could
give me seed or seedlings of any Melastomad which would flower soon to
experiment on! I wrote also to J. Scott to ask if he could give me seed.

Several years ago I raised a lot of seedlings of a Melastomad greenhouse
bush (Monochaetus or some such name) (628/1. Monochaetum.) from stigmas
fertilised separately by the two kinds of pollen, and the seedlings
differed remarkably in size, and whilst young, in appearance; and I never
knew what to think of the case (so you must not use it), and have always
wished to try again, but they are troublesome beasts to fertilise.

On the other hand I could detect no difference in the product from the two
coloured anthers of Clarkia. (628/2. Clarkia has eight stamens divided
into two groups which differ in the colour of the anthers.) If you want to
know further particulars of my experiments on Monochaetum (?) and Clarkia,
I will hunt for my notes. You ask about difference in pollen in the same
species. All dimorphic and trimorphic plants present such difference in
function and in size. Lythrum and the trimorphic Oxalis are the most
wonderful cases. The pollen of the closed imperfect cleistogamic flowers
differ in the transparency of the integument, and I think in size. The
latter point I could ascertain from my notes. The pollen or female organs
must differ in almost every individual in some manner; otherwise the pollen
of varieties and even distinct individuals of same varieties would not be
so prepotent over the individual plant's own pollen. Here follows a case
of individual differences in function of pollen or ovules or both. Some
few individuals of Reseda odorata and R. lutea cannot be fertilised, or
only very rarely, by pollen of the same plant, but can by pollen of any
other individual. I chanced to have two plants of R. odorata in this
state; so I crossed them and raised five seedlings, all of which were self
sterile and all perfectly fertile with pollen of any other individual
mignonette. So I made a self sterile race! I do not know whether these
are the kinds of facts which you require.

Think whether you can help me to seed or better seedlings (not cuttings) of
any Melastomad.

LETTER 629. TO F. MULLER.
Down, March 20th, 1881.

I have received the seeds and your most interesting letter of February 7th.
The seeds shall be sown, and I shall like to see the plants sleeping; but I
doubt whether I shall make any more detailed observations on this subject,
as, now that I feel very old, I require the stimulus of some novelty to
make me work. This stimulus you have amply given me in your remarkable
view of the meaning of the two-coloured stamens in many flowers. I was so
much struck with this fact with Lythrum, that I began experimenting on some
Melastomaceae, which have two sets of extremely differently coloured
anthers. After reading your letter I turned to my notes (made 20 years
ago!) to see whether they would support or contradict your suggestion. I
cannot tell yet, but I have come across one very remarkable result, that
seedlings from the crimson anthers were not 11/20ths of the size of
seedlings from the yellow anthers of the same flowers. Fewer good seeds
were produced by the crimson pollen. I concluded that the shorter stamens
were aborting, and that the pollen was not good. (629/1. "Shorter stamens"
seems to be a slip of the pen for "longer,"--unless the observations were
made on some genus in which the structure is unusual.) The mature pollen
is incoherent, and must be [word illegible] against the visiting insect's
body. I remembered this, and I find it said in my EARLY notes that bees
would never visit the flowers for pollen. This made me afterwards write to
the late Dr. Cruger in the West Indies, and he observed for me the flowers,
and saw bees pressing the anthers with their mandibles from the base
upwards, and this forced a worm-like thread of pollen from the terminal
pore, and this pollen the bees collected with their hind legs. So that the
Melastomads are not opposed to your views.

I am now working on the habits of worms, and it tires me much to change my
subject; so I will lay on one side your letter and my notes, until I have a
week's leisure, and will then see whether my facts bear on your views. I
will then send a letter to "Nature" or to the Linn. Soc., with the extract
of your letter (and this ought to appear in any case), with my own
observations, if they appear worth publishing. The subject had gone out of
my mind, but I now remember thinking that the imperfect action of the
crimson stamens might throw light on hybridism. If this pollen is
developed, according to your view, for the sake of attracting insects, it
might act imperfectly, as well as if the stamens were becoming rudimentary.
(629/2. As far as it is possible to understand the earlier letters it
seems that the pollen of the shorter stamens, which are adapted for
attracting insects, is the most effective.) I do not know whether I have
made myself intelligible.

