754 NATURAL SCIENCE. ^ec. 



other leaves of the plant, though still bearing the original cotyledon 

 at its apex. The accompanying figures of a species of Clarkia will 

 illustrate this (Fig. ii). 



The cotyledons, immediately after germination, are oblong- 

 orbicular, slightly auricled at the base and sessile ; they then enlarge 

 and become orbicular or broadly ovate, somewhat cuneate at the 

 base, and shortly petiolate, as seen in Fig. A, which represents a 

 specimen three days after germination. The new basal portion 

 grows as represented in B and C, five and ten days after germination 

 respectively, until it ultimately forms the greater part of the leaf 

 from which it is easily distinguished by its irregular ciliate margin, 

 as seen in D. Sireptocarpus (Fig. 12) (Gesneraceae) presents a some- 

 what similar case, but here one of the cotyledons only shows an 

 extraordinary development, and becomes the first foliage leaf. 



In the introduction, the author also discusses the shapes of 

 cotyledons, showing how the length, breadth, symmetry, equality, 

 crenation, emargination, lobing or fission, and presence of auricles 

 may often be explained by their having to be packed into a seed of 

 a given shape or successfully to make their exit from the same in 

 germination. It is, of course, open for one to say that the seed is 

 made to fit the embryo, not the embryo the seed, but, as Sir John 

 points out, the shape of the seed is often largely predetermined by 

 that of the fruit or the way in which they are packed ; and, more- 

 over, the devices for distribution or protection against injury by beast or 

 climate must often have had considerable influence. This last con- 

 sideration is well exemplified in two species of Galium. The shape of 

 the seeds is the same, but those of G. apanne have only a thin coat, as 

 there are no conditions of extraordinary severity to be endured, while 

 the seeds of G. saccharatum are provided with a tough corky coat, 

 to prevent drying up in the hot climate of North Africa. The 

 thin coat of apavine will tear easily in germination to allow the exit 

 of the cotyledons, which can therefore broaden out in the endosperm, 

 but the thick coat of saccharatum is too tough to be thus easily torn, 

 and the cotyledons therefore remain narrow, so that they can be 

 drawn out through a much smaller opening (Figs. 13 and 14). 



Narrow cotyledons are frequently a device for easily getting clear 

 of the seed, thus in Santalaceae " their narrowness seems to be due 

 to a difficulty of exit from the small opening of the woody fruit, for 

 there is plenty of space inside to allow of their attaining a much 

 greater width," and even then they often become torn in the process. 

 Compare also the acicular cotyledons of the pines (Fig. 15) and 

 firs, which seem to shake off the seed with difficulty. Broad, flattened 

 seeds often contain narrow cotyledons from the fact that the embryo 

 lies transversely, often curving round the edge as in Menispermum 

 or many Chenopodiaceae (Fig. 16) and Amarantaceae. In the case of 

 the sycamore, the strap-shaped cotyledon can be easily rolled up into 

 a ball and packed in the round seed (Fig, 17). 



