ons. 



392 SIB J. XTJBBOCK — PHTTOBIOLOGICAL OBSERTATIO^^S. 



make it aa advantage that the number of seeds produced by a 

 flower should be reduced, as, for instance, in the case of the 

 Cqmpositae, where the agglomeration of a number of flowers into 

 a single head, as in the Daisy, and their consequent diminution 

 in size, renders it an advantage that each floret should produce 



but one seed. 



The species with large seeds may, as already mentioned, be 

 divided into two groups — first, those in "which the embryo is 

 surrounded by perisperm, and, secondly, those in which it occupies 

 the whole seed. 



In the former the arrangement of the embryo presents no spe- 

 cial difficulties, as the perisperm simply fills up all vacant spaces. 

 In the Litter, on the contrary, Nature has to exercise much 

 ingenuity, and adopts various devices to fill up the whole space. 



One plan is to arrange the cotyledons face to face, and then 

 roll them up in a ball. This is adopted, amongst other cases, in 

 the Sycamore (fig. 52), and hence the long strap-like form of the 

 cotyled 



Another is to arrange the cotyledons face to fiice, and then double 

 them up, as in the Cabbage, Mustard^ Eadi^h (figs. 75-77), &c. 

 In a third the cotyledons are convolute edgeways, as in Calf/- 



can thus. 



In Lepidium sativum the cotyledons are trifid (figs. 4 and 126) ; 

 in Cordia they are thrown into plaits (fig. 87). In others we have 

 still more complex folds, as in the Beech. 



In such cases as the Lupine the cotyledons become so fleshy and 

 thickened, that they almost lose the ai)pearance of leaves ; in 

 this instance they are set free by the splitting of the testa. 

 "When, however, the testa does not readily split, and where m 

 large seeds there is no perisperm, the difficulty of unfolding the 

 cotyledons and extricating them from the seed becomes greater, 

 and we arrive at cases where Nature seems to have abandoned 

 the attempt, and, as in the Oak and Horse Chestnut, the coty- 

 ledons never quit the seed.' Thus, ^ among the Juglandese, 

 Fterocarya has leaf-like cotyledons, while those of the Walnut 

 never quit the shell. Every one, however, must have observed 

 the elaborate folds into which the two cotyledons are thrown 

 folds which seem to have no significance or importance now, and 

 which carry us back to a time when the Walnut, like the Ptero- 

 carya^ had foliaceous cotyledons. 



