My dear Sir

I hope you will excuse my troubling you with the following ideas with regard to the fertilisation of flowers:—

It has long been a source of speculation with me to account for the enormous proportion of flowers which do not produce seeds, notwithstanding abundant pollination. Take for instance such trees as the Laburnum & such common fruit trees as the apple, pear, & peach; I suppose it would be a great exaggeration to say that in the best of seasons as much as 5 percent of the flowers “set”; this must be presumably in consequence of the non-fertilisation of the ovules, notwithstanding abundant pollination; the stigmas being in all these cases abundantly covered with pollen when the flowers are out; & altho this may commonly be, (especially in bad seasons, when there are comparatively few insects) principally pollen from the same flower; yet I think we have little warrant for supposing that the pollen of its own flowers is absolutely inoperative in fertilising the seeds, at all events when the seeds are few in number, as in all the instances named.

During the present season the idea has come to me with the force of conviction that the cause of this sterility is the access of too great a quantity of pollen. It seems fair to argue, from the analogy of the animal kingdom that the subjection of a female to impregnation from a number of males in succession has an almost inevitable tendency to produce sterility; my chief difficulty is that if so simple a cause as this operates in the vegetable kingdom, it is almost incredible that I should not find some reference to it in your writings, altho it is quite possible that if such reference does occur, I may have overlooked it. My principal object in writing to you now is to ask you whether your own experience supports or contradicts this hypothesis, or whether you are able to refer me to practical cultivators with whom you may have been in correspondence & who may have paid special attention to this question.¹

My hypothesis seems to me to derive some support from the following consideration:— Although it is very frequently the case that say not one blossom in a hundred of an apple or pear tree produces even the rudiment of fruit, yet, when they are fertilised, is it not rare to find less than the normal number of impregnated seeds in them? This would seem to point to the conclusion that the cause of failure is something that affects the whole ovary & not the particular ovules, & this would be the case if we suppose that the sterility is caused by the too great accumulation of pollen on the stigmas.

The theory would lead to the conclusion that the efforts of Nature to fructify the plant must be based on a balancing of two efforts, the access of pollen to the stigma & the prevention of the access of too much pollen. We should have then to regard insect agency as directed to both these ends; & although in many cases the primary object of the visits of insects is cross-fertilisation, & this may always be a secondary object, yet in other cases the primary object will be the absolute removal of the excess of pollen.² The same object is of course gained by the unisexuality of flowers, by the contrivances of protandry & protogyny, & of the successive approach of the stamens to the pistil, as in Ruta³ & Saxifraga, in their occasional approach, as in Berberis. Is not the quantity of pollen in self-fertilised cleistogamic flowers always small; & are not these almost always fertile?⁴

Will you also allow me to ask you whether you have yourself observed, what we often see drawn, the passage of a bundle of pollen-tubes from the base of the style into the cavity of the ovary, & what magnifying power & mode of preparation you find most convenient for observing the passage of the tubes?⁵

Pray do not take the trouble to write me a long reply; my chief motive in writing to you is the chance that you will be able, by refering me either to your own observations or to those of some practical horticulturalists; to set the question entirely at rest, or to indicate some series of research by which I may myself decide it one way or the other.

Believe me | very truly y^rs | Alfred W. Bennett

C. Darwin Esq