PTERIDOPHYTA— LYCOPOBIN^ 165 



externally adds to the cortex of the stem. Isoetes thus increases 

 in thickness, but somewhat irregularly, the distribution of the 

 new cortical tissue being interrupted at places in the ring, so 

 causing the furrows noticeable on its exterior. 



The leaves have a single bundle, which is of a somewhat re- 

 duced type. It runs down the middle of the blade into the stem, 

 and joins the ring of the stele. Four large air-spaces run down 

 the whole length of the leaf in the parenchymatous tissue (fig. 

 922). 



Fig. 922. 



Fig. 923. Transverse section of leaf of Iscetes lacnstHs. a. Air-chamber. I; 

 FibrOTascular bundle. 



The root differs from the stem in having a small-celled 

 stratified apical meristem. Its stele contains only one or two 

 bundles of xylem and phloem. 



The sporangia are much more bulky than in any other of the 

 Lyoopodinae. They arise in a depression or pit on the upper 

 surface of the leaf just above its base, a little below the insertion 

 of the ligule. Isoetes is eusporangiate, the sporangium arising 

 from a. group of cells. In the miicrosporangium the arohesporial 

 tissue, which becomes bulky, has its cells arranged in radial rows, 

 springing from a sort of pad or cushion at its base. There takes 



