More Letters of Charles Darwin Volume II

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Mr. Bentham wrote a paper (590/5. "Linn. Soc. Journal," 1869, page 203.)
on the style and stigma in the Goodenovieae, where he speaks of Mr.
Darwin's belief that fertilisation takes place outside the indusium. This
statement, which we imagine Mr. Bentham must have had from an unpublished
source, was incomprehensible to him as long as he confined his work to such
genera as Goodenia, Scaevola, Velleia, Coelogyne, in which the mechanism is
much as above described; but on examining Leschenaultia the meaning became
clear. Bentham writes of this genus:--"The indusium is usually described
as broadly two-lipped, without any distinct stigma. The fact appears to be
that the upper less prominent lip is stigmatic all over, inside and out,
with a transverse band of short glandular hairs at its base outside, while
the lower more prominent lip is smooth and glabrous, or with a tuft of
rigid hairs. Perhaps this lower lip and the upper band of hairs are all
that correspond to the indusium of other genera; and the so-called upper
lip, outside of which impregnation may well take place, as observed by Mr.
Darwin, must be regarded as the true stigma."

Darwin's interest in the Goodeniaceae was due to the mechanism being
apparently fitted for self-fertilisation. In 1871 a writer signing himself
F.W.B. made a communication to the "Gardeners' Chronicle" (590/6. 1871,
page 1103.), in which he expresses himself as "agreeably surprised" to find
Leschenaultia adapted for self-fertilisation, or at least for
self-pollinisation. This led Darwin to publish a short note in the same
journal, in which he describes the penetration of pollen-tubes into the
viscid surface on the outside of the indusium. (590/7. 1871, page 1166.
He had previously written in the "Journal of Horticulture and Cottage
Gardener," May 28th, 1861, page 151:--"Leschenaultia formosa has apparently
the most effective contrivance to prevent the stigma of one flower ever
receiving a grain of pollen from another flower; for the pollen is shed in
the early bud, and is there shut up round the stigma within a cup or
indusium. But some observations led me to suspect that nevertheless insect
agency here comes into play; for I found by holding a camel-hair pencil
parallel to the pistil, and moving it as if it were a bee going to suck the
nectar, the straggling hairs of the brush opened the lip of the indusium,
entered it, stirred up the pollen, and brought out some grains. I did this
to five flowers, and marked them. These five flowers all set pods; whereas
only two other pods set on the whole plant, though covered with innumerable
flowers...I wrote to Mr. James Drummond, at Swan River in Australia,...and
he soon wrote to me that he had seen a bee cleverly opening the indusium
and extracting pollen.") He also describes how a brush, pushed into the
flower in imitation of an insect, presses "against the slightly projecting
lower lip of the indusium, opens it, and some of the hairs enter and become
smeared with pollen." The yield of pollen is therefore differently
arranged in Leschenaultia; for in the more typical genera it depends on the
growth of the style inside the indusium. Delpino, however (see
Hildebrand's version, loc. cit.), describes a similar opening of the cup
produced by pressure on the hairs in some genera of the order.)

Down, June 7th [1860].

Best and most beloved of men, I supplicate and entreat you to observe one
point for me. Remember that the Goodeniaceae have weighed like an incubus
for years on my soul. It relates to Scaevola microcarpa. I find that in
bud the indusium collects all the pollen splendidly, but, differently from
Leschenaultia, cannot be afterwards easily opened. Further, I find that at
an early stage, when the flower first opens, a boat-shaped stigma lies at
the bottom of the indusium, and further that this stigma, after the flower
has some time expanded, grows very rapidly, when the plant is kept hot, and
pushes out of the indusium a mass of pollen; and at same time two horns
project at the corners of the indusium. Now the appearance of these horns
makes me suppose that these are the stigmatic surfaces. Will you look to
this? for if they be by the relative position of the parts (with indusium
and stigma bent at right angles to style) [I am led to think] that an
insect entering a flower could not fail to have [its] whole back (at the
period when, as I have seen, a whole mass of pollen is pushed out) covered
with pollen, which would almost certainly get rubbed on the two horns.
Indeed, I doubt whether, without this aid, pollen would get on to the
horns. What interests me in the case is the analogy in result with the
Lobelia, but by very different means. In Lobelia the stigma, before it is
mature, pushes by its circular brush of hairs the pollen out of the
conjoined anthers; here the indusium collects pollen, and then the growth
of the stigma pushes it out. In the course of about 1 1/2 hour, I found an
indusium with hairs on the outer edge perfectly clogged with pollen, and
horns protruded, which before the 1 1/2 hour had not one grain of pollen
outside the indusium, and no trace of protruding horns. So you will see
how I wish to know whether the horns are the true stigmatic surfaces. I
would try the case experimentally by putting pollen on the horns, but my
greenhouse is so cold, and my plant so small, and in such a little pot,
that I suppose it would not seed...

