SEED-BEARING PLANTS 



423 



larger cell, the tube-cell, composing the larger portion of 

 what is now the mature male gametophyte or pollen-grain. 

 Commonly one of the two prothallial cells disintegrates, 

 so that only one is visible (Fig. 313). Thus it is seen that 

 the vegetative portion of the male gametophyte is reduced 

 to nearly its lowest terms — only one or two cells of no 

 known function; the antheridium is also represented only 

 by its mother-cell. At this stage pollination occurs. 



375. Distribution of Pollen. — In all cone-bearing trees 

 pollination is accomplished by the wind. At about the 



Fig. 313. — The white pine {Pinus Strobns). Sections through mature 

 pollen grains; at the left the remnants of two prothallial cells can be seen, 

 while at the right all signs of the first cell have disappeared. Pollen col- 

 lected June 9, 1898. X about 600. (After Margaret C. Ferguson.) 



time the pollen-grain is mature the axes of the carpellate 

 cones elongate, thus separating the scales from each 

 other (Fig. 309, E), At the same period the axes of the 

 staminate cones elongate, separating the anthers from 

 each other (Fig. 306). The sporangial walls now become 

 opened by a longitudinal slit, and the least jarring of the 

 branch is sufficient to shake out the dry pollen-grains, 

 which appear in countless millions as a fine yellow dust, 

 the ''pollen.'' 



376. Abundance of Pollen. — The pollen is so abundant 

 that it forms a really dense cloud that is easily seen in a 

 photograph of a tree shedding its polien (Fig. 314). The 



