[Chap. XXXm SEXUAL REPRODUCTION IN FLOWERING PLANTS 377 



Fig. 168. Stages in the development of the embryo of shepherd's pmse (Bursa) 

 from the fertiUzed egg to the matme seed. Drawings by Mabel Schaffner. 



of the ovule coats result in the seed coats which enclose the endosperm 

 and embryo. These three structures — embryo, endosperm, and seed coat 

 — are the principal parts of the seed. The seeds of numerous plants, how- 

 ever, consist of an embrvo surrounded bv seed coats onlv. Growth and 

 various other changes in the ovulary result in the formation of a fruit. 



