LIFE HISTORY OF A GYMNOSPERM 
25 
sheath. These foliage leaves persist until the end of their second 
year when they are shed with the spur shoot which bears them. 
The white pine, like most of its allies among the Coniferae, bears 
cones. These structures are of two kinds, viz.: staminate and car- 
pellate. Both kinds are produced on the same tree. 
Fig. 6. — -Transverse section of pine stem in third year of growth showing 
xylem (a), cork (6), cortex (c). secretion reservoir (d), cambium ( e ), phloem (/), 
and pith (g). (Photomicrograph.) 
Staminate Cones. — 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 (micro sporophylls or stamens). On the 
under surface of each scale are the spore-cases (micros por angia) , 
which develop the microspores (pollen grains). Each pollen grain 
when mature consists of a central fertile cell and a pair of air-sacs 
