Chapter 311

I must have the pleasure of thanking you for your long and interesting letter. The cause and means of the transition from an hermaphrodite to a unisexual condition seems to me a very perplexing problem, and I shall be extremely glad to read your remarks on Smilax, whenever I receive the essay which you kindly say that you will send me. (280/1. "Monographiae Phanerogamarum," Volume I. In his treatment of the Smilaceae, De Candolle distinguishes:--Heterosmilax which has dioecious flowers without a trace of aborted stamens or pistils, Smilax with sterile stamens in the female flowers, and Rhipogonum with hermaphrodite flowers.) There is much justice in your criticisms (280/2. The passage criticised by De Candolle is in "Forms of Flowers" (page 7): "It is a natural inference that their corollas have been increased in size for this special purpose." De Candolle goes on to give an account of the "recherche linguistique," which, with characteristic fairness, he undertook to ascertain whether the word "purpose" differs in meaning from the corresponding French word "but.") on my use of the terms object, end, purpose; but those who believe that organs have been gradually modified for Natural Selection for a special purpose may, I think, use the above terms correctly, though no conscious being has intervened. I have found much difficulty in my occasional attempts to avoid these terms, but I might perhaps have always spoken of a beneficial or serviceable effect. My son Francis will be interested by hearing about Smilax. He has dispatched to you a copy of his paper on the glands of Dipsacus (280/3. "Quart. Journ. Mic. Sci." 1877.), and I hope that you will find time to read it, for the case seems to me a new and highly remarkable one. We are now hard at work on an attempt to make out the function or use of the bloom or waxy secretion on the leaves and fruit of many plants; but I doubt greatly whether our experiments will tell us much. (280/4. "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." (See letter to Sir W. Thiselton-Dyer, "Life and Letters," III., page 341. A paper on the same subject by Francis Darwin was published in the "Journ. Linn. Soc." XXII.)) If you have any decided opinion whether plants with conspicuously glaucous leaves are more frequent in hot than in temperate or cold, in dry than in damp countries, I should be grateful if you would add to your many kindnesses by informing me. Pray give my kind remembrances to your son, and tell him that my son has been trying on a large scale the effects of feeding Drosera with meat, and the results are most striking and far more favourable than I anticipated.