REPRODUCTIVE CELLS. 



131 



a. The Microspore : its Origin and Maturation {Finns). — The 

 origin of the microspore takes place, as in the Angiosperms, by 

 the setting apart of an archesporium in the anther, from which 

 are produced an outer sheathing layer, the tapetnm, and an inner 

 mass of pollen mother-cells ; each of the latter gives rise to four 

 rudimentary microspores, which are set free later on into the 

 cavity of the pollen-sac or microsporangium(see Fig. 100) 



Pig. 100.— A Section (Longitudinal) of two Pollen-sacs (Micro- 

 sporangia) of Pinna. — sk, Miorosporophyll ; pa, pollen-sao; epa, wall 

 of the pollen-sac : several pollen-grains are seen inside ; /, /j, fibro- 

 vascular bundles. 



The tapetal cells in this case may give rise to a secondary 

 tapetum. The young unmatured microspore (pollen-grain) has 

 the following structure : — 



