1873-1882.
[The present chapter contains a series of miscellaneous letters on
botanical subjects. Some of them show my father's varied interests in
botanical science, and others give account of researches which never
reached completion.]
BLOOM ON LEAVES AND FRUIT.
[His researches into the meaning of the "bloom," or waxy coating found
on many leaves, was one of those inquiries which remained unfinished at
the time of his death. He amassed a quantity of notes on the subject,
part of which I hope to publish at no distant date. (A small instalment
on the relation between bloom and the distribution of the stomata on
leaves has appeared in the 'Journal of the Linnean Society,' 1886.
Tschirsch ("Linnaea", 1881) has published results identical with
some which my father and myself obtained, viz. that bloom diminishes
transpiration. The same fact was previously published by Garreau in
1850.)
One of his earliest letters on this subject was addressed in August,
1873, to Sir Joseph Hooker:--
"I want a little information from you, and if you do not yourself know,
please to enquire of some of the wise men of Kew.
"Why are the leaves and fruit of so many plants protected by a thin
layer of waxy matter (like the common cabbage), or with fine hair, so
that when such leaves or fruit are immersed in water they appear as if
encased in thin glass? It is really a pretty sight to put a pod of the
common pea, or a raspberry into water. I find several leaves are thus
protected on the under surface and not on the upper.
"How can water injure the leaves if indeed this is at all the case?"
On this latter point he wrote to Sir Thomas Farrer:--
"I am now become mad about drops of water injuring leaves. Please ask
Mr. Paine (Sir Thomas Farrer's gardener.) whether he believes, FROM
HIS OWN EXPERIENCE, that drops of water injure leaves or fruit in his
conservatories. It is said that the drops act as burning-glasses; if
this is true, they would not be at all injurious on cloudy days. As
he is so acute a man, I should very much like to hear his opinion. I
remember when I grew hot-house orchids I was cautioned not to wet their
leaves; but I never then thought on the subject.
"I enjoyed my visit greatly with you, and I am very sure that all
England could not afford a kinder and pleasanter host."
Some years later he took up the subject again, and wrote to Sir Joseph
Hooker (May 25, 1877):--
"I have been looking over my old notes about the "bloom" on plants,
and I think that the subject is well worth pursuing, though I am very
doubtful of any success. Are you inclined to aid me on the mere chance
of success, for without your aid I could do hardly anything?"]
CHARLES DARWIN TO ASA GRAY. Down, June 4 [1877].
... I am now trying to make out the use or function of "bloom," or the
waxy secretion on the leaves and fruit of plants, but am VERY doubtful
whether I shall succeed. Can you give me any light? Are such plants
commoner in warm than in colder climates? I ask because I often walk out
in heavy rain, and the leaves of very few wild dicotyledons can be here
seen with drops of water rolling off them like quick-silver. Whereas in
my flower garden, greenhouse, and hot-houses there are several. Again,
are bloo-protected plants common on your DRY western plains? Hooker
THINKS that they are common at the Cape of Good Hope. It is a puzzle
to me if they are common under very dry climates, and I find bloom very
common on the Acacias and Eucalypti of Australia. Some of the Eucalypti
which do not appear to be covered with bloom have the epidermis
protected by a layer of some substance which is dissolved in boiling
alcohol. Are there any bloo-protected leaves or fruit in the Arctic
regions? If you can illuminate me, as you so often have done, pray do
so; but otherwise do not bother yourself by answering.
Yours affectionately, C. DARWIN.
CHARLES DARWIN TO W. THISELTON DYER. Down, September 5 [1877].
My dear Dyer,
One word to thank you. I declare had it not been for your kindness, we
should have broken down. As it is we have made out clearly that with
some plants (chiefly succulent) the bloom checks evaporation--with
some certainly prevents attacks of insects; with SOME sea-shore plants
prevents injury from salt-water, and, I believe, with a few prevents
injury from pure water resting on the leaves. This latter is as yet
the most doubtful and the most interesting point in relation to the
movements of plants...
CHARLES DARWIN TO F. MULLER. Down, July 4 [1881].
My dear Sir,
Your kindness is unbounded, and I cannot tell you how much your last
letter (May 31) has interested me. I have piles of notes about the
effect of water resting on leaves, and their movements (as I supposed)
to shake off the drops. But I have not looked over these notes for a
long time, and had come to think that perhaps my notion was mere fancy,
but I had intended to begin experimenting as soon as I returned home;
and now with your INVALUABLE letter about the position of the leaves of
various plants during rain (I have one analogous case with Acacia from
South Africa), I shall be stimulated to work in earnest.
