264 BOTANY 



upon the back of the sporophyll. A sorus bears a definite relation 

 to the veins of the leaf, usually standing above one of these, or at 

 its extremity (Fig. 223). At this point a more active growth of the 

 superficial tissue results in a slight elevation, or receptacle, into 

 which sometimes passes a short branch from the vascular bundle, 

 above which the sorus is situated. In most Leptosporangiates the 

 sorus is more or less completely covered by a membranaceous out- 

 growth of the epidermis, the Indusium. 



In the Polypodiaceae, each sporangium (Figs. 227, 228) arises 

 from a single superficial cell of the receptacle. Sometimes one or 

 two transverse walls are formed before the tetrahedral apical cell, 

 from which the young sporangium grows, is developed. Usually the 

 first wall in the young sporangium is nearly vertical, and is followed 

 by two similar ones which intersect the first wall so as to include a 

 tetrahedral apical cell like that in the stem-apex. From the apical 

 cell are cut off several series of lateral segments, the earlier ones 

 giving rise to the three-rowed stalk which is found in the sporan- 

 gium of the Polypodiacese. Finally a periclinal wall separates a 

 terminal segment from the apical cell, and the longitudinal growth 

 of the sporangium is stopped. 



The upper part of the young sporangium rapidly increases in 

 diameter and forms the sporogenous capsule. 



After the apical cell has formed the terminal segment, it becomes 

 the archesporium. From it are cut off four more segments, which 

 may divide into two layers, so that the young capsule consists of a 

 central cell and two or three outer layers. Of the latter, the outer- 

 most persists, and forms the wall of the ripe sporangium. The 

 inner layer or layers constituting the tapetum later have their walls 

 broken down, and form a mass of nucleated protoplasm in which the 

 sporogenous cells lie. 



The primary archesporial cell divides repeatedly, until about twelve 

 to sixteen cells are formed. These contain very dense protoplasm 

 and large nuclei. Finally the division-walls are partially absorbed, 

 and the sporogenous cells separate completely. Each cell then divides 

 into the four spores. 



The young spores have a thin cellulose membrane, which later 

 becomes differentiated into an inner (intine) and outer (exine) 

 layer. As the spores approach maturity, there is usually deposited 

 upon the outside, largely through the activity of the tapetal proto- 

 plasm, an outer sculptured membrane, the epispore, or perinium. 



The wall of the sporangium consists of a single layer of large, 

 thin-walled cells, except for the peculiar annulus, or ring of thick- 

 ened cells running over its apex, and ending just above the "Sto- 

 mium " (Fig. 228), which is composed of two narrow cells, between 

 which the transverse opening occurs. The stornium is formed in the 



