CHAPTER 4 

 THE FEMALE GAMETOPHYTE 1 



Depending on the number of megaspore nuclei taking part in 

 the development, the female gametophytes of angiosperms may be 

 classified into three main types: monosporic, bisporic, and tetra- 

 sporic. In the first, only one of the four megaspores takes part in 

 the development of the gametophyte. In the second, two mega- 

 spore nuclei take part in its formation; and in the third, all four of 

 them. A further subdivision is based on the number of nuclear 

 divisions intervening between the time of megaspore formation and 

 the time of differentiation of the egg, and the total number of nuclei 

 present in the gametophyte at the moment when such differentiation 

 takes place. A secondary increase in their number, which sometimes 

 takes place at a later stage, is not taken into account in this classi- 

 fication. 



The monosporic female gametophytes or embryo sacs fall under 

 two types: 8-nucleate and 4-nucleate. In the development of the 

 8-nucleate embryo sacs, the first division of the functioning mega- 

 spore gives rise to 2 nuclei : the primary micropylar and the primary 

 chalazal. The second division produces one pair of nuclei at the 

 micropylar end and one at the chalazal, and the third results in two 

 groups of 4 nuclei lying at the opposite poles of the elongated embryo 

 sac. The micropylar quartet differentiates into a three-celled egg 

 apparatus and the upper polar nucleus, and the chalazal quartet 

 into a group of three antipodal cells (or nuclei) and the lower polar 

 nucleus. The 2 polar nuclei fuse to give rise to a secondary nucleus. 



This type of embryo sac is the most common and is, therefore, 

 commonly designated as the "Normal type." However, since the 

 others are by no means so infrequent as was once supposed, it will 

 be designated here as the "Polygonum type," for it was in Polygo- 

 num divaricatum that Strasburger (1879) gave the first clear and well- 



1 In writing this chapter the author has drawn freely upon some of his review 

 articles (Maheshwari, 1937, 1941, 1946a,6; 1947, 1948), to which reference may be 

 made for fuller information. 



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