My dear Sir

I should think you could make an interesting paper about spiny trees.² I had a vague notion about plants varying by becoming more spinose in dry & hot climates, but I never put two & two together, as you have done, in relation to the species of such countries being spinose. This is a point after my own heart.—   It is quite new to me about species of far removed orders being spinose in deserts.— The labour would be, I suppose, great; but I can hardly doubt that you might make a very interesting paper on the subject.— The simple fact of bushes & trees being in so marked a manner spinose in deserts alone struck me, from what I have myself seen & read. Livingstone, for instance, was much struck with the contrast in this respect between the plains of the southern parts of Africa & the more humid & intra-tropical parts.³

The explanation of the fact, seemed to me to be probably, that where vegetation was scanty, those plants alone could withstand the injury from browsing quadrupeds, which were protected by spines. Even our gorze is an instance of this, as being when bruised & chopped so eminently liked by horses.—   I have fancied that desert-plants were often strongly aromatic or strong-tasted for a similar purpose, viz protection.— This struck me much on stony mountains of Chile.—⁴

You allude to another interesting point, about unisexual plants having very fragrant & conspicuous plants.⁵ This would be a very curious point, but I sh^d. suppose very difficult to determine, as fragrancy is so indefinite.—   I have imagined or seen stated, that night-blowing flowers are often fragrant to attract insects. & often white. By your excluding orders in which all the species are unisexual you exclude in fact one source of doubt, viz the cases, where wind is sole agent of fertilisation— I have just thought of one strong exception the common-Holly, which is not fragrant or has conspicuous flowers, & is yet essentially unisexual.— Bees can certainly smell any sweet excretion. I fear there would be too many elements of doubts.—

I am not Botanist enough to follow out your ideas about definite & indefinite inflorescence.—   I am not sure that I understand “definite” & “indefinite”.—   I have long thought that dimorphous flowers (& told D^r. Hooker so)⁶ would be a very interesting subject for experiment.—   I once got the Russian violet to commence on; but want of time & my poor health prevented me doing anything. This very summer I have had Venus Looking glass⁷ under cover partly with this object, but the wet season spoiled this & several other little experiments. I shall never take it up now.—   The view which I meant to test was this: that the apetalous forms was self fertilised & therefore seeded abundantly; whilst the flowering kind was alone visited by insects, & either required (as in case of Violets & I believe all Campanulas) & was much benefitted by the visits of insects for its self-fertilisation; & thus incidentally received the benefit of an occasional cross.

This year I tried V. tricolor & it was marvellous the difference in seeding of the flowers, which were visited by Bees or artificially fertilised by me, & those which were untouched.—⁸ I think that you would find this an easy & interesting line of experiment.—

Very many thanks for your interesting letter | Yours sincerely | C. Darwin

There is an N. American Campanula with nearly apetalous flower, which seeds largely. How does pollen get on stigma?—