PTERIDOPHYTES 



161 



develops a wall of several layers of cells, the innermost layer or two 

 functioning as the tapetum; while the inner cell develops a large mass 

 of sporogenous tissue. The number of mother cells in the different 

 genera of Marattiaceae ranges from 128 to 2048, which means a theoreti- 

 cal output of 2048-8192 spores. Most of the Marattiaceae are fur- 

 ther distinguished by the fact that the sporangia form what is called a 



FIG. 380. Thepolystele of Pteris: in the ill-defined stele there occur several (in 

 this case thirteen) vascular bundles of varying size and form, each one of which is con- 

 centric (xylem surrounded by phloem) and invested by a distinct endodermis; in some 

 of them the mesarch character is evident, the group of protoxylem elements (of small 

 caliber) being more or less centrally placed (at least surrounded by metaxylem); ,the 

 heavy-walled tissue represented by the three irregular masses associated with the vas- 

 cular bundles is mechanical tissue. 



synangium (fig. 386). These synangia appear like small groups of 

 coalescent sporangia distributed over the surface of the leaf, as are the 

 son (see p. 165) in other ferns. In fact, in some of the Marattiaceae the 

 sporangia are not in synangia, but form son of distinct sporangia. It 

 is evident that synangia and son are equivalents ; and it is probable 

 that synangia are not coalescent sporangia, but rather sporangia that 

 have not become completely separated from one another. 



C. B. & C. BOTANY II 



