More Letters of Charles Darwin Volume II

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In one of two species of Phyllanthus, growing as weeds near my house, the
leaves of the erect branches bend upwards at night, while in the second
species, with horizontal branches, they sleep like those of Phyllanthus
Niruri or of Cassia. In this second species the tips of the branches also
are curled downwards at night, by which movement the youngest leaves are
yet better protected. From their vertical nyctitropic position the leaves
of this Phyllanthus might return to horizontality, traversing 90 deg, in
two ways, either to their own or to the opposite side of the branch; on the
latter way no rotation would be required, while on the former each leaf
must rotate on its own axis in order that its upper surface may be turned
upwards. Thus the way to the wrong side appears to be even less
troublesome. And indeed, in some rare cases I have seen three, four or
even almost all the leaves of one side of a branch horizontally expanded on
the opposite side, with their upper surfaces closely appressed to the lower
surfaces of the leaves of that side.

This Phyllanthus agrees with Cassia not only in its manner of sleeping, but
also by its leaves being paraheliotropic. (687/2. Paraheliotropism is the
movement by which some leaves temporarily direct their edges to the source
of light. See "Movements of Plants," page 445.) Like those of some
Cassiae its leaves take an almost perfectly vertical position, when at
noon, on a summer day, the sun is nearly in the zenith; but I doubt whether
this paraheliotropism will be observable in England. To-day, though
continuing to be fully exposed to the sun, at 3 p.m. the leaves had already
returned to a nearly horizontal position. As soon as there are ripe seeds
I will send you some; of our other species of Phyllanthus I enclose a few
seeds in this letter.

In several species of Hedychium the lateral halves of the leaves when
exposed to bright sunshine, bend downwards so that the lateral margins
meet. It is curious that a hybrid Hedychium in my garden shows scarcely
any trace of this paraheliotropism, while both the parent species are very
paraheliotropic.

Might not the inequality of the cotyledons of Citrus and of Pachira be
attributed to the pressure, which the several embryos enclosed in the same
seed exert upon each other? I do not know Pachira aquatica, but [in] a
species, of which I have a tree in my garden, all the seeds are
polyembryonic, and so were almost all the seeds of Citrus which I examined.
With Coffea arabica also seeds including two embryos are not very rare; but
I have not yet observed whether in this case the cotyledons be inequal.

I repeated to-day Duval-Jouve's measurements on Bryophyllum calycinum
(687/3. "Power of Movement in Plants," page 237. F. Muller's measurements
show, however, that there is a tendency in the leaves to be more highly
inclined at night than in the middle of the day, and so far they agree with
Duval-Jouve's results.); but mine did not agree with his; they are as
follows:--

Distances in mm. between the tips of the upper pair of leaves.

January 9th, 1881 3 A.M. 1 P.M. 6 P.M.
1st plant 54 43 36
2nd plant 28 25 23
3rd plant 28 27 27
4th plant 51 46 39
5th plant 61 52 45
_______________________________________________

222 193 170

LETTER 688. TO F. MULLER.
Down, February 23rd, 1881.

Your letter has interested me greatly, as have so many during many past
years. I thought that you would not object to my publishing in "Nature"
(688/1. "Nature," March 3rd, 1881, page 409.) some of the more striking
facts about the movements of plants, with a few remarks added to show the
bearing of the facts. The case of the Phyllanthus (688/2. See Letter
687.), which turns up its leaves on the wrong side, is most extraordinary
and ought to be further investigated. Do the leaflets sleep on the
following night in the usual manner? Do the same leaflets on successive
nights move in the same strange manner? I was particularly glad to hear of
the strongly marked cases of paraheliotropism. I shall look out with much
interest for the publication about the figs. (688/3. F. Muller published
on Caprification in "Kosmos," 1882.) The creatures which you sketch are
marvellous, and I should not have guessed that they were hymenoptera.
Thirty or forty years ago I read all that I could find about caprification,
and was utterly puzzled. I suggested to Dr. Cruger in Trinidad to
investigate the wild figs, in relation to their cross-fertilisation, and
just before he died he wrote that he had arrived at some very curious
results, but he never published, as I believe, on the subject.

I am extremely glad that the inundation did not so greatly injure your
scientific property, though it would have been a real pleasure to me to
have been allowed to have replaced your scientific apparatus. (688/4. See
Letter 687.) I do not believe that there is any one in the world who
admires your zeal in science and wonderful powers of observation more than
I do. I venture to say this, as I feel myself a very old man, who probably
will not last much longer.

