THE MICROSPORANGIUM 



217 



presented by those forms included under the general heading of 

 Pteridophyta need not occupy our attention in the present con- 

 nection, important as these structures are from the standpoint of 

 taxonomy and the evolution of particular groups. 



Returning to the seed plants, we find that the Cycadales are 

 the only living forms in which the organization of the mechanical 

 tissues of the sporangium corresponds with that generally existing 



FIG. 161. Sporangium of Selaginella spc. and of Poly podium vulgare 



in the Pteridophyta; and in accordance with this general situation 

 it will be made clear that the epidermis has not an important rela- 

 tion to the distribution of the microspores of plants producing 

 seeds. The interesting genus Ginkgo will serve to illustrate advan- 

 tageously the situation for the lower living seed plants. Fig. 162 

 shows the organization of one of the two sporangia of the micro- 

 sporophyll of this genus as viewed in longitudinal vertical section. 

 Clearly the cells of the epidermis are thin-walled and can perform 

 no important office in the openings of the spore cavities. Beneath 

 the epidermis is found a broad zone of cells provided with barred 