VARIABILITY.
[The following letter refers to a subject on which my father felt the
strongest interest:--the experimental investigation of the causes of
variability. The experiments alluded to were to some extent planned out,
and some preliminary work was begun in the direction indicated below,
but the research was ultimately abandoned.]
CHARLES DARWIN TO J.H. GILBERT. (Dr. Gilbert, F.R.S., joint author
with Sir John Bennett Lawes of a long series of valuable researches in
Scientific Agriculture.) Down, February 16, 1876.
My dear Sir,
When I met you at the Linnean Society, you were so kind as to say that
you would aid me with advice, and this will be of the utmost value to me
and my son. I will first state my object, and hope that you will excuse
a long letter. It is admitted by all naturalists that no problem is so
perplexing as what causes almost every cultivated plant to vary, and no
experiments as yet tried have thrown any light on the subject. Now
for the last ten years I have been experimenting in crossing and
self-fertilising plants; and one indirect result has surprised me much;
namely, that by taking pains to cultivate plants in pots under glass
during several successive generations, under nearly similar conditions,
and by self-fertilising them in each generation, the colour of the
flowers often changes, and, what is very remarkable, they became in some
of the most variable species, such as Mimulus, Carnation, etc., quite
constant, like those of a wild species.
This fact and several others have led me to the suspicion that the cause
of variation must be in different substances absorbed from the soil by
these plants when their powers of absorption are not interfered with
by other plants with which they grow mingled in a state of nature.
Therefore my son and I wish to grow plants in pots in soil entirely, or
as nearly entirely as is possible, destitute of all matter which plants
absorb, and then to give during several successive generations to
several plants of the same species as different solutions as may be
compatible with their life and health. And now, can you advise me how
to make soil approximately free of all the substances which plants
naturally absorb? I suppose white silver sand, sold for cleaning
harness, etc., is nearly pure silica, but what am I to do for alumina?
Without some alumina I imagine that it would be impossible to keep the
soil damp and fit for the growth of plants. I presume that clay washed
over and over again in water would still yield mineral matter to the
carbonic acid secreted by the roots. I should want a good deal of soil,
for it would be useless to experimentise unless we could fill from
twenty to thirty moderately sized flower-pots every year. Can you
suggest any plan? for unless you can it would, I fear, be useless for us
to commence an attempt to discover whether variability depends at all
on matter absorbed from the soil. After obtaining the requisite kind of
soil, my notion is to water one set of plants with nitrate of potassium,
another set with nitrate of sodium, and another with nitrate of lime,
giving all as much phosphate of ammonia as they seemed to support, for
I wish the plants to grow as luxuriantly as possible. The plants watered
with nitrate of Na and of Ca would require, I suppose, some K; but
perhaps they would get what is absolutely necessary from such soil as I
should be forced to employ, and from the rain-water collected in tanks.
I could use hard water from a deep well in the chalk, but then all the
plants would get lime. If the plants to which I give Nitrate of Na and
of Ca would not grow I might give them a little alum.
I am well aware how very ignorant I am, and how crude my notions are;
and if you could suggest any other solutions by which plants would be
likely to be affected it would be a very great kindness. I suppose that
there are no organic fluids which plants would absorb, and which I could
procure?
I must trust to your kindness to excuse me for troubling you at such
length, and,
I remain, dear Sir, yours sincerely, CHARLES DARWIN.
[The next letter to Professor Semper (Professor of Zoology at Wurzburg.)
bears on the same subject:]
FROM CHARLES DARWIN TO K. SEMPER. Down, July 19, 1881.
My dear Professor Semper,
I have been much pleased to receive your letter, but I did not expect
you to answer my former one... I cannot remember what I wrote to you,
but I am sure that it must have expressed the interest which I felt in
reading your book. (Published in the 'International Scientific Series,'
in 1881, under the title, 'The Natural Conditions of Existence as they
affect Animal Life.') I thought that you attributed too much weight to
the DIRECT action of the environment; but whether I said so I know not,
for without being asked I should have thought it presumptuous to have
criticised your book, nor should I now say so had I not during the last
few days been struck with Professor Hoffmann's review of his own work in
the 'Botanische Zeitung,' on the variability of plants; and it is really
surprising how little effect he produced by cultivating certain plants
under unnatural conditions, as the presence of salt, lime, zinc, etc.,
etc., during SEVERAL generations. Plants, moreover, were selected which
were the most likely to vary under such conditions, judging from the
existence of closely-allied forms adapted for these conditions. No
doubt I originally attributed too little weight to the direct action of
conditions, but Hoffmann's paper has staggered me. Perhaps hundreds of
generations of exposure are necessary. It is a most perplexing subject.
