390 MAXUAL OF BOTANY 



limited amount of soft tissue, continuous with the bast. The 

 endodermis of the steles or meristeles can often be traced for- 

 wards almost or quite to the termination of the bundles. 



The character of the fibro- vascular bundles usually remains 

 unchanged ; in most Ferns, however, the originally concentric 

 bundle loses the bast upon its upper face, and so becomes col- 

 lateral. 



The leaf-blade so constructed shows an epidermis which is 

 continuous with that of the petiole, and which extends over the 

 whole of its surface. It is usually only one layer of cells thick ; 

 the cells are somewhat brick-shaped on section, and have their 

 outer walls generally cuticularised to a greater or less extent. 

 In thick coriaceous leaves the outer part of the cell-walls of the 

 epidermis can often be stripped off in a continuous layer, known 



Fig. 774, Fia. 775. 



cys~^ 



\pa 



Fig. 774. Vertical section of the epidermis of Hoya carnosa treated with 

 caustic potash, a. Tlip detacheri cuticle, 'h. The partially cuticularised 



layers of the outer walls of the epidermal cells. After Yon Mohl. /-'/V/. 



775. Three-layered epidermis of leaf of Flcn^ fiastica, 



as the cuticle (fig. 774). In some leaves the epidermal cells 

 di^dde by walls parallel to the surface, and the epidermis 

 becomes composed of severallayers of cells (fg. 775). Viewed 

 from the surface the epidermal cells are sometimes square or 

 oblong, sometimes much sinuated in shape. Generally in the 

 higher plants the cells of the epidermis contain little more than 

 water ; in some plants, especially in the Ferns, ehloroplasts are 

 present. 



In aquatic plants the external walls of the epidermal cells 

 are not cuticularised. 



The epidermis in all the higher plants is furnished with 

 stomata, which are variously disposed. In the leaves of soft 

 herbaceous plants these openings are present on both surfaces of 

 the leaf ; those of hard woody plants have them generally only 



