DIV. n 



SPERM ATOPHYTA 



571 



shape 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, 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. 



mi 



FIG. 550. Development of the embryo-sac in Polygonum diraricatum. m, Mother cell of the 

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

 nuclei ; a, antipodal cells ; fc, secondary nucleus of the embryo-sac ; cTia, chalaza ; mi, micro- 

 pyle ; ai, ii, outer and inner integuments ; fun, funiculus. (1-7 x 320, 8 x 135. After 

 STRASBURQER.) 



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

 the indispensably necessary parts remain. 



(b) MACROSPORES. -- The characteristic differences which the 

 Angiosperms show from the general course of development of the 

 MACROSPORANGIUM in the Gymnosperms commence with the cell 

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

 from the tetrad division (Fig. 550, 1-5). The "PRIMARY NUCLEUS 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. 550, 6-8). Both at the upper or micropylar end of the embryo- 



