146 



ELEMENTARY STUDIES IN BOTANY 



been noted that the mixture of white and red also appeared 

 in the very ears that had been pollinated, without waiting 

 for the embryo to develop a new plant. This was a mystery 

 until double fertilization was discovered, and it was found that 

 the red color of the pollen parent was in the endosperm, and 

 had been introduced by the sperm that 

 fertilized the endosperm nucleus. 



87. The embryo. The fertilized egg 

 develops the embryo, but in Angiosperms 

 two distinct types of embryo are developed, 

 which give names to the great groups of 

 Angiosperms. 



In one type of embryo, the tip of the 

 hypocotyl (see 75, p. 126), which is to 

 give rise to the root, is at one end of the 

 embryo, the stem tip is at the other end, 

 and the cotyledons (see 75), usually two 

 in number, develop on the side of the 

 embryo just below the stem tip (Fig. 116, 

 A). The Angiosperms having this kind of 

 an embryo are called Dicotyledons, and they 

 are very much the more numerous group. 



In the other type Of embryo, the hy- 

 pocotyl tip is at one end, the solitary 

 cotyledon is at the other, and the stem tip 

 develops on the side of the embryo (Fig. 116, B). The 

 Angiosperms having this kind of embryo are called Mono- 

 cotyledons. 



It must not be supposed that the difference between Di- 

 cotyledons and Monocotyledons depends upon the number of 

 cotyledons, as the names might imply, but on the relative 

 position of the stem tip and cotyledon in the two cases. For 

 example, a Dicotyledon, while it usually has two cotyledons, 

 may have more, or it may have only one ; bul if the stem tip 

 is terminal rather than lateral, it is a Dicotyledon. On the 



FIG. 118. Pod of 

 sweet pea burst- 

 ing open to dis- 

 charge its seeds. 

 After GRAY. 



