[Vol. 9 



86 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



which appear later except for the absence of the sporangium. 



The leaves consist of two regions, a long narrow extension 

 widening into a sheathing base, the margins of which are mem- 

 branaceous in character. The lateral extension of the base and 

 the longitudinal extension of the transparent edges may serve 

 in some degree as diagnostic characters, though there is variation 

 according to the size of the plant, position of the leaf, and es- 

 pecially the depth of the corm in the soil. 



On the upper or inner face, each leaf bears a delicate little 

 extension of tissue, free end uppermost, lying parallel to the leaf 

 surface. This ligule in face view appears most often triangular, 

 sometimes much elongated, sometimes subulate or rounded. To 

 some extent, the form and size of the ligule may prove of value in 

 diagnosis, though not constant enough to be of determining 



caliber. Especially in older leaves, it may be imperfect through 



tearing of the delicate tissue. The ligule was called the 

 "calyx" by Linnaeus, and "processus glandulae" by Cesati and 

 DeNotaris ('58). The former interpretation may be understood 

 in view of the attempt to homologize structures in Isoetes with 

 those in flowering plants. The latter term is clear when one sees 

 the swollen imbedded portion of the ligule, called the "glos- 

 sopodium" by A. Braun (*64). This, in section, is sharply dis- 

 tinguishable from the adjacent tissues of the leaf, though in- 

 timately grown to them, and might readily be compared to a 

 glandular structure. 



Wilson-Smith further terms the layer of large glandular cells 

 about the glossopodium and next the leaf tissues proper the 

 "sheath" and distinguishes between the dead marginal and apical 

 cells, and the living central region of the tongue-like extension 

 of the ligule. 



The origin of the ligule has been shown to be a single super- 

 ficial cell (Hofmeister, '62, and Wilson-Smith, '00) of the leaf, 



which develops very rapidly to form a short row of cells, soon 

 becoming a plate, except in the basal portion which, as indicated 

 above, becomes massive. 



The insertion of the ligule is sometimes marked by a fold of 

 tissue immediately below the base, to which Braun applied the 

 name "labium". This is apt to be fairly consistent in the degree 

 of development within the species, but is so small a character 

 that it is difficult to use it in a diagnostic fashion. 