I wish I was not so old, and had more strength, for I see lines of
research to follow. Hoffmann even doubts whether plants vary more
under cultivation than in their native home and under their natural
conditions. If so, the astonishing variations of almost all cultivated
plants must be due to selection and breeding from the varying
individuals. This idea crossed my mind many years ago, but I was
afraid to publish it, as I thought that people would say, "how he does
exaggerate the importance of selection."
I still MUST believe that changed conditions give the impulse to
variability, but that they act IN MOST CASES in a very indirect manner.
But, as I said, it is a most perplexing problem. Pray forgive me for
writing at such length; I had no intention of doing so when I sat down
to write.
I am extremely sorry to hear, for your own sake and for that of Science,
that you are so hard worked, and that so much of your time is consumed
in official labour.
Pray believe me, dear Professor Semper, Yours sincerely, CHARLES DARWIN.
GALLS.
[Shortly before his death, my father began to experimentise on the
possibility of producing galls artificially. A letter to Sir J.D. Hooker
(November 3, 1880) shows the interest which he felt in the question:--
"I was delighted with Paget's Essay ('Disease in Plants,' by Sir
James Paget.--See "Gardeners' Chronicle", 1880.); I hear that he has
occasionally attended to this subject from his youth... I am very glad he
has called attention to galls: this has always seemed to me a profoundly
interesting subject; and if I had been younger would take it up."
His interest in this subject was connected with his ever-present wish
to learn something of the causes of variation. He imagined to himself
wonderful galls caused to appear on the ovaries of plants, and by these
means he thought it possible that the seed might be influenced, and thus
new varieties arise. He made a considerable number of experiments by
injecting various reagents into the tissues of leaves, and with some
slight indications of success.]
AGGREGATION.
[The following letter gives an idea of the subject of the last of his
published papers. ('Journal of the Linnean Society.' volume xix, 1882,
pages 239 and 262.) The appearances which he observed in leaves and
roots attracted him, on account of their relation to the phenomena of
aggregation which had so deeply interested him when he was at work on
Drosera:]
CHARLES DARWIN TO S.H. VINES. (Reader in Botany in the University of
Cambridge.) Down, November 1, 1881.
My dear Mr. Vines,
As I know how busy you are, it is a great shame to trouble you. But you
are so rich in chemical knowledge about plants, and I am so poor, that
I appeal to your charity as a pauper. My question is--Do you know of
any solid substance in the cells of plants which glycerine and water
dissolves? But you will understand my perplexity better if I give you
the facts: I mentioned to you that if a plant of Euphorbia peplus is
gently dug up and the roots placed for a short time in a weak solution
(1 to 10,000 of water, suffices in 24 hours) of carbonate of ammonia the
(generally) alternate longitudinal rows of cells in every rootlet, from
the root-cap up to the very top of the root (but not as far as I have
yet seen in the green stem) become filled with translucent, brownish
grains of matter. These rounded grains often cohere and even become
confluent. Pure phosphate and nitrate of ammonia produce (though more
slowly) the same effect, as does pure carbonate of soda.
Now, if slices of root under a cover-glass are irrigated with glycerine
and water, every one of the innumerable grains in the cells disappear
after some hours. What am I to think of this.?...
Forgive me for bothering you to such an extent; but I must mention
that if the roots are dipped in boiling water there is no deposition of
matter, and carbonate of ammonia afterwards produces no effect. I should
state that I now find that the granular matter is formed in the cells
immediately beneath the thin epidermis, and a few other cells near the
vascular tissue. If the granules consisted of living protoplasm (but
I can see no traces of movement in them), then I should infer that
the glycerine killed them and aggregation ceased with the diffusion of
invisibly minute particles, for I have seen an analogous phenomenon in
Drosera.
If you can aid me, pray do so, and anyhow forgive me. Yours very
sincerely, CH. DARWIN.
MR. TORBITT'S EXPERIMENTS ON THE POTATO-DISEASE.
[Mr. James Torbitt, of Belfast, has been engaged for the last twelve
years in the difficult undertaking, in which he has been to a large
extent successful, of raising fungus-proof varieties of the potato. My
father felt great interest in Mr. Torbitt's work, and corresponded with
him from 1876 onwards. The following letter, giving a clear account of
Mr. Torbitt's method and of my father's opinion of the probability of
its success, was written with the idea that Government aid for the work
might possibly be obtainable:]
CHARLES DARWIN TO T.H. FARRER. Down, March 2, 1878.
