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Down, Beckenham, Kent.

July 5th [1873]

Dear Sir

I have read about 2/3 of your book with much interest. The power of pollen in exciting the skin & mucous membrane seems to me an astonishing fact.— Would it not be worth while to kill the pollen by a dry heat rather above the boiling point of water, & see if it retains its injurious properties? But my object in writing now is to say that I imagine that you are not fully aware that plants may be divided into 2 great classes, — those with incoherent pollen & those with coherent pollen.

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The former are called by Delpino "anemophilous" plants, as they are fertilized by the wind; & the latter "entomophilous plants", as they are fert. by insects. Perhaps where grass is cut & dried, some pollen of the entomophilous division may be blown about; but naturally hardly any could thusly be blown. Whereas the pollen of anemophilous plants cannot fail to be largely blown in every direction. In your list the Graminae, Cyperaceae, Amentaceae, Urticaceae, some Polygonaceae & some Plantagineae are the sole anemophilous plants. The Coniferae entirely belong to the same class, & lakes in the Tyrol are sometimes coated with the pollen of fir trees. I do not know whether you will care to receive these few hasty remarks. With my best respects I remain Dear Sir Yours faithfully

Ch. Darwin

Dr. Burdon Sanderson, who has been here, & to whom I showed your book, was much interested by what he had time to read.

P.S. I have now read a little more. Your investigations in the upper regions seem to me most ingenious & profoundly interesting. I add one or two trifling remarks

p. 148. I have seen an account of buckets full of coniferous pollen having been swept off the deck of a ship off the American coast; but I cannot give my reference.

p. 152. I shd think grains of pollen, after having forced their contents, through the tubes into the plumose stigmata of grasses, could be easily blown away by a strong wind, & would then consist of mere empty cases.

p. 157. Buckwheat is certainly an entomophilous plant & as it is dimorphic depends on Bees for its perfect fertilization. The wind cd rarely or never carry away much of its pollen.

C. D.

P.S. 2d. Riley (a good observer) says in his "Fifth Annual Report of the Noxious Insects of Missouri" that near St. Louis the ground appeared one day as if sprinkled with sulphur from the quantity of coniferous pollen, which must come from the fir-trees, 400 miles distant, then in flower in the S. States.