$83 BOTANY. 



constitute the endosperm, and serve somewliat later to 

 nourish the growing emhryo. This nourishing tissue is 

 considered to be the homologue of the first stage (prothal- 

 lium) of the Fernworts, here greatly belated. 



490. The embryo in its growth gradually absorbs the en- 

 dosperm. In many cases growth is checked in the ripening 

 of the seed, before much of the endosperm is used up (Fig. 

 183, AtoD); in such seeds the embryo is small and poorly 

 developed. In other cases more (Fig. 133, .©to G), or in 

 still others all (Fig. 133, ^to J), of the endosperm is ab- 

 sorbed; in these the embryos are much larger and better 

 developed. Where endosperm remains in a seed, its cells 

 are generally filled with starch, or less frequently with 

 oily matters; where no endosperm remains, there is always 

 a storage of starch or oily matter in some part 'of the em- 

 bryo. While the embryo is growing inside of the ovule, 

 the outer ovule-coat generally becomes thicker and harder, 

 all the ovule-tissues become drier, and at last the hard, 

 dry ovule, now called a seed, separates at its base and falls 

 to the ground. 



491. The seed in germinating absorbs moisture, swells 

 up, and generally bursts its coat. The embryo resumes its 

 growth, sending out its root into the soil, and its stem and 

 leaves upward into the air. Where there is endosperm, 

 the embryo grows by absorbing food from it; where there 

 is no endosperm, the large embryo is strong enough to grow 

 for a time by using the store of food contained within itself. 

 In some cases (e.g., bean, squash, melon, etc.) all the leaves 

 withdraw from the seed-coat and appear above ground, 

 while in others the first one or two leaves (cotyledons) re- 

 main in the seed in the ground, only the succeeding leaves 

 coming up into the light and air, as in peas, wheat, etc. 



