GROUPS OF PLANTS. 



89 



forest trees. After the deposition of the pollen grain on the stigma, 

 the tube-cell begins to form a tubular process (pollen tube) which 

 carries the male nuclei to the egg-cell (Fig. 85, i). It pierces 

 the tissue of the stigma (Fig. 85, /i) and traverses the style (Fig. 



jjtQlOSPERMS 



Fig. 58. Hypothetical tree of relationship and descent of the leading groups of plants. — 



After Ganong. 



85, g) until it reaches the micropyle of the ovule, which it enters 

 (Fig. 85, m), then reaching the nucellus it penetrates this, enter- 

 ing the embryo-sac. The tip of the tube breaks and one of the 

 generative nuclei which has been carried downward unites with the 

 egg, after which a wall is formed, giving rise to an oospore. The 



