SECT. II 



THANEROGAMIA 



503 



sterile sister-cell (.s) adjoining the vegetative cells (Fig. 459 B, D). It is by the 



Fig. ib9.—A-E, Pollen-grains of Gymno.sperms and stages in the development of the pollen-tube. 

 A, Pollen-grain of Gi»i/.!/o 6i?(i6a still enclosed in the sporangium (x 360). B, Pollen-grain of 

 Larix europaea germinating on the tip of the nucellus ( x 160). C, D, Firms Laricio (after Coulter 

 and Chamberlain ; x 300). E, Pinus strohus (after M. 0. Ferguson ; x 270). Tip of a pollen- 

 tube which lias readied two-thirds of the way to the archegoniuin. c, Vegetative, prothallial 

 cell ; sp, spermatogenous cell which gives rise to the antheridial mother-cell (/n) and its sterile 

 sister-cell (s) ; ;/, the two generative cells derived by the division of the antheridial mother-cell. 

 They are of unequal size and enclosed in the same mass of protoplasm ; k, nucleus of the 

 pollen-tube. 



breaking down of this latter cell that the antheridial mother-cell becomes free to 

 pass into the pollen-tube. On this taking place, or while the latter cell is still in 





Fio. iCO.—Ceratozainia longifoUa. Longitudinal 

 section through a young ovule, showing the 

 nucellus (n), including a large group of 

 sporogenous cells (spor.) and the integument 

 (i). (After Treub. x 33.) 



its original position, it divides into two daughter-cells (</) ; these are the genera- 



FiG. AiH.—Taxushaccata. Longitudinal section 

 through the sporogenous tissue, showing an 

 embryo-sac mother-cell which has under- 

 gone the tetrad division ; three of the facul- 

 tative macrospores are degenerating', while 

 the fourth is undergoing further develop- 

 ment, (x 250.) 



