674 A TEXTBOOK OF THEORETICAL BOTANY 



are formed. These microspores are equivalent to the pollen grains of the 

 Angiosperms. 



The female cone, which takes three years to develop, is a more substantial 

 structure, consisting of a woody, central axis bearing lateral appendages in 

 a close spiral. These bear the megasporangia in which are developed the 

 megaspores. 



The Male Cone 



The male cone is produced in the axil of a scale leaf at the base of the 

 developing shoot of the current year, and thus replaces a short shoot (Fig. 

 673). It consists of an axis about 5 mm. long, at the base of which are a 



Fig. 673. — Pimis sylrestris. Young shoot bearing a group 



of male cones. 



number of small scales, above which are arranged, spirally, between sixty and 

 one hundred specialised leaves or microsporophylls. In longitudinal section 

 it is seen that each sporophyll arises from the central axis (Fig. 674) and passes 

 out horizontally to terminate in a flattened head, part of which projects 

 upwards and overlaps the sporophyll above, while part turns down and 

 covers the two microsporangia or pollen sacs, which are attached along the 

 under side of the sporophyll. A single vascular bundle enters the sporophyll 

 but stops short before reaching the head. 



Within the w^all of the sporangium we can recognize at an early stage a 

 peripheral tapetum and a central archesporial tissue. From the latter a 

 number of microspore mother cells are differentiated. The nucleus of 

 each divides twice, meiotically, whereby the chromosome number is halved. 

 Thus from each mother cell four microspores are produced, each of which 

 possesses the monoploid chromosome number. These microspores or pollen 



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