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 are the spore-cases, 

 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 {Piniis sylveslris). 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). 

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