THE SPERMATOPHYTES 447 
The white pine, like most of its allies among the Conifer, 
bears cones. ‘These structures are of two kinds, viz.: staminate 
and carpellate. Both kinds are produced on the same tree. 
STAMINATE ConeEs.—The yellow, ovate, staminate cones 
appear about May and are clustered at the base of the new 
growth of the current season. Each consists of a main axis 
(modified branch) which bears spirals of scales called microspor- 
ophylls or stamens. On the under surface of each scale are the 
Fic. 335.—Staminate cones of the Austrian pine (Pinus austriaca). Below, before - 
shedding pollen; above, after shedding. (Gager.) 
spore-cases (microsporangia), which develop the microspores (pollen 
grains). A section of the microsporangium shows it to consist of 
a wall beneath which is a layer of cells called the tapetum and in 
the interior a large number of spheroidal spore mother-cells. 
Each of the spore mother cells divides into four spores. During 
this process, reduction division takes place and the chromosomes 
are reduced to one-half the number, so that the spores are haploid 
as those of the fern. Each pollen grain when mature consists of 
a central fertile cell and a pair of air-sacs or wings, one on either 
side of the fertile cell. The purpose of the latter is to give greater 
buoyancy in the air to the microspore. 