LETTER 630. TO W. THISELTON-DYER.
Down, March 21st [1881].

I have had a letter from Fritz Muller suggesting a novel and very curious
explanation of certain plants producing two sets of anthers of different
colour. This has set me on fire to renew the laborious experiments which I
made on this subject, now 20 years ago. Now, will you be so kind as to
turn in your much worked and much holding head, whether you can think of
any plants, especially annuals, producing 2 such sets of anthers. I
believe that this is the case with Clarkia elegans, and I have just written
to Thompson for seeds. The Lythraceae must be excluded, as these are
heterostyled.

I have got seeds from Dr. King of some Melastomaceae, and will write to
Veitch to see if I can get the Melastomaceous genera Monochaetum and
Heterocentron or some such name, on which I before experimented. Now, if
you can aid me, I know that you will; but if you cannot, do not write and
trouble yourself.

2.X.III. CORRESPONDENCE WITH JOHN SCOTT, 1862-1871.

"If he had leisure he would make a wonderful observer, to my judgment; I
have come across no one like him."--Letter to J.D. Hooker, May 29th [1863].

(631/1. The following group of letters to John Scott, of whom some account
is given elsewhere (Volume I., Letters 150 and 151, and Index.) deal
chiefly with experimental work in the fertilisation of flowers. In
addition to their scientific importance, several of the letters are of
special interest as illustrating the encouragement and friendly assistance
which Darwin gave to his correspondent.)

LETTER 631. JOHN SCOTT TO CHARLES DARWIN.
Edinburgh Botanic Gardens, November 11th, 1862.

I take the liberty of addressing you for the purpose of directing your
attention to an error in one of your ingenious explanations of the
structural adaptations of the Orchidaceae in your late work. This occurs
in the genus Acropera, two species of which you assume to be unisexual, and
so far as known represented by male individuals only. Theoretically you
have no doubt assigned good grounds for this view; nevertheless,
experimental observations that I am now making have already convinced me of
its fallacy. And I thus hurriedly, and as you may think prematurely, direct
your attention to it, before I have seen the final result of my own
experiment, that you might have the longer time for reconsidering the
structure of this genus for another edition of your interesting book, if
indeed it be not already called for. I am furthermore induced to
communicate the results of my yet imperfect experiments in the belief that
the actuating principle of your late work is the elicitation of truth, and
that you will gladly avail yourself of this even at the sacrifice of much
ingenious theoretical argumentation.

Since I have had an opportunity of perusing your work on orchid
fertilisation, my attention has been particularly directed to the curiously
constructed floral organs of Acropera. I unfortunately have as yet had
only a few flowers for experimental enquiry, otherwise my remarks might
have been clearer and more satisfactory. Such as they are, however, I
respectfully lay [them] before you, with a full assurance of their
veracity, and I sincerely trust that as such you will receive them.

Your observations seem to have been chiefly directed to the A. luteola,
mine to the A. Loddigesii, which, however, as you remark, is in a very
similar constructural condition with the former; having the same narrow
stigmatic chamber, abnormally developed placenta, etc. In regard to the
former point--contraction of stigmatic chamber--I may remark that it does
not appear to be absolutely necessary that the pollen-masses penetrate this
chamber for effecting fecundation. Thus a raceme was produced upon a plant
of A. Loddigesii in the Botanic Gardens here lately; upon this I left only
six flowers. These I attempted to fertilise, but with two only of the six
have I been successful: I succeeded in forcing a single pollen-mass into
the stigmatic chamber of one of the latter, but I failed to do this on the
other; however, by inserting a portion of a pedicel with a pollinium
attached, I caused the latter to adhere, with a gentle press, to the mouth
of the stigmatic chamber. Both of these, as I have already remarked, are
nevertheless fertilised; one of them I have cut off for examination, and
its condition I will presently describe; the other is still upon the plant,
and promises fair to attain maturity. In regard to the other four flowers,
I may remark that though similarly fertilised--part having pollinia
inserted, others merely attached--they all withered and dropped off without
the least swelling of the ovary. Can it be, then, that this is really an
[andro-monoecious] species?--part of the flowers male, others truly
hermaphrodite.