My dear Farrer,
Mr. Torbitt's plan of overcoming the potato-disease seems to me by far
the best which has ever been suggested. It consists, as you know
from his printed letter, of rearing a vast number of seedlings from
cross-fertilised parents, exposing them to infection, ruthlessly
destroying all that suffer, saving those which resist best, and
repeating the process in successive seminal generations. My belief in
the probability of good results from this process rests on the fact of
all characters whatever occasionally varying. It is known, for instance,
that certain species and varieties of the vine resist phylloxera better
than others. Andrew Knight found in one variety or species of the apple
which was not in the least attacked by coccus, and another variety has
been observed in South Australia. Certain varieties of the peach resist
mildew, and several other such cases could be given. Therefore there is
no great improbability in a new variety of potato arising which would
resist the fungus completely, or at least much better than any existing
variety. With respect to the cross-fertilisation of two distinct
seedling plants, it has been ascertained that the offspring thus raised
inherit much more vigorous constitutions and generally are more prolific
than seedlings from self-fertilised parents. It is also probable that
cross-fertilisation would be especially valuable in the case of the
potato, as there is reason to believe that the flowers are seldom
crossed by our native insects; and some varieties are absolutely sterile
unless fertilised with pollen from a distinct variety. There is some
evidence that the good effects from a cross are transmitted for several
generations; it would not, therefore be necessary to cross-fertilise the
seedlings in each generation, though this would be desirable, as it is
almost certain that a greater number of seeds would thus be obtained. It
should be remembered that a cross between plants raised from the tubers
of the same plant, though growing on distinct roots, does no more good
than a cross between flowers on the same individual. Considering the
whole subject, it appears to me that it would be a national misfortune
if the cros-fertilised seeds in Mr. Torbitt's possession produced by
parents which have already shown some power of resisting the disease,
are not utilised by the Government, or some public body, and the process
of selection continued during several more generations.
Should the Agricultural Society undertake the work, Mr. Torbitt's
knowledge gained by experience would be especially valuable; and
an outline of the plan is given in his printed letter. It would be
necessary that all the tubers produced by each plant should be collected
separately, and carefully examined in each succeeding generation.
It would be advisable that some kind of potato eminently liable to the
disease should be planted in considerable numbers near the seedlings so
as to infect them.
Altogether the trial would be one requiring much care and extreme
patience, as I know from experience with analogous work, and it may be
feared that it would be difficult to find any one who would pursue the
experiment with sufficient energy. It seems, therefore, to me highly
desirable that Mr. Torbitt should be aided with some small grant so as
to continue the work himself.
Judging from his reports, his efforts have already been crowned in so
short a time with more success than could have been anticipated; and
I think you will agree with me, that any one who raises a fungus-proof
potato will be a public benefactor of no common kind.
My dear Farrer, yours sincerely, CHARLES DARWIN.
[After further consultation with Sir Thomas Farrer and with Mr. Caird,
my father became convinced that it was hopeless to attempt to obtain
Government aid. He wrote to Mr. Torbitt to this effect, adding, "it
would be less trouble to get up a subscription from a few rich leading
agriculturists than from Government. This plan I think you cannot object
to, as you have asked nothing, and will have nothing whatever to do with
the subscription. In fact, the affair is, in my opinion, a compliment
to you." The idea here broached was carried out, and Mr. Torbitt was
enabled to continue his work by the aid of a sum to which Sir T. Farrer,
Mr. Caird, my father, and a few friends, subscribed.
My father's sympathy and encouragement were highly valued by Mr.
Torbitt, who tells me that without them he should long ago have given up
his attempt. A few extracts will illustrate my father's fellow feeling
with Mr. Torbitt's energy and perseverance:--
"I admire your indomitable spirit. If any one ever deserved success,
you do so, and I keep to my original opinion that you have a very good
chance of raising a fungus-proof variety of the potato.
"A pioneer in a new undertaking is sure to meet with many
disappointments, so I hope that you will keep up your courage, though we
have done so very little for you."
Mr. Torbitt tells me that he still (1887) succeeds in raising varieties
possessing well-marked powers of resisting disease; but this immunity is
not permanent, and, after some years, the varieties become liable to the
attacks of the fungus.]
THE KEW INDEX OF PLANT-NAMES, OR 'NOMENCLATOR DARWINIANUS.'
[Some account of my father's connection with the Index of Plant-names
now (1887) in course of preparation at Kew will be found in Mr. B.
Daydon Jackson's paper in the 'Journal of Botany,' 1887, page 151. Mr.
