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. The polystele 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 
f 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 
sori (see p. 165) in other ferns. In fact, in some of the Marattiaceae the 
sporangia are not in synangia, but form sori of distinct sporangia. It 
is evident that synangia and sori are equivalents ; and it is probable 
that synangia are not coalescent sporangia, but rather sporangia that 
have not become completely separated from one another. 