The little length of stigmatic horns at the moment when pollen is forced
out of the indusium, compared to what they ultimately attain, makes me
fancy that they are not then mature or ready, and if so, as in Lobelia,
each flower must be fertilised by pollen from another and earlier flower.

How curious that the indusium should first so cleverly collect pollen and
then afterwards push it out! Yet how closely analogous to Campanula
brushing pollen out of the anther and retaining it on hairs till the stigma
is ready. I am going to try whether Campanula sets seed without insect
agency.

LETTER 591. TO J.D. HOOKER.

(591/1. The following letters are given here rather than in chronological
order, as bearing on the Leschenaultia problem. The latter part of Letter
591 refers to the cleistogamic flowers of Viola.)

Down, May 1st [1862].

If you can screw out time, do look at the stigma of the blue Leschenaultia
biloba. I have just examined a large bud with the indusium not yet closed,
and it seems to me certain that there is no stigma within. The case would
be very important for me, and I do not like to trust solely to myself. I
have been impregnating flowers, but it is rather difficult...

I have just looked again at Viola canina. The case is odder: only 2
stamens which embrace the stigma have pollen; the 3 other stamens have no
anther-cells and no pollen. These 2 fertile anthers are of different shape
from the 3 sterile others, and the scale representing the lower lip is
larger and differently shaped from the 4 other scales representing 4 other
petals.

In V. odorata (single flower) all five stamens produce pollen. But I
daresay all this is known.

LETTER 592. TO J.D. HOOKER.
November 3rd [1862].

Do you remember the scarlet Leschenaultia formosa with the sticky margin
outside the indusium? Well, this is the stigma--at least, I find the
pollen-tubes here penetrate and nowhere else. What a joke it would be if
the stigma is always exterior, and this by far the greatest difficulty in
my crossing notions should turn out a case eminently requiring insect aid,
and consequently almost inevitably ensuring crossing. By the way, have you
any other Goodeniaceae which you could lend me, besides Leschenaultia and
Scaevola, of which I have seen enough?

I had a long letter the other day from Crocker of Chichester; he has the
real spirit of an experimentalist, but has not done much this summer.

LETTER 593. TO F. MULLER.
Down, April 9th and 15th [1866].

I am very much obliged by your letter of February 13th, abounding with so
many highly interesting facts. Your account of the Rubiaceous plant is one
of the most extraordinary that I have ever read, and I am glad you are
going to publish it. I have long wished some one to observe the
fertilisation of Scaevola, and you must permit me to tell you what I have
observed. First, for the allied genus of Leschenaultia: utterly
disbelieving that it fertilises itself, I introduced a camel-hair brush
into the flower in the same way as a bee would enter, and I found that the
flowers were thus fertilised, which never otherwise happens; I then
searched for the stigma, and found it outside the indusium with the pollen-
tubes penetrating it; and I convinced Dr. Hooker that botanists were quite
wrong in supposing that the stigma lay inside the indusium. In Scaevola
microcarpa the structure is very different, for the immature stigma lies at
the base within the indusium, and as the stigma grows it pushes the pollen
out of the indusium, and it then clings to the hairs which fringe the tips
of the indusium; and when an insect enters the flower, the pollen (as I
have seen) is swept from these long hairs on to the insect's back. The
stigma continues to grow, but is not apparently ready for impregnation
until it is developed into two long protruding horns, at which period all
the pollen has been pushed out of the indusium. But my observations are
here at fault, for I did not observe the penetration of the pollen-tubes.
The case is almost parallel with that of Lobelia. Now, I hope you will get
two plants of Scaevola, and protect one from insects, leaving the other
uncovered, and observe the results, both in the number of capsules
produced, and in the average number of seeds in each. It would be well to
fertilise half a dozen flowers under the net, to prove that the cover is
not injurious to fertility.

With respect to your case of Aristolochia, I think further observation
would convince you that it is not fertilised only by larvae, for in a
nearly parallel case of an Arum and a Aristolochia, I found that insects
flew from flower to flower. I would suggest to you to observe any cases of
flowers which catch insects by their probosces, as occurs with some of the
Apocyneae (593/1. Probably Asclepiadeae. See H. Muller, "Fertilisation of
Flowers," page 396.); I have never been able to conceive for what purpose
(if any) this is effected; at the same time, if I tempt you to neglect your
zoological work for these miscellaneous observations I shall be guilty of a
great crime.

