528 MOSSES AND FERNS CHAP. 



posed of similar cells, e. g. f S. rupestris, or of cells of somewhat 

 different form on the two surfaces of the leaf, e. g., S, Mar- 

 tensiL Some of the epidermal cells may have the form of 

 sclerenchymatous fibres (S. suberosa). The mesophyll is com- 

 posed of a loose network of cells, which may be all alike (S. 

 rupestris) or less frequently, there is developed below the upper 

 epidermis, a palisade parenchyma (S. Lyallii). As a rule 

 stomata are formed only upon the lower epidermis, but there 

 are some exceptions. 



The single median vascular bundle is concentric in struc- 

 ture, and the leaf-traces join the vascular cylinder of the stem, 

 as they do in Lycopodium. The xylem consists of a single row 

 of annular tracheids, and three or four spiral ones. The 

 phloem is mainly composed of elongated parenchyma cells, but 

 one or two sieve-tubes can usually be demonstrated. Sur- 

 rounding the bundle is a pericycle consisting of a single layer 

 of cells, or in some cases more, but no definite endodermis is 

 present. 



There is always developed at the base of the leaf the char- 

 acteristic ligula (Fig. 303, /). This develops at an early 

 period, and seems to be an organ for retaining moisture, as its 

 young cells develop abundant mucilage. In its fully developed 

 condition it shows a basal portion (glossopodium) composed 

 of large cells which are surrounded by a sort of sheath which is 

 continuous with the epidermis of the leaf. It varies in form in 

 different species. Thus in 5\ Vogelii it is tongue-shaped; in 

 S. Martensii, fan-shaped ; in 5". cuspidata, fringed ( for further 

 details of its structure and development see Gibson (4)). 



Simple hairs are of frequent occurrence in various parts of 

 the sporophyte. 



The Chloroplasts 



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 (9), 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 re- 

 mains undivided (S. Martensii) , or it may become more or less 

 completely divided into two (S. Kraussiana). In S. Willde- 

 nowii there may be as many as eight. In the cortical paren- 



