GROUP IV. — PHANEROGAMIA. 



449 



-pr 



however {e.g. Larch, Ginkgo, Fir, Ephedra), generally three cells 

 are successively cut off by parallel septa (Fig. 290) : of these, 

 the two first formed are merely vegetative prothallial cells, and 

 undergo disorganisation, whilst the last is the antheridial cell, and 

 undergoes division into a generative cell and a stalk-cell. In 

 some cases (Ginkgo, Ephedra, Cycads) the second prothallial cell 

 persists ; and it appears that in the Gnet^ceae the antheridial cell 

 does not divide to form a stalk-cell but is actually the generative 

 cell. 



In both Angiosperms and Gymnosperms, the pollen-tube is 

 formed by 

 the o u t- 

 growth of 

 the large 

 vegetative 

 cell : in both 

 cases the 

 generative 

 cell (after 

 being set 

 free when 

 necessary) 

 enters the 

 pollen-tube, 

 together 

 with the ve- 

 getative nu- 

 cleus, and, 

 in Gymno- 

 sperms, 

 with the nu- 

 cleus of the 

 stalk - cell ; 

 the vegeta- 

 tivenucleus 



becomes disorganised (Fig. 289 B, n), whilst the generative cell 

 undeigoes division into two; either into two equal generative 

 cells, as is generally the case, or into two unequal cells only one 

 of which is generative {e.g. Taxas). More than one pollen- tube 

 may be developed from the microspore (Fig. 283). 



Thus the male individual in the Phanerogams is a prothallium 



71 



pr 



Fig. 290.— Development of the male prothallium of Ginkgo hiloha. A 

 Pollen-gian in which cell- division is proceeding whilst still in the 

 pollen-sac; ji nucleus: pr three prothallial cells, the innermost of 

 which is the antheridial cell. B Older i)olIen-grain developing a 

 poUen-iube ; i>i* prothallial colls; u nucleus of pollen-tube, (x 5U): 

 after Strasburger.) 



