Volume Ii Part 44 (1/2)
One of his earliest letters on this subject was addressed in August, 1873, to Sir Joseph Hooker (736/2. Published in ”Life and Letters,”
III., page 339.):
”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 gla.s.s? 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 Darwin wrote to the late Lord Farrer:
”I am now become mad about drops of water injuring leaves. Please ask Mr. Payne (736/3. Lord 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-gla.s.ses; 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 hothouse orchids I was cautioned not to wet their leaves; but I never then thought on the subject.”
The next letter, though of later date than some which follow it, is printed here because it briefly sums his results and serves as guide to the letters dealing with the subject.)
LETTER 736. TO W. THISELTON-DYER.
(736/4. Published in ”Life and Letters,” III., page 341.)
Down, September 5th [1877].
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-sh.o.r.e 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.
(736/5. Modern research, especially that of Stahl on transpiration (”Bot. Zeitung,” 1897, page 71) has shown that the question is more complex than it appeared in 1877. Stahl's point of view is that moisture remaining on a leaf checks the transpiration-current; and by thus diminis.h.i.+ng the flow of mineral nutriment interferes with the process of a.s.similation. Stahl's idea is doubtless applicable to the whole problem of bloom on leaves. For other references to bloom see letters 685, 689 and 693.)
LETTER 737. TO J.D. HOOKER. Down, August 19th, 1873.
The next time you walk round the garden ask Mr. Smith (737/1. Probably John Smith (1798-1888), for some years Curator, Royal Gardens, Kew.), or any of your best men, what they think about injury from watering during suns.h.i.+ne. One of your men--viz., Mr. Payne, at Abinger, who seems very acute--declares that you may water safely any plant out of doors in suns.h.i.+ne, and that you may do the same for plants under gla.s.s if the sashes are opened. This seems to me very odd, but he seems positive on the point, and acts on it in raising splendid grapes. Another good gardener maintains that it is only COLD water dripping often on the same point of a leaf that ever injures it. I am utterly perplexed, but interested on the point. Give me what you learn when you come to Down.
I should like to hear what plants are believed to be most injured by being watered in suns.h.i.+ne, so that I might get such.
I expect that I shall be utterly beaten, as on so many other points; but I intend to make a few experiments and observations. I have already convinced myself that drops of water do NOT act as burning lenses.
LETTER 738. TO J.D. HOOKER. December 20th [1873].
I find that it is no use going on with my experiments on the evil effects of water on bloom-divested leaves. Either I erred in the early autumn or summer in some incomprehensible manner, or, as I suspect to be the case, water is only injurious to leaves when there is a good supply of actinic rays. I cannot believe that I am all in the wrong about the movements of the leaves to shoot off water.
The upshot of all this is that I want to keep all the plants from Kew until the spring or early summer, as it is mere waste of time going on at present.
LETTER 739. TO W. THISELTON-DYER. Down, July 22nd [1877].
Many thanks for seeds of the Malva and information about Averrhoa, which I perceived was sensitive, as A. carambola is said to be; and about Mimosa sensitiva. The log-wood [Haematoxylon] has interested me much.
The wax is very easily removed, especially from the older leaves, and I found after squirting on the leaves with water at 95 deg, all the older leaves became coated, after forty-eight hours, in an astonis.h.i.+ng manner with a black Uredo, so that they looked as if sprinkled with soot and water. But not one of the younger leaves was affected. This has set me to work to see whether the ”bloom” is not a protection against parasites. As soon as I have ascertained a little more about the case (and generally I am quite wrong at first) I will ask whether I could have a very small plant, which should never be syringed with water above 60 deg, and then I suspect the leaves would not be spotted, as were the older ones on the plant, when it arrived from Kew, but nothing like what they were after my squirting.
In an old note of yours (which I have just found) you say that you have a sensitive Schrankia: could this be lent me?
I have had lent me a young Coral-tree (Erythrina), which is very sickly, yet shows odd sleep movements. I suppose I could buy one, but Hooker told me first to ask you for anything.