222 



REPRODUCTION 



ps ppr 



1 o 



Fig. 127. — Showing stages in the formation of anthers and pollen grains or micro- 

 spores of Silphium. A, a, archesporium ; B, ps, primary sporogenous cell, ppi, primary 

 parietal layer ; C, i, inner layer, o, outer layer formed from primary parietal layer ; s, sporog- 

 enous cells formed from division of primary sporogenous cell; D, s, sporogenous cells; i, 

 tapetum; 00, two parietal layers; E, cross section of one lobe of mature anther; g, spore 

 grandmother cells, 00, and ( as in D; F, portion of longitudinal section of young anther 

 showing a, archesporium; G, showing longitudinal section of same stage as D, parts 

 lettered the same as D; H, longitudinal section of about the same stage as E, with parts 

 lettered the same; 1 and /, pollen grains beginning to germinate of two species of Sil- 

 phium; m, male or sperm nuclei; K, division of mother cell nuclei leading to microspores 

 in L. (After Merrell.) 



