SFERMATOPHYTA 



379 



remain separate. When the stamen is mature, the microsporangia, 

 regardless of their number, are called pollen sacs. 



As in gymnosperms, the development of the microsporangia is euspo- 

 rangiate, A very young anther is made up of uniform meristematic tissue 



Fig. 324. Cross section of a young and of a mature anther of lily {Lilium). A, young 

 anther, the four microsporangia with sporogenous tissue, X60; B, mature anther with two 

 pollen sacs containing pollen grains, X30. The tapetum, surrounding the sporogenous 

 tissue and conspicuous in A, has broken down in B, while the endothecium has developed 

 bands of thickening. {B, after Chamberlain.) 



surrounded by an epidermis. As seen in cross section, four lobes soon 

 appear and a conducting strand becomes differentiated in the center. 

 The cells forming the hypodermal layer are probably all potentially spo- 

 rogenous but, as a rule, are actively so only in four regions, viz., under the 

 lobes. Only one longitudinal row of hypodermal cells may be differen- 

 tiated under each lobe as sporangium initials, as in the Malvaceae and 



