THE SPERM A TOPII VTA 



347 



structure (the stigma) which is nearby. Since the egg cells 

 are buried in the nuccllar tissue, it is evident that the male 

 gametes cannot approach them directly, as in the lower plants, 

 and a new structure has accordingly been developed which 

 conveys them to the egg. This is the pollen-tube. It arises 

 from the pollen-grain as a slender, thin-walled projection and 

 into it the contents of the gi'ain passes, led by the tube-nucleus. 



Fig. 218. — Development of the male gametophyte in a gymnosperm (Pine). 

 A, longitudinal section (diagrammatic) of a staminate or "male" cone. B, one 

 of the sporophylls from A, much enlarged. The sporangium is filled with micro- 

 spores or pollen, in various stages of development. C, section across the sporo- 

 phyll at right angles to that in B, showing the two sporangia which are borne on 

 each sporophyll. D, a mature pollen grain. The outer wall (in the case of pine 

 pollen) is p^iffed out somewhat at two points, forming balloon-like "wings" 

 which add to the buoyancy of the grain. The single nucleus of the megaspore 

 has now divided to form the generative cell, lying next the wall, and the tube 

 nucleus, in the center of the cell. E, a germinating pollen grain. The tube 

 nucleus follows close behind the end of the growing tube. The generative cell 

 has divided into a stalk cell (lighter) and a body cell (darker). F, the end of the 

 pollen-tube just before fertilization. The body cell has developed into the 

 two male cells or gametes, which have now come down the tube and are ready to 

 effect fertilization. 



Fertilization and Seed-production (Figs. 217 and 229). — The 

 poll(Mi tube grows rapidly and penetrates the tissues, much as a 

 fungus filament penetrates the tissues of its host, until it reaches 

 the embryo-sac and the egg-cells. A large nucleus in the tube, 

 often the only one at this point except the tube nucleus, now 

 divides into two male gametes, and as the end of the tube bursts 