To return for a moment to the indusium: how curious it is that the pollen
should be thus collected in a special receptacle, afterwards to be swept
out by insects' agency!

I am surprised at what you tell me about the fewness of the flowers of your
native orchids which produce seed-capsules. What a contrast with our
temperate European species, with the exception of some species of Ophrys!--
I now know of three or four cases of self-fertilising orchids, but all
these are provided with means for an occasional cross.

I am sorry to say Dr. Cruger is dead from a fever.

I received yesterday your paper in the "Botanische Zeitung" on the wood of
climbing plants. (593/2. Fritz Muller, "Ueber das Holz einiger um
Desterro wachsenden Kletterpflanzen." "Botanische Zeitung," 1866, pages
57, 65.) I have read as yet only your very interesting and curious remarks
on the subject as bearing on the change of species; you have pleased me by
the very high compliments which you pay to my paper. I have been at work
since March 1st on a new English edition (593/3. The 4th Edition.) of my
"Origin," of which when published I will send you a copy. I have much
regretted the time it has cost me, as it has stopped my other work. On the
other hand, it will be useful for a new third German edition, which is now
wanted. I have corrected it largely, and added some discussions, but not
nearly so much as I wished to do, for, being able to work only two hours
daily, I feared I should never get it finished. I have taken some facts
and views from your work "Fur Darwin"; but not one quarter of what I should
like to have quoted.

LETTER 594. TO A.G. MORE.
Down, June 24th, 1860.

I hope that you will forgive the liberty which I take in writing to you and
requesting a favour. Mr. H.C. Watson has given me your address, and has
told me that he thought that you would be willing to oblige me. Will you
please to read the enclosed, and then you will understand what I wish
observed with respect to the bee-orchis. (594/1. Ophrys apifera.) What I
especially wish, from information which I have received since publishing
the enclosed, is that the state of the pollen-masses should be noted in
flowers just beginning to wither, in a district where the bee-orchis is
extremely common. I have been assured that in parts of Isle of Wight,
viz., Freshwater Gate, numbers occur almost crowded together: whether
anything of this kind occurs in your vicinity I know not; but, if in your
power, I should be infinitely obliged for any information. As I am
writing, I will venture to mention another wish which I have: namely, to
examine fresh flowers and buds of the Aceras, Spiranthes, marsh Epipactis,
and any other rare orchis. The point which I wish to examine is really
very curious, but it would take too long space to explain. Could you
oblige me by taking the great trouble to send me in an old tin canister any
of these orchids, permitting me, of course, to repay postage? It would be
a great kindness, but perhaps I am unreasonable to make such a request. If
you will inform me whether you have leisure so far to oblige me, I would
tell you my movements, for on account of my own health and that of my
daughter, I shall be on the move for the next two or three weeks.

I am sure I have much cause to apologise for the liberty which I have
taken...

LETTER 595. TO A.G. MORE.
Down, August 3rd, 1860.

I thank you most sincerely for sending me the Epipactis [palustris]. You
can hardly imagine what an interesting morning's work you have given me, as
the rostellum exhibited a quite new modification of structure. It has been
extremely kind of you to take so very much trouble for me. Have you looked
at the pollen-masses of the bee-Ophrys? I do not know whether the
Epipactis grows near to your house: if it does, and any object takes you
to the place (pray do not for a moment think me so very unreasonable as to
ask you to go on purpose), would you be so kind [as] to watch the flowers
for a quarter of an hour, and mark whether any insects (and what?) visit
these flowers.

I should suppose they would crawl in by depressing the terminal portion of
the labellum; and that when within the flower this terminal portion would
resume its former position; and lastly, that the insect in crawling out
would not depress the labellum, but would crawl out at back of flower.
(595/1. The observations of Mr. William Darwin on Epipactis palustris
given in the "Fertilisation of Orchids," Edition II., 1877, page 99, bear
on this point. The chief fertilisers are hive-bees, which are too big to
crawl into the flower. They cling to the labellum, and by depressing it
open up the entrance to the flower. Owing to the elasticity of the
labellum and its consequent tendency to spring up when released, the bees,
"as they left the flower, seemed to fly rather upwards." This agrees with
Darwin's conception of the mechanism of the flower as given in the first
edition of the Orchid book, 1862, page 100, although at that time he
imagined that the fertilising insect crawled into the flower. The extreme
flexibility and elasticity of the labellum was first observed by Mr. More
(see first edition, page 99). The description of the flower given in the
above letter to Mr. More is not quite clear; the reader is referred to the
"Fertilisation of Orchids," loc. cit.) An insect crawling out of a
recently opened flower would, I believe, have parts of the pollen-masses
adhering to the back or shoulder. I have seen this in Listera. How I
should like to watch the Epipactis.

