SECT. 11 



THANEROGAMIA 



481 



The macrosporophylls form the female, the microsporophylls the male 

 flowers. The two sexes are found either on the same individual 

 (monoecious), or each plant bears either male or female flowers 

 (dioecious). Leaves forming an envelope around the group of 

 sporophylls are only found in a few flowers of the Gymnospermae. 

 (Gnetaceae). 



The MALE flowers are shoots of limited length, the axis of which 

 bears the closely crowded and usually spirally arranged sporophylls. 

 The scales which invested the flower in the bud often persist at the 

 base of the axis (Fig. 424). The microsporangia are borne on the 



Pio. 424. — rinus montanu. A, Longitudinal section of a ripe male flower (x 10). B, Longitudinal 

 section of a single stamen ( X 20). C, Transverse section of a stamen (x 27). Z>, a ripe pollen- 

 grain of Piniis silvestris ( x 400). 



lower surface of the sporophylls, two or more being present on each. 

 Their opening is determined as in the sporangia of the Pteridophyta 

 by the peculiar construction of the outer layer of cells of the wall 

 (exothecium). The pollen-grains are spherical, and are frequently 

 provided with two sacs filled with air which increase their buoyancy 

 and assist in their distribution by the wind (Fig. 424). On germina- 

 tion the outer firm layer of the wall of the pollen grain (exine) is 

 completely lost, being fractured by the increase in size of the proto- 

 plasmic body (cf. Fig. 459 D). 



In many Gymnogperms the FEMALE flowers or CONES resemble 

 the male flowers in being composed of an axis bearing numerous 

 spirally arranged sporophylls. In other cases they differ from this 



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