204 EVOLUTION OF PLANTS 



organs, stem, root, and cotyledons, are readily made out. 

 The embryo may be imbedded in the endosperm and 

 not occupy the whole of the seed-cavity (Fig. 48, 

 D), but more often, perhaps, the endosperm is com- 

 pletely absorbed before the seed ripens, and the large 

 embryo fills the seed completely, as we see in all legu- 

 minous plants. In such embryos the cotyledons are 

 very large and thick, and their cells are filled with 

 starch and other food-substances which are used up in 

 the early stages of germination (Fig. 48, C). 



The cotyledons usually differ a good deal in shape 

 from the later leaves (Fig. 48, A), which gradually 

 acquire their perfect form. The cotyledons, where the 

 embryo fills the seed, are, as we have seen, thick and 

 fleshy, with obscure veins ; but where the embryo does 

 not fill the seed, and endosperm is present, they are 

 usually more like the later leaves, being thin with 

 prominent veins, as in the morning-glory. 



None of the Dicotyledons occur as submersed marine 

 plants, but otherwise they are found in nearly every 

 situation where plants can grow at all. They may be 

 completely immersed in fresh water, e.g. bladder-weed 

 (Utricularia), or the leaves may float as in the water- 

 lilies, while many of them are inhabitants of swamps, 

 where they are more or less completely submerged. 

 Many of them live in the sand of the seashore, while 

 others are desert plants. The various forms of sage- 

 brush and cacti of our own Western arid regions are 

 excellent types of these " xerophytic ' Dicotyledons. 

 In these the evaporating surface is greatly reduced by 

 the minute size of the leaves, and loss of water is further 

 retarded by excessive thickening of the outer tissues of 



