THE MICROSPORANGIUM 



33 



appearance and behavior between the cells of the tapetum and the 

 micrcsporogenous tissue, earlier botanists supposed that the former 

 is derived by a sterilization of the outer sporogenous cells. Develop- 

 mental studies of a precise nature have, however, nearly always 

 confirmed its parietal origin. 4 



The nuclear divisions in the tapetum were formerly believed to 

 be amitotic, but recent studies (Bonnet, 1912; Cooper, 1933; Wit- 

 kus, 1945) have shown that this is incorrect and that appearances 

 suggesting amitosis are really caused by mitotic irregularities and 



G H I J K 



Fig. 27. Nuclear divisions in tapetal cells of Zea mays (A-F), Lilium canadense 

 (G-H), and Podophyllum peltatum (I-K). (After Cooper, 1933.) 



nuclear fusions. According to present conceptions, the nucleus of a 

 tapetal cell may divide in any of the following ways: 5 



1. By normal mitosis. The division takes place in the ordinary 



nuclei of Lactuca in the synizesis stage, and Moissl (1941) reports a similar condi- 

 tion in some members of the Caprifoliaceae. 



4 Recently, Capoor (19376) has reported that in Holoptelea the tapetal cells are 

 almost indistinguishable from the adjacent cells of the sporogenous tissue. He 

 cautiously adds, however, that this fact alone is insufficient to justify any inference 

 regarding the sporogenous origin of the tapetum. 



6 It is to be noted that in a few families and orders, viz., Mimosaceae, Cras- 

 sulaceae, Gentianaceae, Boraginaceae, Hydrophyllaceae, Juncaceae, Orchidaceae, 

 and Helobiales, the tapetal cells usually remain uninucleate from the time of their 

 formation to their eventual disintegration. 



