510 



BOTANY 



PART II 



The MICROSPORES of Angiosperms (") before they are shed from the pollen-sac 

 form an antlieridial motlier-cell (Fig. 471 m) wliich is clearly delimited from the 

 large pollen-tube cell, but is not enclosed by a cell wall. When the pollen-grain 

 germinates on the stigma the antheridial cell passes into the pollen-tube, and its 

 nucleus sooner or later divides into two generative :uiclei which lie free in the 

 protoplasm within the pollen-tube. They are of an elongated oval or ellipsoidal 

 shajae and pass one after another down the pollen-tube. The nucleus of the pollen- 

 tube {k) is usually visible in the neighbourhood of the generative nuclei. The 

 absence of the small prothallial cells, and of a sterile sister-cell of the antheridium. 



Fio. 472.— Development of the embryo-sac in Polygonum, divaricatum. tn, Mother-cell of tlie 

 embryo-sac; emb, embryo-sac; st, sterile sister-cells; e, egg-cell ; s, synergidae ; j), polar- 

 nuclei ; n, antipodal cells; k, secondary nucleus of the embryo-sac; cha, chalaza ; nil, micro- 

 pyle ; ai, ii, outer and inner integuments ; fun, funiculus. (1-7 x 3-20, S x 135.) 



as well as the absence of a cell wall from the mother-cell of the antheridium, and 

 lastly the presence of naked generative nuclei instead of generative cells in the 

 pollen-tube, are points in which the Angiosperms differ from Gymnosperms. The 

 reduction of the male prothallium has thus gone so far that only the indispensably 

 necessary parts remain. 



The characteristic differences which the Angiosperms show from the general 

 course of development of the macrospokangium in the Gymnosperms commence 

 with the cell divisions in the single, functional, macrospore mother-cell resulting 

 from the tetrad division (Fig. 472, 5). The "ruiMAUY nuclecs of the embryo- 

 sac " divides and the daughter-nuclei separate from one another. They divide twice 

 in succession so that eight nuclei are present. After this, cell-formation commences 

 around these nuclei (Fig. 472, 6-8). Both at the upper or micropylar end of the 



k 