P.S.--With respect to Phyllanthus, I think that it would be a good
experiment to cut off most of the leaflets on one side of the petiole, as
soon as they are asleep and vertically dependent; when the pressure is thus
removed, the opposite leaflets will perhaps bend beyond their vertically
dependent position; if not, the main petiole might be a little twisted so
that the upper surfaces of the dependent and now unprotected leaflets
should face obliquely the sky when the morning comes. In this case
diaheliotropism would perhaps conquer the ordinary movements of the leaves
when they awake, and [assume] their diurnal horizontal position. As the
leaflets are alternate, and as the upper surface will be somewhat exposed
to the dawning light, it is perhaps diaheliotropism which explains your
extraordinary case.

LETTER 689. TO F. MULLER.
Down, April 12th, 1881.

I have delayed answering your last letter of February 25th, as I was just
sending to the printers the MS. of a very little book on the habits of
earthworms, of which I will of course send you a copy when published. I
have been very much interested by your new facts on paraheliotropism, as I
think that they justify my giving a name to this kind of movement, about
which I long doubted. I have this morning drawn up an account of your
observations, which I will send in a few days to "Nature." (689/1.
"Nature," 1881, page 603. Curious facts are given on the movements of
Cassia, Phyllanthus, sp., Desmodium sp. Cassia takes up a sunlight
position unlike its own characteristic night-position, but resembling
rather that of Haematoxylon (see "Power of Movement," figure 153, page
369). One species of Phyllanthus takes up in sunshine the nyctitropic
attitude of another species. And the same sort of relation occurs in the
genus Bauhinia.) I have thought that you would not object to my giving
precedence to paraheliotropism, which has been so little noticed. I will
send you a copy of "Nature" when published. I am glad that I was not in
too great a hurry in publishing about Lagerstroemia. (689/2.
Lagerstraemia was doubtfully placed among the heterostyled plants ("Forms
of Flowers," page 167). F. Muller's observations showed that a totally
different interpretation of the two sizes of stamen is possible. Namely,
that one set serves merely to attract pollen-collecting bees, who in the
act of visiting the flowers transfer the pollen of the longer stamens to
other flowers. A case of this sort in Heeria, a Melastomad, was described
by Muller ("Nature," August 4th, 1881, page 308), and the view was applied
to the cases of Lagerstroemia and Heteranthera at a later date ("Nature,"
1883, page 364). See Letters 620-30.) I have procured some plants of
Melastomaceae, but I fear that they will not flower for two years, and I
may be in my grave before I can repeat my trials. As far as I can
imperfectly judge from my observations, the difference in colour of the
anthers in this family depends on one set of anthers being partially
aborted. I wrote to Kew to get plants with differently coloured anthers,
but I learnt very little, as describers of dried plants do not attend to
such points. I have, however, sowed seeds of two kinds, suggested to me as
probable. I have, therefore, been extremely glad to receive the seeds of
Heteranthera reniformis. As far as I can make out it is an aquatic plant;
and whether I shall succeed in getting it to flower is doubtful. Will you
be so kind as to send me a postcard telling me in what kind of station it
grows. In the course of next autumn or winter, I think that I shall put
together my notes (if they seem worth publishing) on the use or meaning of
"bloom" (689/3. See Letters 736-40.), or the waxy secretion which makes
some leaves glaucous. I think that I told you that my experiments had led
me to suspect that the movement of the leaves of Mimosa, Desmodium and
Cassia, when shaken and syringed, was to shoot off the drops of water. If
you are caught in heavy rain, I should be very much obliged if you would
keep this notion in your mind, and look to the position of such leaves.
You have such wonderful powers of observation that your opinion would be
more valued by me than that of any other man. I have among my notes one
letter from you on the subject, but I forget its purport. I hope, also,
that you may be led to follow up your very ingenious and novel view on the
two-coloured anthers or pollen, and observe which kind is most gathered by
bees.

LETTER 690. TO F. MULLER.
[Patterdale], June 21st, 1881.

