LETTER 675. TO F. MULLER. Down [received February 24th, 1867].

Your letter of November 2nd contained an extraordinary amount of interesting matter. What a number of dimorphic plants South Brazil produces: you observed in one day as many or more dimorphic genera than all the botanists in Europe have ever observed. When my present book is finished I shall write a final paper upon these plants, so that I am extremely glad to hear of your observations and to see the dried flowers; nevertheless, I should regret MUCH if I prevented you from publishing on the subject. Plumbago (675/1. Plumbago has not been shown to be dimorphic.) is quite new to me, though I had suspected it. It is curious how dimorphism prevails by groups throughout the world, showing, as I suppose, that it is an ancient character; thus Hedyotis is dimorphic in India (675/2. Hedyotis was sent to Darwin by F. Muller; it seems possible, therefore, that Hedyotis was written by mistake for some other Rubiaceous plant, perhaps Oldenlandia, which John Scott sent him from India.); the two other genera in the same sub-family with Villarsia are dimorphic in Europe and Ceylon; a sub-genus of Erythroxylon (675/3. No doubt Sethia.) is dimorphic in Ceylon, and Oxalis with you and at the Cape of Good Hope. If you can find a dimorphic Oxalis it will be a new point, for all known species are trimorphic or monomorphic. The case of Convolvulus will be new, if proved. I am doubtful about Gesneria (675/4. Neither Convolvulus nor Gesneria have been shown to be dimorphic.), and have been often myself deceived by varying length of pistil. A difference in the size of the pollen-grains would be conclusive evidence; but in some cases experiments by fertilisation can alone decide the point. As yet I know of no case of dimorphism in flowers which are very irregular; such flowers being apparently always sufficiently visited and crossed by insects.