NARROW COTYLEDONS 



23 



The Sycamore (Acer Pseudo-Platanus) (fig. 9) has also 

 narrow cotyledons ; but the arrangement is very different. 

 The fruit (fig. 50) is winged, the seed somewhat obovoid and 

 exalbuminous 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. The mode of folding, 

 however, as shown in fig. 50, A and B, is not always the 

 same. I shall suggest a reason for the difference further on. 



FIG. 50. Acer Pseudo-Platanus. Fruit, nat. size. A, B, embryo, showing 

 two modes of arrangement of cotyledons. 



In other species the narrowness of the cotyledons is perhaps 

 an advantage in facilitating the exit from the seed. See, for 

 instance, the cases of Galium (p. 28) and Symplocos. 



BROAD COTYLEDONS. 



I now pass to species with broad cotyledons. The Acorn 

 Nut, Bean (Phaseolus) (fig. 51), and Pea afford familiar cases, 

 in which the two broad, fleshy, thickened cotyledons conform 

 to and occupy the whole seed. In the Castor-oil plant 

 (Ricinus) (fig. 11) the seed (fig. 52) is ovoid-oblong, somewhat 

 compressed dorso-ventrally, and beautifully mottled, while the 

 projecting knob at the hilum gives it very much the appear- 

 ance of a beetle or large tick. The endosperm is abundant, 

 fleshy, white, and surrounds the embryo. The embryo is 

 straight, flat, large, central, and white; the cotyledons broad, 



