222 - Multicellular Plants 



cells that represents the embryo sporophyte 

 (Fig. 12-19). 



While the embryo is growing, many 

 changes also occur in the several tissues sur- 

 rounding the embryo: (1) the endosperm cell 

 gives rise to a mass of endosperm tissue, 

 which lies in direct contact with the embryo 

 and provides it with organic food; (2) the 

 ovule, taken as a whole, becomes the seed; 

 and (3) the ovule chamber, also taken as a 

 whole, enlarges and becomes the fruit (Fig. 

 12-1-1). Accordingly the embryo sporophyte 

 (Fig. 12-19) lies near the center of the ripe 

 seed, surrounded by an endosperm, unless 

 this tissue is used up before the seed is ripe. 

 In the seed, however, the only body of cells 

 that arises from the fertilized egg is the 

 embryo itself; and only these diploid cells 

 are represented in the body of the mature 

 sporophyte, which develops after the seed has 

 sprouted. 



The Seed. Seeds are uniquely distinctive 

 of the Gymnospermae and Angiospermae. A 

 seed is a dormant embryo sporophyte, en- 

 closed in a cover that is derived chiefly from 

 the outer wall of the ovule. In fact, the ovule 

 as a whole gives rise to the seed as a whole. 

 Some seeds contain gametophyte tissue (the 

 endosperm), directly surrounding the embryo 

 and separating the embryo from the seed 

 cover (Fig. 12-19). But in many seeds, all the 

 substance of the endosperm is absorbed by 

 the growing embryo during the period of 



ripening, leaving no trace of endosperm in 

 the mature seed (Fig. 12-19). 



Seeds have played an important role in 

 permitting the seed plants to scatter them- 

 selves over wide areas of the earth. The 

 dormant embryo within some seeds may sur- 

 vive for more than a hundred years, and is 

 able to endure adverse conditions of dryness, 

 temperature, etc. Many seeds are equipped 

 with devices (wings, spines, etc.) that play 

 a part in successfully dispersing the species. 

 Moreover, because the embryo has reached a 

 fairly advanced stage of development in the 

 ripe seed, and because rapid growth is as- 

 sured by organic food stored in the seed, the 

 new sporophyte very quickly establishes it- 

 self as a sturdy independent plant soon after 

 the seed falls upon new ground, where con- 

 ditions are right for sprouting (Fig. 12-20). 



Structure of the Embryo: the Cotyledons. When 

 the protective coat of a bean seed is removed, 

 the embryo as a whole is exposed. The main 

 bulk of the embno consists of the two 

 cotyledons — that is, the two swollen fleshy 

 "halves of the bean." But between the cotyle- 

 dons, and not fully visible until one cotyle- 

 don is removed, lies the body of the embryo 

 (Fig. 12-21). 



The cotyledons are the storage leaves of 

 the embryo. The swollen fleshiness of these 

 embryonic leaves is due to large amounts of 

 starch and other organic compounds con- 

 tributed to the cotyledons by the endosperm 



Fig. 12-20. Germination of a 

 seed (bean), s, seed coats; r, 

 radicle; h, hypocotyl; c, cotyle- 

 don; e, epicotyl. 



