VASCULAR CRYPTOGAMS 



proceeds a branch into each leaf and one into the root. The layer of 

 meristem which surrounds the axial bundle increases chiefly in the 

 centrifugal direction, fresh layers thus formed replacing the outer ones, 

 which continually die off. The secondary long-enduring increase in 

 thickness of the stem is chiefly due to increase in thickness of the cortical 

 tissue, though new xylem-elements are also produced. The mode of 

 apical growth differs from that in most species of Selaginella. There is 

 no single apical cell, the apex of the stem being occupied by a group of 

 equivalent merismatic cells. 



FIG. 31. Longitudinal section of stem of 7. lacustris. bb 9 , 

 leaves ; r l r 10 , roots : the ligules are shaded ( x 30). 

 (After Hofmeister.) 



FIG. 32. Longitudinal section 

 through lower portion of 

 leaf of /. lacustris (diagram- 

 matic). L, ligule ; J, indu- 

 sium ; S#, microsporange ; 

 TV, trabecules ; Gf, vascu- 

 lar bundle of sporophyll. 

 (After Goebel.) 



The leaves of Isoetes are very elongated, cylindrical, and quill-shaped, 

 and are arranged in a complicated phyllotaxis on the very short stem. 

 They are segmented into a basal portion, the sheath or glossopodt, and 

 an apical portion, the lamina. The sheath is nearly triangular in form 

 with a very broad insertion, and does not completely embrace the stem. 

 It is convex behind and concave in front, where it bears the sporange 

 in a large depression known as thefovea ; the margin of this depression 

 rises in the form of a thin membranous outgrowth, the veil or indusium^ 

 which, in many species, extends above and beyond the sporange. Above 



