PTERIDOPHYTA 



317 



The leaves are developed in a compact spiral. Each season's 

 growth is separated from the next by a series of sterile leaves, 

 which are more or less rudimentary. In the mature sporophyte all 

 of the foliage leaves are sporophylls, each having a single large 

 sporangium upon the inner face of its expanded base (Fig. 282, B). 

 The sporangium is oval in outline, and sunk in a depression (Fovea), 

 whose margin (Velum) may almost completely cover the sporan- 

 gium, suggesting a structure comparable to the integument of an 

 ovule. Above the Fovea is a small scalelike outgrowth, the Ligula. 



B. 



FIG. 284. Isoetes echinospora. A, young embryo (x 450). B, older embryo, still 

 enclosed in the gametophyte (X 150) ; ar, archegonia. 



The Stem 



The stem is composed of a central vascular cylinder, made up largely of 

 short tracheids. Outside of this is a layer of prismatic cells, which perhaps 

 represent the phloem. Outside this is a layer of meristematic cells, which add 

 to the prismatic cells on the inner side, and to the cortex outside. A true sec- 

 ondary thickening thus takes place, but it is quite different from that of other 

 plants. 



The Leaf 



The leaf is traversed by a single very simple vascular bundle, surrounding 

 which are four large air-channels, separated at intervals by diaphragms. Where 



