CHAPTER 2.XV.
MISCELLANEOUS BOTANICAL LETTERS.
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 bloom-
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 bloom-
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 cross-
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.]
