1890.] on the Shapes of Leaves and Cotyledons. 107 



narrow seed containing 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 number 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 arrangement is 

 very different. The fruit is winged, the seed an oblate spheroid 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 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 (Ricinus), 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. 



Emarginate Cotyledons. 

 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 the walls of the 

 seed being somewhat thickened at the end, and projecting slightly 

 into the hollow of the seed, cause a corresponding depression in the 

 cotyledons. 



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

 tion 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 arrauged one over the other like two sheets of note-paper, the 

 radicle being folded along the edge. To this folding the emargma- 

 tion is due. If a piece of paper be taken, folded 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 w aimo w ev (Cheiranthus) the seed has 

 a similar outline, and yet the cotyledons are not emarginate. The 

 reason of this is that in the wallflower, the seed is more com- 

 pressed than in the mustard and radish, and the cotyledons are 

 not folded ; so that the whole, not the half, of each cotyledon, cor- 

 responds to the form of the seed. 