If you can it any time send me Spiranthes or Aceras or O. ustulata, you
would complete your work of kindness.

P.S.--If you should visit the Epipactis again, would you gather a few of
the lower flowers which have been opened for some time and have begun to
wither a little, and observe whether pollen is well cleared out of anther-
case. I have been struck with surprise that in nearly all the lower
flowers sent by you, though much of the pollen has been removed, yet a good
deal of pollen is left wasted within the anthers. I observed something of
this kind in Cephalanthera grandiflora. But I fear that you will think me
an intolerable bore.

LETTER 596. TO A.G. MORE.
Down, August 5th, 1860.

I am infinitely obliged for your most clearly stated observations on the
bee-orchis. It is now perfectly clear that something removes the pollen-
masses far more with you than in this neighbourhood. But I am utterly
puzzled about the foot-stalk being so often cut through. I should suspect
snails. I yesterday found thirty-nine flowers, and of them only one
pollen-mass in three flowers had been removed, and as these were extremely
much-withered flowers I am not quite sure of the truth of this. The wind
again is a new element of doubt. Your observations will aid me extremely
in coming to some conclusion. (596/1. Mr. More's observations on the
percentage of flowers in which the pollinia were absent are quoted in
"Fertilisation of Orchids," Edition I., page 68.) I hope in a day or two
to receive some day-moths, on the probosces of which I am assured the
pollen-masses of the bee-orchis still adhere (596/2. He was doomed to
disappointment. On July 17th, 1861, he wrote to Mr. More:--"I found the
other day a lot of bee-Ophrys with the glands of the pollinia all in their
pouches. All facts point clearly to eternal self-fertilisation in this
species; yet I cannot swallow the bitter pill. Have you looked at any this
year?")...

I wrote yesterday to thank you for the Epipactis. For the chance of your
liking to look at what I have found: take a recently opened flower, drag
gently up the stigmatic surface almost any object (the side of a hooked
needle), and you will find the cap of the hemispherical rostellum comes off
with a touch, and being viscid on under-surface, clings to needle, and as
pollen-masses are already attached to the back of rostellum, the needle
drags out much pollen. But to do this, the curiously projecting and fleshy
summits of anther-cases must at some time be pushed back slightly. Now
when an insect's head gets into the flower, when the flap of the labellum
has closed by its elasticity, the insect would naturally creep out by the
back-side of the flower. And mark when the insect flies to another flower
with the pollen-masses adhering to it, if the flap of labellum did not
easily open and allow free ingress to the insect, it would surely rub off
the pollen on the upper petals, and so not leave it on stigma. It is to
know whether I have rightly interpreted the structure of this whole flower
that I am so curious to see how insects act. Small insects, I daresay,
would crawl in and out and do nothing. I hope that I shall not have
wearied you with these details.

If you would like to see a pretty and curious little sight, look to Orchis
pyramidalis, and you will see that the sticky glands are congenitally
united into a saddle-shaped organ. Remove this under microscope by pincers
applied to foot-stalk of pollen-mass, and look quickly at the spontaneous
movement of the saddle-shaped organs and see how beautifully adapted to
seize proboscis of moth.

LETTER 597. TO J.D. HOOKER
December 4th [1860].

Many thanks about Apocynum and Meyen.