I should be much obliged if you could without much trouble send me seeds of
any heterostyled herbaceous plants (i.e. a species which would flower
soon), as it would be easy work for me to raise some illegitimate seedlings
to test their degree of infertility. The plant ought not to have very
small flowers. I hope that you received the copies of "Nature," with
extracts from your interesting letters (690/1. "Nature," March 3rd, 1881,
Volume XXIII., page 409, contains a letter from C. Darwin on "Movements of
Plants," with extracts from Fritz Muller's letter. Another letter, "On the
Movements of Leaves," was published in "Nature," April 28th, 1881, page
603, with notes on leaf-movements sent to Darwin by Muller.), and I was
glad to see a notice in "Kosmos" on Phyllanthus. (690/2. "Verirrte
Blatter," by Fritz Muller ("Kosmos," Volume V., page 141, 1881). In this
article an account is given of a species of Phyllanthus, a weed in Muller's
garden. See Letter 687.) I am writing this note away from my home, but
before I left I had the satisfaction of seeing Phyllanthus sleeping. Some
of the seeds which you so kindly sent me would not germinate, or had not
then germinated. I received a letter yesterday from Dr. Breitenbach, and
he tells me that you lost many of your books in the desolating flood from
which you suffered. Forgive me, but why should you not order, through your
brother Hermann, books, etc., to the amount of 100 pounds, and I would send
a cheque to him as soon as I heard the exact amount? This would be no
inconvenience to me; on the contrary, it would be an honour and lasting
pleasure to me to have aided you in your invaluable scientific work to this
small and trifling extent. (690/3. See Letter 687, also "Life and
Letters," III., page 242.)

LETTER 691. TO F. MULLER.

(691/1. The following extract from a letter to F. Muller shows what was
the nature of Darwin's interest in the effect of carbonate of ammonia on
roots, etc. He was, we think, wrong in adhering to the belief that the
movements of aggregated masses are of an amoeboid nature. The masses
change shape, just as clouds do under the moulding action of the wind. In
the plant cell the moulding agent is the flowing protoplasm, but the masses
themselves are passive.)

September 10th, 1881.

Perhaps you may remember that I described in "Insectivorous Plants" a
really curious phenomenon, which I called the aggregation of the protoplasm
in the cells of the tentacles. None of the great German botanists will
admit that the moving masses are composed of protoplasm, though it is
astonishing to me that any one could watch the movement and doubt its
nature. But these doubts have led me to observe analogous facts, and I
hope to succeed in proving my case.

LETTER 692. TO F. MULLER.
Down, November 13th, 1881.

I received a few days ago a small box (registered) containing dried
flower-heads with brown seeds somewhat sculptured on the sides. There was
no name, and I should be much obliged if some time you would tell me what
these seeds are. I have planted them.

I sent you some time ago my little book on earthworms, which, though of no
importance, has been largely read in England. I have little or nothing to
tell you about myself. I have for a couple of months been observing the
effects of carbonate of ammonia on chlorophyll and on the roots of certain
plants (692/1. Published under the title "The Action of Carbonate of
Ammonia on the Roots of Certain Plants and on Chlorophyll Bodies," "Linn.
Soc. Journ." XIX., 1882, pages 239-61, 262-84.), but the subject is too
difficult for me, and I cannot understand the meaning of some strange facts
which I have observed. The mere recording new facts is but dull work.

Professor Wiesner has published a book (692/2. See Letter 763.), giving a
different explanation to almost every fact which I have given in my "Power
of Movement in Plants." I am glad to say that he admits that almost all my
statements are true. I am convinced that many of his interpretations of
the facts are wrong, and I am glad to hear that Professor Pfeffer is of the
same opinion; but I believe that he is right and I wrong on some points. I
have not the courage to retry all my experiments, but I hope to get my son
Francis to try some fresh ones to test Wiesner's explanations. But I do
not know why I have troubled you with all this.

LETTER 693. TO F. MULLER.
[4, Bryanston Street], December 19th, 1881.

I hope that you may find time to go on with your experiments on such plants
as Lagerstroemia, mentioned in your letter of October 29th, for I believe
you will arrive at new and curious results, more especially if you can
raise two sets of seedlings from the two kinds of pollen.

