LYCOPODINEAE. LIGULA TAE. 



289 



which surrounds the central vascular group and is continually producing new layers 

 of parenchyma on the outside ; this formation takes place especially in two or three 

 directions on the transverse section, and gives rise to two or three projecting masses 

 of tissue which die off slowly on the outside, and between which lie as many deep 

 furrows meeting on the under side of the stem ; from these furrows numerous roots 

 arise in rows in acropetal succession. 



In the Selaginelleae the stem remains thin, but increases rapidly in length forming 

 distinct internodes, and displays copious monopodial branching, which from the 

 vigorous growth of the lateral shoots often appears to be dichotomous. The 

 extremity of the stem rises as a slender cone above the youngest leaves. The systems 

 of shoots with their many branches 

 are developed bilaterally in one plane 

 in such a manner that they frequently 

 have a definite outline and resemble 

 a multi-pinnate leaf. As the leaves 

 of Selaginella are small, the general 

 habit is determined chiefly by the sys- 

 tems of branches; the primary shoots 

 are creeping rhizomes, or they ascend 

 obliquely, or climb erect, or are the 

 primary stems of small tree-like or 

 shrub-like plants. But in all these 

 cases the branches are all in one 

 plane and spring from the sides of 

 the primary axis ; and the dorsiven- 

 trality so strikingly displayed in the 

 position of the branches and leaves 

 is present from the first in the growing r j 



point of the shoot. 



The leaves are always simple 

 and unbranched, and traversed by 

 a single vascular bundle ; they ter- 

 minate above in a simple point, in the Selaginelleae not unfrequently in a delicate 

 awn. The largest leaves occur in Isoetes, where they may be from four to 

 sixty centimetres long. In this genus they are divided into a basal portion, the 

 sheath, and into an upper portion, the lamina. The sheath does not entirely 

 embrace the stem, but rising from a very broad insertion ends in a long point 

 above and is therefore nearly triangular in form; it is convex behind, concave in 

 front, and on that side has a large depression, ihefovea, in which the sporangium is 

 fixed ; the margin of this depression rises into a thin membranous outgrowth which 

 in many species lays itself over and covers the sporangium, and is termed the velum 

 (indushim}. Above the fovea and separated from it by the 'saddle' is a smaller 

 depression, \htfoveola, the lower margin of which forms a lip, the labium, while from 

 its cavity rises a membranous structure, the Iigule, which is acuminate above from a 

 cordate base and projects beyond the foveola (Fig. 241 Z). The lamina into which the 

 sheath passes above contains chlorophyll and is thick and narrow, almost circular in 



FIG. 233. Isoetes laaistris. Longitudinal section at right angles to 

 the bifurcation of the stem, 10 months old ; ^ stem, b\ to &* leaves, r\ to 

 r 10 roots ; the ligule of the two developed leaves is shaded. Magn. 

 30 times. 



