160 



SYSTEMATIC BOTANY. 



■when fertilization is to take place, allows the entrance of 

 the pollen-tube. 



i8i. Simultaneously with the growth of the integuments 

 is developed, near the upper end of the body, or nucleus of 

 the ovule (Fig. 276), the embryo-sac (Fig. 276, em. s). 

 This, when first distinguishable from the other tissue, 

 consists of a cell somewhat larger 

 than the adjacent ones. It then 

 enlarges greatly ; voluminous vacu- 

 oles appear, and the protoplasm con- 

 denses at the two extremities. In 

 its apical portion, one or more 

 roundish masses of protoplasm be- 

 come differentiated, and these are 

 the germ-cells, the impregnation of 

 one of which results iu the pro- 

 duction of the embryo. The firm, 

 outer coat of the pollen grain (Fig. 

 276, p) becomes ruptured upon 

 absorption of moisture, the inner 

 coat protrudes, and lengthens into a 

 slender tube (Fig. 276, tu), which 

 grows (Fig. 276, pol. tu) down 

 through the micropyle (Fig. 276, mie) till it comes in 

 contact with the embryo-sac (Fig. 276, em. s). Fertiliza- 

 tion is the result of this contact, whereupon an embryo is 

 formed (Fig. 277, i). This consists of a short stem 

 (caulicle. Fig. 277, cau), bearing one or more rudimentary 

 leaves, called the cotyledons (Fig. 277, cot), at one end, and 



Fig. 2'77. Successive stages (/, //, ///) in the development of the Bean plantlet ; 

 cpi, cotyledons; cau. caulicle; //.plumule; rd^ radicle; /y, hypocotyledonary 

 stem ; /vs, the first set of leaves ; /K, a portion of root, with root-hairs slightly 

 magnified. 



