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 
