1404 A TEXTBOOK OF THEORETICAL BOTANY 



the average number in C. siiherosa to be about 300 (Fig. 1298). The lower 

 hmit of this mass is not sharply defined and some of the lowermost cells 

 may be of chalazal origin. They sometimes assume the character of 

 tracheids, forming a continuation of the end of the funicular vascular bundle. 

 Only a limited number of the sporogenous cells undergo meiosis, some 

 forming megaspore tetrads, and some, it appears, developing into embryo 

 sacs. The rest remain sterile, become elongated and are eventually sup- 

 pressed. A group of mature embryo sacs may be produced but generally 

 only one is fertilized. In Ouerciis, the growing nucellus is occupied by a 

 mass of between twenty and sixty archesporial cells (Fig. 1299). Several of 

 these may begin development but usually only one, four or five cells deep, 

 becomes a mother cell, the neighbouring cells disintegrating. The two 

 meiotic divisions follow each other rapidly, without cell divisions, and a four- 

 nucleate embryo sac results without the formation of separate megaspores. 

 The mature embryo sac is of a normal eight-nucleate type and, by the time 

 it is mature, all the nucellus, except at the base, has been resorbed and the 

 synergidae push up into the micropyle. 



The Development of the Embryo Sac 



The angiospermic embryo sac is the product of the germination of a 

 megaspore and is therefore the greatly reduced homologue of the female 

 prothallus. Although ten dififerent types are recognized, they are all vari- 

 ants of one basic pattern, a somewhat complex organization, which occurs 

 with surprising regularity in all the main groups of the Angiosperms and 

 constitutes one of the arguments in favour of their unity as an evolutionary 

 class. 



This basic pattern consists of an oval, thin-walled sac containing eight 

 nuclei and, when mature, somewhat scanty cytoplasm. Three of the nuclei 

 form a group at the micropylar end and are generally isolated by delicate 

 surrounding pellicles. The median nucleus functions as the oosphere and 

 gives rise to the embryo after fertilization. The two lateral nuclei were called 

 the synergidae by Strasburger, a name which signifies their function as 

 assistants or co-workers of the oosphere. These three are collectively called 

 the egg-apparatus. At the chalazal end is another group of three nuclei, also 

 generally surrounded by pellicles, which are called collectively the antipodals. 

 They play no part in fertilization and sometimes disappear quickly. In 

 other cases they may multiply or may become enlarged. They may have 

 some glandular or nutritive function or they may be simply vestigial. Their 

 status is uncertain. 



In the centre of the sac, between the two other groups, is formed a pair 

 of nuclei lying close together, usually called the polar nuclei. They lie free 

 in the cytoplasm of the sac and sometimes unite before fertilization. In all 

 cases they unite jointly with one of the two male generative nuclei from the 

 pollen tube, an act known as triple fusion, the product of which is the 

 primary endosperm nucleus. From this develops the endosperm, which, 