Many thanks for the facts about the effect of rain and mud in relation to
the waxy secretion. I have observed many instances of the lower side being
protected better than the upper side, in the case, as I believe, of bushes
and trees, so that the advantage in low-growing plants is probably only an
incidental one. (693/1. The meaning is here obscure: it appears to us
that the significance of bloom on the lower surface of the leaves of both
trees and herbs depends on the frequency with which all or a majority of
the stomata are on the lower surface--where they are better protected from
wet (even without the help of bloom) than on the exposed upper surface. On
the correlation between bloom and stomata, see Francis Darwin "Linn. Soc.
Journ." XXII., page 99.) As I am writing away from my home, I have been
unwilling to try more than one leaf of the Passiflora, and this came out of
the water quite dry on the lower surface and quite wet on the upper. I
have not yet begun to put my notes together on this subject, and do not at
all know whether I shall be able to make much of it. The oddest little
fact which I have observed is that with Trifolium resupinatum, one half of
the leaf (I think the right-hand side, when the leaf is viewed from the
apex) is protected by waxy secretion, and not the other half (693/2. In
the above passage "leaf" should be "leaflet": for a figure of Trifolium
resupinatum see Letter 740.); so that when the leaf is dipped into water,
exactly half the leaf comes out dry and half wet. What the meaning of this
can be I cannot even conjecture. I read last night your very interesting
article in "Kosmos" on Crotalaria, and so was very glad to see the dried
leaves sent by you: it seems to me a very curious case. I rather doubt
whether it will apply to Lupinus, for, unless my memory deceives me, all
the leaves of the same plant sometimes behaved in the same manner; but I
will try and get some of the same seeds of the Lupinus, and sow them in the
spring. Old age, however, is telling on me, and it troubles me to have
more than one subject at a time on hand.

(693/3. In a letter to F. Muller (September 10, 1881) occurs a sentence
which may appropriately close this series: "I often feel rather ashamed of
myself for asking for so many things from you, and for taking up so much of
your valuable time, but I can assure you that I feel grateful.")

2.XI.III. MISCELLANEOUS, 1868-1881.

LETTER 694. TO G. BENTHAM.
Down, April 22nd, 1868.

I have been extremely much pleased by your letter, and I take it as a very
great compliment that you should have written to me at such length...I am
not at all surprised that you cannot digest pangenesis: it is enough to
give any one an indigestion; but to my mind the idea has been an immense
relief, as I could not endure to keep so many large classes of facts all
floating loose in my mind without some thread of connection to tie them
together in a tangible method.

With respect to the men who have recently written on the crossing of
plants, I can at present remember only Hildebrand, Fritz Muller, Delpino,
and G. Henslow; but I think there are others. I feel sure that Hildebrand
is a very good observer, for I have read all his papers, and during the
last twenty years I have made unpublished observations on many of the
plants which he describes. [Most of the criticisms which I sometimes meet
with in French works against the frequency of crossing I am certain are the
result of mere ignorance. I have never hitherto found the rule to fail
that when an author describes the structure of a flower as specially
adapted for self-fertilisation, it is really adapted for crossing. The
Fumariaceae offer a good instance of this, and Treviranus threw this order
in my teeth; but in Corydalis Hildebrand shows how utterly false the idea
of self-fertilisation is. This author's paper on Salvia (694/1.
Hildebrand, "Pringsheim's Jahrbucher," IV.) is really worth reading, and I
have observed some species, and know that he is accurate]. (694/2. The
passage within [] was published in the "Life and Letters," III., page 279.)
Judging from a long review in the "Bot. Zeitung", and from what I know of
some the plants, I believe Delpino's article especially on the Apocynaea,
is excellent; but I cannot read Italian. (694/3. Hildebrand's paper in
the "Bot. Zeitung," 1867, refers to Delpino's work on the Asclepiads,
Apocyneae and other Orders.) Perhaps you would like just to glance at such
pamphlets as I can lay my hands on, and therefore I will send them, as if
you do not care to see them you can return them at once; and this will
cause you less trouble than writing to say you do not care to see them.
With respect to Primula, and one point about which I feel positive is that
the Bardfield and common oxlips are fundamentally distinct plants, and that
the common oxlip is a sterile hybrid. (694/4. For a general account of
the Bardfield oxlip (Primula elatior) see Miller Christy, "Linn. Soc.
Journ." Volume XXXIII., page 172, 1897.) I have never heard of the common
oxlip being found in great abundance anywhere, and some amount of
difference in number might depend on so small a circumstance as the
presence of some moth which habitually sucked the primrose and cowslip. To
return to the subject of crossing: I am experimenting on a very large
scale on the difference in power and growth between plants raised from
self-fertilised and crossed seeds, and it is no exaggeration to say that
the difference in growth and vigour is sometimes truly wonderful. Lyell,
Huxley, and Hooker have seen some of my plants, and been astonished; and I
should much like to show them to you. I always supposed until lately that
no evil effects would be visible until after several generations of
self-fertilisation, but now I see that one generation sometimes suffices,
and the existence of dimorphic plants and all the wonderful contrivances of
orchids are quite intelligible to me.

LETTER 695. TO T.H. FARRER (Lord Farrer).
Down, June 5th, 1868.

