84 



NATURE 



[May 2 2, 1890 



similar outline, and yet the cotyledons are not emarginate. 

 The reason of this is that in the wall-flower, Cheiranthus, 

 the seed is more compressed than in the mustard and 

 radish, and consequently the cotyledons are not folded ; 

 so that the whole, not the half, of each cotyledon corre- 

 sponds to the form of the seed. 



Lobed Cotyledons. 



The great majority of cotyledons are entire, but some 

 are more or less lobed. For instance, those of the mallow 

 are broadly ovate, minutely emarginate, cordate at the 

 base, and three-lobed or angled towards the apex, with 

 three veins, each running into one of the lobes. 



The embryo is green, curved, and occupies a great 

 part of the seed. The cotyledons are applied face to 

 face ; then, as growth continues, the tip becomes curved 

 and depressed into a median longitudinal furrow, the fold 

 of the one lying in that of the other. 



[Sir John then showed clearly by diagrams and paper 

 how the emargination arises, but it cannot be made clear 

 without illustrations.] 



The cotyledons of the lime are very peculiar. They 

 are deeply fivelobed, the central lobe being the longest ; 

 so that they are roughly shaped like a hand. The seed 

 is an oblate spheroid, resembling an orange in form, and 

 the embryo is embedded in semi-transparent albumen. 



The embryo is at first straight ; the radicle is stout 

 and obtuse ; the cotyledons ovate-obtuse, plano-convex, 

 fleshy, pale green, and applied face to face. They grow, 

 however, considerably, and when they meet the wall of 

 the seed, they bend back on themselves, and then curve 

 round, following the general outline of the seed. If 

 anyone will take a common tea-cup and try to place 

 in it a sheet of paper, the paper will, of course, be thrown 

 into ridges. If these ridges be removed and so much left 

 as will lie smoothly inside the cup, it will be found that 

 the paper has been cut into lobes more or less resembling 

 those of the cotyledons of Tilia. Or if, conversely, a 

 piece of paper be cut into lobes resembling those of the 

 cotyledons, it will be found that the paper will fit the 

 concavity of the cup. The case is almost like that of our 

 own hand, which can be opened and closed conveniently 

 owing to the division of the five fingers. 



Unequal Cotyledons. 

 In most cases the two cotyledons are equal, but there 

 are several cases in which one of them is larger than the 

 other. They had not escaped the attention of Darwin, 

 who attributed the difference to the fact '• of a store of 

 nutriment being laid up in some other part, as in the 

 hypocotyl, or one of the cotyledons." I confess that I do 

 not quite see how this affords any explanation of the 

 fact. The suggestion I have thrown out is that the differ- 

 ence is due to the relative position of the two cotyledons in 

 the seed, which in some cases favours one of them at the 

 expense of the other. Thus in the mustard they are 

 unequal, and, as we have already seen, they are folded 

 up, one inside the other. The outer one, therefore, has 

 more space, and becomes larger. In many other Crucifers, 

 though the cotyledons are not folded, they are what is 

 called "incumbent" — that is to say, they are folded on 

 the radicle, and the outer one has therefore more room 

 than the other. 



Unsyinmetrical Cotyledons. 

 In other cases, as in the geraniums, laburnum, lupines, 

 &c., there is inequality, not between the two cotyledons, 

 but between the two halves of each cotyledon. In the 

 geraniums this is due to the manner in which the cotyle- 

 dons are folded. In cabbage and mustard we have seen 

 that one cotyledon is folded inside the other ; in the 

 geranium they are convolute, one half of each being folded 

 inside one half of the other, the two inner halves being 

 the smaller, the two outer the larger ones. 



NO. 1073, vol- 42] 



In the laburnum, where the arrangement is very similar, 

 the inequality in the two sides of the cotyledon is due to 

 the inequality between the two sides of the seed. 



Subterranean Cotyledons. 



I have already observed that in some cases the cotyle- 

 dons occupy the whole of the seed, which, in more or less 

 spherical seeds is effected either by a process of folding 

 and packing, or by the cotyledons becoming themselves 

 more or less thickened, as in peas and beans, nuts and 

 chestnuts. This is the reason why such seeds fall more " 

 or less readily into two halves, the radicle or plumule 

 being so small in comparison as generally to escape 

 notice, though, if a horse-chestnut is peeled, the radicle 

 appears as a sort of tail. 



In some beans the cotyledons sometimes emerge from 

 the seed, sometimes remain underground. In others, 

 as also in the oak and horse-chestnut, they never leave 

 the seed, or come above ground : they have lost the 

 function of leaves and become mere receptacles of 

 nourishment. 



Did it ever occur to you to think, when you have been 

 eating walnuts, why their structure is so complex, and 

 why the edible part is thrown into those complicated 

 lobes and folds 1 The history is very interesting. 



In the walnut, the cotyledons now never leave the 

 seed, but in an allied genus, Pterocarya, they come 

 above ground as usual, and are very peculiar in form, 

 being deeply four-lobed. The reason of this is very 

 curious. The fruit is originally much larger than the 

 seed, but, as it approaches maturity, the hard woody 

 tissue disintegrates at four places, leaving thus four hol- 

 low spaces. Into these spaces the seed sends four pro- 

 jections, and into these four projections each cotyledon 

 sends a lobe. Hence the four lobes. 



Now in the walnut a very similar process takes place, 

 only the hollow spaces are much larger, so that, instead 

 of a solid wall, with 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 Httle 

 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 cotyle- 

 don being often somewhat shorter than the other. On the 

 other hand, the nuts of the beech are more or less tri- 

 angular : an arrangement like that of the sycamore would 

 therefore 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. 



