110 Sir John Luhhoclc [April 25, 



hollow spaces occupied by the seed, it gives the impression as if the 

 seed was thrown into folds occupied by the wall of the fruit. To 

 occupy these spaces fully, the cotyledons themselves were thrown 

 into folds as we now see them. The fruit of Pterocarya is much 

 smaller than that of the horse-chestnut, which doubtless was itself 

 formerly not so large as it now is. As it increased, the cotyledons 

 became fleshier and fleshier, and found it more and more difficult to 

 make their exit from the seed, until at last they have given up any 

 attempt to do so. Hence these curious folds, with which we are so 

 familiar, are the efforts made by the originally leafy cotyledons to 

 occupy the interior of the nut. If you separate them, you will easily 

 find the little rootlet, and the plumule with from five to seven pairs 

 of minute leaves. 



But perhaps you will ask me why I have assumed that in these 

 cases the cotyledons have conformed to the seeds ? May it not be 

 that the seed is determined, on the contrary, with reference to the 

 cotyledons? The size, form, &c., of the seeds, however, evidently 

 have relation to the habits, conditions, &c., of the parent plant. 



Let me, in illustration, take one case. The cotyledons of the 

 sycamore are long, narrow, and strap-like ; those of the beech are 

 short, very broad, and fan-like. Both species are aperispermic, the 

 embryo occupying the whole interior of the seed. 



Now, in the sycamore, the seed is more or less an oblate spheroid, 

 and the long ribbon-like cotyledons, being rolled up into a ball, fit 

 it closely, the inner cotyledon being often somewhat shorter than the 

 other. On the other hand, the nuts of the beech are more or less 

 triangular; an arrangement like that of the sycamore would there- 

 fore be utterly unsuitable, as it would necessarily leave great gaps. 

 The cotyledons, however, are folded up like a fan, but with more 

 complication, and in such a manner that they fit beautifully into the 

 triangular nut. 



Can we, however, carry the argument one stage further ? Why 

 should the seed of the sycamore be globular, and that of the beech 

 triangular? Is it clear that the cotyledons are constituted so as to 

 suit the seed ? May it not be that it is the seed which is adapted to 

 the cotyledons ? In answer to this we must examine the fruit, and 

 we shall find that in both cases the cavity of the fruit is approxi- 

 mately spherical. That of the sycamore, however, is comparatively 

 small, say ^ inch in diameter, and contains one seed, which exactly 

 conforms to the cavity in which it lies. In the beech, on the 

 contrary, the fruit is at least twice the size, and contains from two to 

 four seeds, which consequently, in order to occupy the space, are 

 compelled (to give a familiar illustration, like the segments of an 

 orange) to take a more or less triangular form. 



Thus, then, in these cases, starting with the form of the fruit, we 

 sec that it governs that of the seed, and that of the seed, again, 

 determines that of the cotyledons. But though the cotyledons often 

 f(dl()W the form of the seed, this is not invariably the case : other 



