GERMINATION OF POLLEN 



349 



of two very flat cells, which soon break down (Fig. 204, B, v.c). 

 The remaining and larger portion of the pollen grain forms the 

 so-called tube cell (/.».). On reaching the nucellus the outer 

 membrane of the microspore is ruptured, and the tube cell 

 grows out to form the pollen tube (Fig. 203, A, p.t.), into whose 



mi. 



uc. 



Fig. 203. — Structure of the Ovule of Piniis. A, Diagrammatic longitudinal 

 section of mature ovule (after Co^ilter and Chamberlain). B, Front 

 portion of same greatly enlarged, showing a pollen tube penetrating 

 the nucellus (after Strasburgcr). C, Young archegonium showing the 

 layer of nutritive cells investing the egg (after Ferguson), a., neck 

 of archegonium; ar., archegonium; i., integument; mi., micropyle ; 

 n., nucellus ; 0., egg ; p., female prothallus ; p.t., pollen tube ; s., stony 

 layer of integument ; v.c, ventral canal cell. 



tip its nucleus wanders, whilst simultaneously the antheridial 

 cell divides into two, the inner half {body cell) subsequently 

 forming the two male cells. 



The growth of the pollen tube into the nucellus soon ceases, 

 and is not resumed till a short time before fertilisation. This 

 interval between pollination and fertilisation is comparatively 



