LIFE HISTORY OF AN ANGIOSPERM 55 



the exit door for the hypocotyl of the embryo after the fertilized 

 and ripened ovule becomes a seed. Within the nucellus, if the sec- 

 tions examined have been properly fixed, will be found a megaspore 

 or embryo sac. 



Development of the Female Gametophyte Through the Matura- 

 tion of the Embryo Sac. In its immature condition the embryo 

 sac (megaspore) contains a mass of protoplasm surrounding a nucleus. 

 This nucleus undergoes three divisions forming as a result eight 

 nuclei which ultimately arrange themselves within the protoplasm 

 of the embryo sac as follows: three of them occupy a position at the 

 apex, the lower nucleus of the group being that of the egg or ovum, the 

 other two nuclei being the synergids or assisting nuclei; at the oppo- 

 site end of the sac three nuclei known as antipodals take their posi- 

 tion; the two remaining nuclei called polar nuclei take up a position 

 near the center of the embryo sac. In this condition the contents 

 of the embryo sac constitutes the female gametophyte. See Fig. 

 28 (1-8). 



MATURATION OF THE POLLEN GRAIN AND FORMATION OF THE 

 MALE GAMETOPHYTE 



The pollen grains (microspores), within the anther sacs, all arise 

 from a number of tetrads (groups of four) which are formed by the 

 division and redi vision of pollen mother-cells preceeding them. Each 

 pollen grain, after the tetrads have separated into their components, 

 consists of an outer firm wall or exosporium, an inner wall or endo- 

 sporium, within which will be found the. region called the fovilla, 

 which is nothing other than a mass of protoplasm containing a 

 nucleus. Before the pollen is shed from the anther its protoplasmic 

 contents undergo a series of changes leading up to the development 

 of the male gametophyte. The nucleus and protoplasm enveloping 

 it divides to form two cells, one a generative-cell containing a genera- 

 tive nucleus, the other a tube-cell containing a tube nucleus. The 

 generative nucleus then divides to form two sperm nuclei and the 

 partition wall between the two cells disappears. In this condition 

 the protoplasmic contents of the pollen grain constitute the male 

 gametophyte. 



