CYCADALES 



119 



II chromosomes, and in one case 13 chromosomes; in Zamia 25 

 nuclei were examined and all showed 12 chromosomes. Chamber- 

 lain (46, 63) found 12 chromosomes in the pollen mother cells of 

 Dioon edule, 12 at the mitosis which gives rise to the ventral canal 

 and egg nuclei, and 12 in cells of the endosperm. 



Figs. 105-109. — Stangeria paradoxa: development of microsporangium; fig. 105, 

 section showing two of the four cells forming the archesporial plate; fig. 106, each 

 cell of the plate divided into an inner primary sporogenous cell (shaded) and an outer 

 primary wall cell; fig. 107, a more advanced stage; fig. 108, still more advanced stage, 

 showing tapetum (shaded) and sporogenous cells (j); fig. 109, mature microsporan- 

 gium; <, tapetum; w, crushed cells of the wall; figs. 105-108, X266; fig. 109, X66. — 

 After Lang (22). 



The output of spores has been estimated by Miss F. Grace Smith 

 (56) and by Chamberlain (63). When spores are very small and 

 closely packed together, and the spore mass is nearly spherical, the 

 number in a sporgjigium may be estimated by the formula ^ttR^. 

 To apply the formula it is necessary only to count the number of spores 

 in a radius from the center of the sporangium to the tapetum, sub- 

 stitute this number for R in the formula, and make the calculation. 

 There is some error because the spore mass may not be exactly 



