May 2 2, 1890] 



NATURE 



will take, for instance, the Spanish chestnut and the 

 black poplar. In the Spanish chestnut the stem is much 

 stronger than that of the beech. Consequently it can carry 

 a greater leaf-surface. But the distance between the 

 buds being about the same the leaves cannot be much 

 wider ; hence they are much longer in proportion, and 

 this gives them their peculiar sword-blade-like shape. 



Now, if we look at the end of a branch of black poplar 

 and compare it with one of white poplar, we are struck 

 with two things : in the first place, the branch cannot be 

 laid out on a sheet of paper so that the leaves shall not 

 overlap ; the leaves are too numerous and large. 

 Secondly, in the white poplar the upper and under 

 surfaces of the leaf are very difterent, the lower one 

 being covered with a thick felt of hair, which gives it its 

 white colour ; in the black poplar, on the other hand, the 

 two surfaces are nearly similar. 



These two characteristics are correlated, for while in 

 the white poplar the leaves are horizontal, in the black 

 poplar, on the contrary, they hang vertically. Hence the 

 I two surfaces are under very similar conditions, and con- 

 sequently present a similar structure ; while for the same 

 reason they hang free from one another. 



Let us again look for a moment at the great group of 

 Conifers. Why, for instance, do some have long leaves 

 and some short ones .' This, I believe, depends on the 

 strength of the twigs and the number of years which the 

 leaves last ; long leaves dropping after one, two, or 

 three years, while species with shorter ones retained 

 them many years — the spruce fir, for instance, 8 or 10, 

 Abies Pinsapo even as many as 18. 



[Here Sir John dwelt on and explained the forms of 

 several familiar leaves.] 



Seedlings. 



I now come to the second part of my lecture — the 

 forms of cotyledons. Anyone who has ever looked at a 

 seedling plant must have been struck by the fact that the 

 first leaves dififer entirely from those which follow — not 

 merely from the final form, but even from those which 

 i immediately follow. These first leaves are called coty- 

 ledons. The forms of many cotyledons have been care- 

 fully described, but no reason had been given for the 

 forms assumed, nor any explanation offered why they 

 should differ so much from the subsequent leaves. Klebs, 

 indeed, in his interesting memoir on " Germination/' 

 characterizes it as quite an enigma. 



Mustard and cress were the delight and wonder of our 

 childhood, but it never then occurred to me at least to 

 ask why they were formed as they are. So they grew, 

 and beyond that it did not occur to me, nor I think to 

 most, that it was possible to inquire. I have, however, I 

 think, suggested plausible reasons in many cases, some 

 of which I will now submit for your consideration. 

 Cotyledons differ greatly in form. 



Some are narrow, in illustration of which I may men- 

 tion the fennel and ferula, in the stalk or ferule of which 

 Prometheus is fabled to have brought down fire from 

 heaven. 



Some are broad, as in the beech and mustard. More- 

 over, some species have narrow cotyledons and broad 

 leaves, while others have broad cotyledons and narrow 

 leaves. 



Some are emarginate, as in the mustard ; lobed, as in 

 the lime ; bifid, as in Eschscholtzia ; trifid, as in the cress ; 

 or with four long lobes, as in Pterocarya. 



Some are unequal, as in the mustard ; or unsymme- 

 trical, as in the geranium. 



Some are sessile, and some are stalked ; some are large, 

 some small. 



Generally, they are green, leaf-like, and aerial, but 

 sometimes they are thick and fleshy, as in the oak, nut, 

 walnut, peas, beans, and many others, in which they never 

 quit the seed at all. 



NO. 1073. VOL. 42] 



Let us see, then, whether we can throw any light on 

 these differences, and why they should be so unlike the 

 true leaves. 



If we cut open a seed, we find within it the future 

 plant : sometimes, as in the larkspur, a very small oval 

 body ; sometimes, as in the ash, or the castor-oil, a lovely 

 little miniature plant, with a short stout root and two 

 well-formed leaves, inclosing between them the rudiment 

 of the future stem ; the whole lying embedded in food- 

 material or perisperm ; while sometimes the emhryo oc- 

 cupies the whole interior of the seed, the food-material 

 being stored up, not round, but in the seed-leaves or 

 cotyledons themselves. Peas and beans, almonds, nuts, 

 and walnuts are familiar cases. In split peas, for in- 

 stance, — who split the peas ? If you look at them you 

 will see that it is too regularly and beautifully done for 

 human hands. In fact, the two halves are the two fleshy 

 cotyledons : strictly speaking, they are not split, for they 

 never were united. 



Narrow Cotyledons. 



Let us now begin with such species as have narrow 

 cotyledons, and see if we can throw any light on this 

 characteristic. The problem is simple enough in such 

 cases as the plane, where we have, on the one hand, 

 narrow cotyledons, and, on the other hand, a long narrow 

 seed fully occupied by a straight embryo. Again, in the 

 ash, the cotyledons lie parallel to the longer axis of the 

 seed, which is narrow and elongated. Such cases are, 

 however, comparatively few ; and there are a large num- 

 ber of species in which the seeds are broad and even 

 orbicular, while yet the cotyledons are narrow. 



In these it will generally be found that the cotyledons 

 lie transversely to the seed. 



The sycamore has also narrow cotyledons, but the 

 arrangenient is very different. The fruit is winged, the 

 seed somewhat obovoid and aperispermic— that is to say, 

 the embryo, instead of lying embedded in food-material, 

 occupies the whole cavity of the seed. Now, if we wished 

 to pack a leaf into a cavity of this form, it would be 

 found convenient to choose one of a long strap-like shape, 

 and then roll it up into a sort of ball. This is, I believe, 

 the reason why this form of cotyledon is most suitable 

 in the case of the sycamore. 



Broad Cotyledons. 



I now pass to species with broad cotyledons. In the 

 castor-oil plant, Euonymus, or the apple, for instance, 

 the young plant lies the broad way of the seed, and the 

 cotyledons conform to it. In the genus Coreopsis, 

 Coreopsis auriculata has broad cotyledons, and Coreopsis 

 filifolia has narrow ones— the first having broad, the 

 second narrow seeds. 



In a great many species the cotyledons are emarginate 

 — that is to say, they are more or less deeply notched at 

 the end. This is due to a variety of causes. One of the 

 simplest cases is that of the oak, where the two fleshy 

 cotyledons fill the seed ; and as the walls of the seed are 

 somewhat thickened at the end, and project slightly into 

 the hollow of the seed, this causes a corresponding 

 depression in the cotyledons. 



In such cases as the mustard, cabbage, and radish, the 

 emargination is due to a very different cause. The seed 

 is oblong, thick, and slightly narrower at one end than 

 the other. There is no perisperm, so that the embryo 

 occupies the whole seed, and as this is somewhat deep,, 

 the cotyledons, in order to occupy the whole space, are 

 folded and arranged one over the other like two sheets of 

 note-paper, the radicle being folded along the edge. To 

 this folding the emargination is due. If a piece of paper 

 be taken, lolded on itself, cut into the form of the seed, 

 and then unfolded, the reason for the form of the cotyledon 

 becomes clear at once. 



But it may be said that in the wallflower the seed has a 



