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 Slrobus}. 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, ). 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 
