POLLEN-GRAINS AND OVULES 



135 



GRAIN, produced within the anther, and the female 

 cell is the oosphere or ovum, produced within the 

 ovule. This leads us to look into the structure of 

 the ovule and of the pollen-grain. 



In Chapter XIV the structure of the ovule has been 

 described so far as the formation of the embryo-sac. 

 The nucleus of the embryo-sac divides first into two 

 parts; next, each of the latter divides into two parts, 

 giving rise to four nuclei; and, lastly, each of these 



Fig-. 1T9. — Successive Changfes in the Embryo-sac of Polygonum prior to Fertilization 



^, Synergidas. o, Oosphere. a, Antipodal cells. ?*, Secondary nucleus 

 (after Strasburger) 



four divides into two; so ultimately eight nuclei are 

 formed. Three of them migrate to the micropylar end 

 of the embryo-sac, forming what is known as the egg 

 APPARATUS, three to the opposite or antipodal end, 

 forming the antipodal cells, and the remaining two 

 fuse together, forming the secondary nucleus of the 

 embryo-sac, which usually remains near the centre of 

 the embryo-sac (fig. 119). The nuclei at both the 

 ends gradually form naked cells. Two cells of the 

 egg apparatus are similar, and known as the guiding 

 cells or SYNERGID.^, and the third, which projects 

 into the cavity of the embryo-sac, is the female cell 

 or oosphere. Usually the centre of the embryo-sac 



