STROBILI 453 
the back or lower side of the microsporophylls are the micro- 
sporangia, usually two, and each contains numerous microspores. 
Nearly opposite each other on the microspore are two air-sacs 
whereby the spores are easily carried by the wind. When the 
spores are mature, the microsporangia or pollen sacs open by 
longitudinal slits, and the pollen shatters out, often like small 
Fic. 402. The staminate structures of the Pine. A, cluster of staminate 
strobili (X about 3). B, astaminate strobilus enlarged, showing the arrange- 
ment of the microsporophylls. C, a microsporophyll, showing the two 
sporangia (m); D, microspore showing the two wings and two cells of the male 
gametophyte. 
clouds of dust. The wind carries the pollen about, and some 
reaches the ovulate strobili, but much the larger part of it is 
wasted. Sometimes pollen accumulates on walks under Pines 
that are shedding their pollen until the walks look as if they had 
been sprinkled with finely powdered sulphur. 
The ovulate strobili or cones appear near the tips of the new 
growths in early spring. Usually they are smaller when they 
first appear than the staminate cones, but they persist and, after 
a growth of two seasons, become the conspicuous scaly cones so 
familiar on or about pine trees. Sometimes several occur to- 