I must write a line to cry peccavi. I have seen the action in Ophrys
exactly as you describe, and am thoroughly ashamed of my inaccuracy.
(695/1. See "Fertilisation of Orchids," Edition II., page 46, where Lord
Farrer's observations on the movement of the pollinia in Ophrys muscifera
are given.) I find that the pollinia do not move if kept in a very damp
atmosphere under a glass; so that it is just possible, though very
improbable, that I may have observed them during a very damp day.

I am not much surprised that I overlooked the movement in Habenaria, as it
takes so long. (695/2. This refers to Peristylus viridis, sometimes known
as Habenaria viridis. Lord Farrer's observations are given in
"Fertilisation of Orchids," Edition II., page 63.)

I am glad you have seen Listera; it requires to be seen to believe in the
co-ordination in the position of the parts, the irritability, and the
chemical nature of the viscid fluid. This reminds me that I carefully
described to Huxley the shooting out of the pollinia in Catasetum, and
received for an answer, "Do you really think that I can believe all that!"
(695/3. See Letter 665.)

LETTER 696. TO J.D. HOOKER.
Down, December 2nd, 1868.

It is a splendid scheme, and if you make only a beginning on a "Flora,"
which shall serve as an index to all papers on curious points in the
life-history of plants, you will do an inestimable good service. Quite
recently I was asked by a man how he could find out what was known on
various biological points in our plants, and I answered that I knew of no
such book, and that he might ask half a dozen botanists before one would
chance to remember what had been published on this or that point. Not long
ago another man, who had been experimenting on the quasi-bulbs on the
leaves of Cardamine, wrote to me to complain that he could not find out
what was known on the subject. It is almost certain that some early or
even advanced students, if they found in their "Flora" a line or two on
various curious points, with references for further investigation, would be
led to make further observations. For instance, a reference to the viscid
threads emitted by the seeds of Compositae, to the apparatus (if it has
been described) by which Oxalis spurts out its seeds, to the sensitiveness
of the young leaves of Oxalis acetosella with reference to O. sensitiva.
Under Lathyrus nissolia it would [be] better to refer to my hypothetical
explanation of the grass-like leaves than to nothing. (696/1. No doubt the
view given in "Climbing Plants," page 201, that L. nissolia has been
evolved from a form like L. aphaca.) Under a twining plant you might say
that the upper part of the shoot steadily revolves with or against the sun,
and so, when it strikes against any object it turns to the right or left,
as the case may be. If, again, references were given to the parasitism of
Euphrasia, etc., how likely it would be that some young man would go on
with the investigation; and so with endless other facts. I am quite
enthusiastic about your idea; it is a grand idea to make a "Flora" a guide
for knowledge already acquired and to be acquired. I have amused myself by
speculating what an enormous number of subjects ought to be introduced into
a Eutopian (696/2. A mis-spelling of Utopian.) Flora, on the quickness of
the germination of the seeds, on their means of dispersal; on the
fertilisation of the flower, and on a score of other points, about almost
all of which we are profoundly ignorant. I am glad to read what you say
about Bentham, for my inner consciousness tells me that he has run too many
forms together. Should you care to see an elaborate German pamphlet by
Hermann Muller on the gradation and distinction of the forms of Epipactis
and of Platanthera? (696/3. "Verhand. d. Nat. Ver. f. Pr. Rh. u. Wesfal."
Jahrg. XXV.: see "Fertilisation of Orchids," Edition II., pages 74, 102.)
It may be absurd in me to suggest, but I think you would find curious
facts and references in Lecoq's enormous book (696/4. "Geographie
Botanique," 9 volumes, 1854-58.), in Vaucher's four volumes (696/5.
"Plantes d'Europe," 4 volumes, 1841.), in Hildebrand's "Geschlechter
Vertheilung" (696/6 "Geschlechter Vertheilung bei den Pflanzen," 1 volume,
Leipzig, 1867.), and perhaps in Fournier's "De la Fecondation." (696/7.
"De la Fecondation dans les Phanerogames," par Eugene Fournier: thesis
published in Paris in 1863. The facts noted in Darwin's copy are the
explosive stamens of Parietaria, the submerged flowers of Alisma containing
air, the manner of fertilisation of Lopezia, etc.) I wish you all success
in your gigantic undertaking; but what a pity you did not think of it ten
years ago, so as to have accumulated references on all sorts of subjects.
Depend upon it, you will have started a new era in the floras of various
countries. I can well believe that Mrs. Hooker will be of the greatest
possible use to you in lightening your labours and arranging your
materials.