In making longitudinal sections of the fertilised ovary before mentioned, I
found the basal portion entirely destitute of ovules, their place being
substituted by transparent cellular ramification of the placentae. As I
traced the placentae upwards, the ovules appeared, becoming gradually more
abundant towards its apex. A transverse section near the apex of the
ovary, however, still exhibited a more than ordinary placental
development--i.e. [congenitally?] considered--each end giving off two
branches, which meet each other in the centre of the ovary, the ovules
being irregularly and sparingly disposed upon their surfaces.

In regard to the mere question of fertilisation, then, I am perfectly
satisfied, but there are other points which require further elucidation.
Among these I may particularly refer to the contracted stigmatic chamber,
and the slight viscidity of its disk. The latter, however, may be a
consequence of uncongenial conditions--as you do not mention particularly
its examination by any author in its natural habitat. If such be the case,
the contracted stigmatic chamber will offer no real difficulty, should the
viscous exudations be only sufficient to render the mouth adhesive. For,
as I have already shown, the pollen-tubes may be emitted in this condition,
and effect fecundation without being in actual contact with the stigmatic
surface, as occurs pretty regularly in the fertilisation of the Stapelias,
for example. But, indeed, your own discovery of the independent
germinative capabilities of the pollen-grains of certain Orchidaceae is
sufficiently illustrative of this.

I may also refer to the peculiar abnormal condition that many at least of
the ovaries present in a comparative examination of the placentae, and of
which I beg to suggest the following explanation, though it is as yet
founded on limited observations. In examining certain young ovaries of A.
Loddigesii, I found some of them filled with the transparent membranous
fringes of more or less distinctly cellular matter, which, from your
description of the ovaries of luteola, appears to differ simply in the
greater development in the former species. Again, in others I found small
mammillary bodies, which appeared to be true ovules, though I could not
perfectly satisfy myself as to the existence of the micropyle or nucleus.
I unfortunately neglected to apply any chemical test. The fact, however,
that in certain of the examined ovaries few or none of the latter bodies
occurred--the placenta alone being developed in an irregular membranous
form, taken in conjunction with the results of my experiments--before
alluded to--on their fertilisation, leads me to infer that two sexual
conditions are presented by the flowers of this plant. In short, that many
of the ovaries are now normally abortive, though Nature occasionally makes
futile efforts for their perfect development, in the production of ovuloid
bodies; these then I regard as the male flowers. The others that are still
capable of fertilisation, and likewise possessing male organs, are
hermaphrodite, and must, I think, from the results of your comparative
examinations, present a somewhat different condition; as it can scarcely be
supposed that ovules in the condition you describe could ever be
fertilised.

This is at least the most plausible explanation I can offer for the
different results in my experiments on the fertilisation of apparently
similar morphologically constructed flowers; others may, however, occur to
you. Here there is not, as in the Catasetum, any external change visible
in the respective unisexual and bisexual flowers. And yet it would appear
from your researches that the ovules of Acropera are in a more highly
atrophied condition than occurs in Catasetum, though, as you likewise
remark, M. Neumann has never succeeded in fertilising C. tridentatum. If
there be not, then, an arrangement of the reproductive structures, such as
I have indicated, how can the different results in M. Neumann's experiments
and mine be accounted for? However, as you have examined many flowers of
both A. luteola and Loddigesii, such a difference in the ovulary or
placental structures could scarcely have escaped your observation. But, be
this as it may, the--to me at least--demonstrated fact still remains, that
certain flowers of A. Loddigesii are capable of fertilisation, and that,
though there are good grounds for supposing that important physiological
changes are going on in the sexual phenomena of this species, there is no
evidence whatever for supposing that external morphological changes have so
masked certain individuals as to prevent their recognition.

I would now, sir, in conclusion beg you to excuse me for this infringement
upon your valuable time, as I have been induced to write you in the belief
that you have had negative results from other experimenters, before you
ventured to propose your theoretical explanation, and consequently that you
have been unknowingly led into error. I will continue, as opportunities
present themselves, to examine the many peculiarities you have pointed out
in this as well as others of the Orchid family; and at present I am looking
forward with anxiety for the maturation of the ovary of A. Loddigesii,
which will bear testimony to the veracity of the remarks I have ventured to
lay before you.