PROBLEM 3. Hom^ Cojnplex Flams 



Stigma 



Pollen tube 

 Micropyle 



zed egg eel 



Fertilized 



ble nucleus 



Fig. 392 Section through pistil after fertiliza- 

 tion has occtirred. Only one ovule and one 

 complete pollen tube are shown. Compare with 

 diagra?7is of ovule and pollen (Figs. ^89 and 

 391). 



do not enter dry up and disappear. 

 When the embryo sac is reached by the 

 pollen tube, the cell wall at the end of 

 the tube dissolves. The sperm nuclei pass 

 into the embryo sac and one of them 

 fuses with the t^^ nucleus, producing 

 the fertilized tg^ cell. 



The second sperm nucleus sometimes 

 unites with the double nucleus lying in 

 the center of the embryo sac. Since the 

 double nucleus is not an egg nucleus, 

 this union does not result in a fertilized 

 Q^^ cell or zygote. The final result in 

 some plants is a large group of food-stor- 

 ing cells called endosperm. Sometimes 

 the second sperm nucleus does not fer- 

 tilize the double nucleus. 



Almost always, no doubt, more grains 

 settle on the stigma than there are ovules; 

 then the first tube to reach the ovule is 



Reproduce 44:5 



the one to enter. Its nucleus fertilizes the 

 Qgg cell in the ovule. If some ovule is not 

 reached by a pollen tube its egg cell will 

 not be fertilized. 



The embryo plant is formed. The fer- 

 tilized tg^ (zygote) becomes surrounded 

 by a cell wall, and a series of divisions 

 begins. Division occurs again and again 

 until a mass of cells is formed; the proc- 

 ess is somewhat like that of cleavage in 

 an animal. However, these cells do not 

 later arrange themselves in the form of 

 a hollow ball or cup; such embr\"onic 

 stages are found in animals only. Differ- 

 entiation does occur; some groups of 

 cells become pith, some become phloem, 

 some xylem, some epidermis. Soon the 

 tiny root, stem, and leaf become appar- 

 ent. Although embryos are small and 

 often curled up, the new plant parts can 

 usually be recognized in the seed. The 

 leaves are still without color since the 

 embryo is located within the ovule, 

 \\'hich is, as you recall, inside the ovary 

 of the pistil where no light can reach it. 

 The embryo, though small, is consider- 

 ably larger than the ^^g cell from which 

 it grew. The food used in this growth 

 enters it from the surrounding tissues. 



In seeds such as corn and wheat the 

 endosperm grows during the develop- 

 ment of the embryo. In other seeds such 

 as beans and peas the endosperm fails to 

 develop. To study seeds with and with- 

 out endosperm, use the bean seed and the 

 kernel of com. See Exercise 7. By doing 

 Exercise 8 you can determine the im- 

 portance of the cotyledons in seeds 

 which lack endosperm. 



Other changes in the flower. The de- 

 velopment of the embryo occupies many 

 days or weeks, and in the meantime the 



