SEED-BEARING PLANTS 417 
branches, consisting of the main axis, bearing the scales 
(microsporophylls, or stamens) arranged in spirals. On 
the underside of the microsporophyll is the spore-case, 
containing the microspores. The staminate cones pass the 
winter inside the bud, and during this period the sporangia 
contain only spore-mother-cells. The tetrad-division of 
FIG. 307. Scotch pine (Pinus syhestris} . Showing carpotropism of 
carpellate cones. Young cones at the left; cones one year old at the right. 
(Cf. Figs. 308 and 309.) 
the spore-mother-cells, occurs early in the spring, the exact 
time varying with the species, the locality, and the 
character of the season, but the young microspores are 
usually all formed by the first of May. On either side 
of the main body of the microspore is a tiny air-sac which 
gives the spore great buoyancy in the air (Fig. 313). 
27 