The latter I want about some strange movements in cells of Drosera, which
Meyen alone seems to have observed. (597/1. No observations of Meyen are
mentioned in "Insectivorous Plants.) It is very curious, but Trecul
disbelieves that Drosera really clasps flies! I should very much wish to
talk over Drosera with you. I did chloroform it, and the leaves which were
already expanded did not recover thirty seconds of exposure for three days.
I used the expression weight for the bit of hair which caused movement and
weighed 1/78000 of a grain; but I do not believe it is weight, and what it
is, I cannot after many experiments conjecture. (597/2. The doubt here
expressed as to whether the result is due to actual weight is interesting
in connection with Pfeffer's remarkable discovery that a smooth object in
contact with the gland produces no effect if the plant is protected from
all vibration; on an ordinary table the slight shaking which reaches the
plant is sufficient to make the body resting on the gland tremble, and thus
produce a series of varying pressures--under these circumstances the gland
is irritated, and the tentacle moves. See Pfeffer, "Untersuchungen aus d.
bot. Institut zu Tubingen," Volume I., 1885, page 483; also "Insectivorous
Plants," Edition II., page 22.) The movement in this case does not depend
on the chemical nature of substance. Latterly I have tried experiments on
single glands, and a microscopical atom of raw meat causes such rapid
movement that I could see it move like hand of clock. In this case it is
the nature of the object. It is wonderful the rapidity of the absorption:
in ten seconds weak solution of carbonate of ammonia changes not the
colour, but the state of contents within the glands. In two minutes thirty
seconds juice of meat has been absorbed by gland and passed from cell to
cell all down the pedicel (or hair) of the gland, and caused the sap to
pass from the cells on the upper side of the pedicel to the lower side, and
this causes the curvature of the pedicel. I shall work away next summer
when Drosera opens again, for I am much interested in subject. After the
glandular hairs have curved, the oddest changes take place--viz., a
segregation of the homogeneous pink fluid and necessary slow movements in
the thicker matter. By Jove, I sometimes think Drosera is a disguised
animal! You know that I always so like telling you what I do, that you
must forgive me scribbling on my beloved Drosera. Farewell. I am so very
glad that you are going to reform your ways; I am sure that you would have
injured your health seriously. There is poor Dana has done actually
nothing--cannot even write a letter--for a year, and it is hoped that in
another YEAR he may quite recover.

After this homily, good night, my dear friend. Good heavens, I ought not
to scold you, but thank you, for writing so long and interesting a letter.

LETTER 598. TO E. CRESY.
Down, December 12th [1860?].

After writing out the greater part of my paper on Drosera, I thought of so
many points to try, and I wished to re-test the basis of one large set of
experiments, namely, to feel still more sure than I am, that a drop of
plain water never produces any effect, that I have resolved to publish
nothing this year. For I found in the record of my daily experiments one
suspicious case. I must wait till next summer. It will be difficult to
try any solid substances containing nitrogen, such as ivory; for two quite
distinct causes excite the movement, namely, mechanical irritation and
presence of nitrogen. When a solid substance is placed on leaf it becomes
clasped, but is released sooner than when a nitrogenous solid is clasped;
yet it is difficult (except with raw meat and flies) to be sure of the
result, owing to differences in vigour of different plants. The last
experiments which I tried before my plants became too languid are very
curious, and were tried by putting microscopical atoms on the gland itself
of single hairs; and it is perfectly evident that an atom of human hair,
1/76000 of a grain (as ascertained by weighing a length of hair) in weight,
causes conspicuous movement. I do not believe (for atoms of cotton thread
acted) it is the chemical nature; and some reasons make me doubt whether it
is actual weight; it is not the shadow; and I am at present, after many
experiments, confounded to know what the cause is. That these atoms did
really act and alter the state of the contents of all the cells in the
glandular hair, which moved, was perfectly clear. But I hope next summer
to make out a good deal more...

LETTER 599. TO J.D. HOOKER.
Down, May 14th [1861].

I have been putting off writing from day to day, as I did not wish to
trouble you, till my wish for a little news will not let me rest...

I have no news to tell you, for I have had no interesting letters for some
time, and have not seen a soul. I have been going through the "Cottage
Gardener" of last year, on account chiefly of Beaton's articles (599/1.
Beaton was a regular contributor to the "Cottage Gardener," and wrote
various articles on cross breeding, etc., in 1861. One of these was in
reply to a letter published in the "Cottage Gardener," May 14th, 1861, page
112, in which Darwin asked for information as to the Compositae and the
hollyhock being crossed by insect visitors. In the number for June 8th,
1861, page 211, Darwin wrote on the variability of the central flower of
the carrot and the peloria of the central flower in Pelargonium. An
extract from a letter by Darwin on Leschenaultia, "Cottage Gardener," May
28th, 1861, page 151, is given in Letter 590, note.); he strikes me as a
clever but d--d cock-sure man (as Lord Melbourne said), and I have some
doubts whether to be much trusted. I suspect he has never recorded his
experiment at the time with care. He has made me indignant by the way he
speaks of Gartner, evidently knowing nothing of his work. I mean to try
and pump him in the "Cottage Gardener," and shall perhaps defend Gartner.
He alludes to me occasionally, and I cannot tell with what spirit. He
speaks of "this Mr. Darwin" in one place as if I were a very noxious
animal.