500 MOSSES AND FERNS chap. 



with deeply-pitted walls. In the creeping stenns, even in poly- 

 stelic species, there is but a single stele, which gradually passes 

 over into the separate steles of the upright stems. 



The leaves show a single very simple concentric bundle, 

 similar to those of the stem, but less developed. The leaf- 

 traces, as in Lycopodium, join the central vascular cylinder 

 (Fig. 261, C). The leaf always develops a ligula just above 

 the base. , This (Fig. 261, L) is a tongue-shaped organ, which 

 cannot be traced back to a single cell. The basal cells are 

 larger than the others, and it is much constricted at the point 

 where it joins the leaf. 



TJie Roots 



The roots in S. Kraussiana are borne upon the special 

 leafless branches or rhizophores, which in structure are much 

 like the stem. Previous to the formation of the first roots 

 upon the rhizophore,^ the apical cell is obliterated and replaced 

 by a group of initial cells. The apical cells of the (usually 

 two) roots formed arise secondarily, and quite independently 

 of each other, from cells lying below the surface, and covered 

 with one or two layers of cells. These cells soon assume a 

 tetrahedral form, and become the apical cells of the primary 

 roots. The branching of the roots, like that of the stem, is 

 really monopodial, although apparently a true dichotomy. 



The vascular bundle of the root is monarch (Fig. 263, B), 

 and does not show a distinct endodermis. The phloem sur- 

 rounds the xylem completely, but apparently sieve -tubes are 

 not developed opposite the protoxylem. The elements of the 

 bundle are in structure like those of the stem-bundles. ^ 



The CJiloroplasts 



The chloroplasts of Selaginella are peculiar, on account of 

 their large size and small numbers, A careful study has been 

 made of these by Haberlandt,^ who found that in each of the 

 meristematic cells of the stem apex a single plastid was 

 present. This in the assimilative cells of the leaves either 

 remains undivided (5. Martensii), or it may become more or less 

 completely divided into two (5. Kraussiana). In 5. Willdenoivii 



1 Sadebeck (6). 2 Haberlandt (9). 



