434 



BOTANY 



which comprise the most highly developed of all plants and, pre- 

 dominating both in the number of species and individuals, have pro- 

 duced the chief part of all the vegetation since the Tertiary Period. 



Class I 

 GYMNOSPERMAE 



The flowers of the Gymnosperms are always unisexual and naked, 

 or in rare cases (Gnetaceae) provided with a small, insignificant 

 perianth. 



The male flowers consist most frequently of long shoots with a 

 larger or smaller number of spiral or whorled scale-like staminal leaves 

 (Fig. 365, A, G), bearing on the under side two or more pollen-sacs. 



Fig. 365. — Pinus Pumilio. A, Longitudinal section of a nearly mature male flower (xlO); B 

 longitudinal section ( X 20), C transverse section ( x 27) of a staminal leaf ; D, pollen-grain of 

 Pinus silvestris ( x 400). 



The pollen-grains are generally spherical, and, in some genera, are pro- 

 vided with two bladder-like protrusions of the exine, which are filled 

 with air and facilitate their dispersal by the wind (D). 



The germinating pollen-grain undergoes division and forms two or 

 more prothallium-cells invested with cellulose walls. One of these 

 cells assumes an antheridial character and divides into two generative 

 cells, corresponding functionally to spermatozoa. 