Jackson quotes the following statement by Sir J.D. Hooker:--
"Shortly before his death, Mr. Charles Darwin informed Sir Joseph Hooker
that it was his intention to devote a considerable sum of money annually
for some years in aid or furtherance of some work or works of practical
utility to biological science, and to make provisions in his will in the
event of these not being completed during his lifetime.
"Amongst other objects connected with botanical science, Mr. Darwin
regarded with especial interest the importance of a complete index
to the names and authors of the genera and species of plants known to
botanists, together with their native countries. Steudel's 'Nomenclator'
is the only existing work of this nature, and although now nearly half a
century old, Mr. Darwin had found it of great aid in his own researches.
It has been indispensable to every botanical institution, whether as a
list of all known flowering plants, as an indication of their authors,
or as a digest of botanical geography."
Since 1840, when the 'Nomenclator' was published, the number of
described plants may be said to have doubled, so that the 'Nomenclator'
is now seriously below the requirements of botanical work. To remedy
this want, the 'Nomenclator' has been from time to time posted up in an
interleaved copy in the Herbarium at Kew, by the help of "funds supplied
by private liberality." (Kew Gardens Report, 1881, page 62.)
My father, like other botanists, had as Sir Joseph Hooker points out,
experienced the value of Steudel's work. He obtained plants from all
sorts of sources, which were often incorrectly named, and he felt the
necessity of adhering to the accepted nomenclature, so that he might
convey to other workers precise indications as to the plants which he
had studied. It was also frequently a matter of importance to him to
know the native country of his experimental plants. Thus it was natural
that he should recognize the desirability of completing and publishing
the interleaved volume at Kew. The wish to help in this object was
heightened by the admiration he felt for the results for which the world
has to thank the Royal Gardens at Kew, and by his gratitude for the
invaluable aid which for so many years he received from its Director and
his staff. He expressly stated that it was his wish "to aid in some
way the scientific work carried on at the Royal Gardens" (Kew Gardens
Report, 1881, page 62.)--which induced him to offer to supply funds for
the completion of the Kew 'Nomenclator.'
The following passage, for which I am indebted to Professor Judd, is of
much interest, as illustrating the motives that actuated my father in
this matter. Professor Judd writes:--
"On the occasion of my last visit to him, he told me that his income
having recently greatly increased, while his wants remained the same,
he was most anxious to devote what he could spare to the advancement
of Geology or Biology. He dwelt in the most touching manner on the fact
that he owed so much happiness and fame to the natural-history
sciences, which had been the solace of what might have been a painful
existence;--and he begged me, if I knew of any research which could be
aided by a grant of a few hundreds of pounds, to let him know, as it
would be a delight to him to feel that he was helping in promoting the
progress of science. He informed me at the same time that he was making
the same suggestion to Sir Joseph Hooker and Professor Huxley with
respect to Botany and Zoology respectively. I was much impressed by
the earnestness, and, indeed, deep emotion, with which he spoke of his
indebtedness to Science, and his desire to promote its interests."
Sir Joseph Hooker was asked by my father "to take into consideration,
with the aid of the botanical staff at Kew and the late Mr. Bentham, the
extent and scope of the proposed work, and to suggest the best means of
having it executed. In doing this, Sir Joseph had further the advantage
of the great knowledge and experience of Professor Asa Gray, of
Cambridge, U.S.A., and of Mr. John Ball, F.R.S." ('Journal of Botany,'
loc. cit.)
The plan of the proposed work having been carefully considered, Sir
Joseph Hooker was able to confide its elaboration in detail to Mr.
B. Daydon Jackson, Secretary of the Linnean Society, whose extensive
knowledge of botanical literature qualifies him for the task. My
father's original idea of producing a modern edition of Steudel's
'Nomenclator' has been practically abandoned, the aim now kept in view
is rather to construct a list of genera and species (with references)
founded on Bentham and Hooker's 'Genera Plantarum.' The colossal nature
of the work in progress at Kew may be estimated by the fact that the
manuscript of the 'Index' is at the present time (1887) believed to
weigh more than a ton. Under Sir Joseph Hooker's supervision the work
goes steadily forward, being carried out with admirable zeal by Mr.
Jackson, who devotes himself unsparingly to the enterprise, in which,
too, he has the advantage of the active interest in the work felt by
Professor Oliver and Mr. Thiselton Dyer.
The Kew 'Index,' which will, in all probability, be ready to go to press
in four or five years, will be a fitting memorial of my father: and his
share in its completion illustrates a part of his character--his ready
sympathy with work outside his own lines of investigation--and his
respect for minute and patient labour in all branches of science.]