on--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.

(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
diminishing the flow of mineral nutriment interferes with the process of
assimilation. 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
sunshine. 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
sunshine, and that you may do the same for plants under glass 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 sunshine, 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 w