SPERMATOPIIYTES : ANGIOSPERMS 



369 



which divides and begins the formation of the endosperm, 

 a tissue that feeds the embryo and is often the nutritive 

 part of seeds. In Angiosperms, therefore, there are two 

 simultaneous acts of fertilization, one starting the embryo, 

 the other the endosperm, and hence in this group " double 

 fertilization '' is said to occur. 



244. The embryo. — When the oospore germinates, a more 

 or less distinct suspensor is usually formed, but never so 

 prominent as in Gymnosperms ; and at the end of the 

 suspensor the embryo is developed, 

 which, when completed, is more or 

 less surrounded by nourishing en- 

 dosperm, or has stored wp within 

 its seed-leaves an abundant food 

 supply. 



The two groups of Angiosperms 

 differ widely in the structure of the 

 embryo. In Monocotyledons the axis 

 of the embryo develops the root-tip at 

 one end and the " seed-leaf " (cotyle- 

 don) at the other, the stem-tip arising 

 from the side of the axis as a lateral 

 member (Fig. 329). 



In Dicotyledons the axis of the 

 embryo develops the root-tip at one 

 end and the stem-tip at the other, 

 the cotyledons (usually two) appear- 

 ing as a pair of opposite lateral 

 members on either side of the stem- 

 tip (Fig. 328). As the cotyledons 

 are lateral members their number 

 may vary. 



The axis of the embryo between the root-tip and the 

 cotyledons is called the hjipocotyl (Figs. 143, 315, 331), which 

 means " under the cotyledon," a region which shows pecul- 

 iar activity in connection with the escape of the embryo 



Fig. 329. Young embryo of 

 water plantain (Alisma), a 

 Monocotyledon, the root 

 being organized at one 

 end (next the suspensor), 

 the single cotyledon (O 

 at the other, and the stem- 

 tip arising from a lateral 

 notch {v). — After Han- 

 stein. 



