50 



BOTANY 



PART I 



without intercellular spaces between them. The lateral walls are 

 often undulated or toothed, which increases the firmness of the union 

 of the cells. In transverse section the cells are of uniform depth and 

 are rectangular or lens-shaped. The protoplasts of the epidermal cells 

 are commonly reduced to thin layers lining the walls and enclosing 

 large vacuoles filled with colourless or coloured cell sap. The 

 epidermis of the parts exposed to light in most Ferns and in many 

 shade-loving Phanerogams is provided with chlorophyll and takes 

 part in assimilation. With progressive division of labour, however, 

 chlorophyll is absent from the epidermis, which then serves merely to 

 protect the more internal tissues especially against desiccation. 



The outer walls of the epidermal cells of all subaerial parts of 

 the plant, which last for a considerable time, are thickened. In this 



FIG. 44. Transverse section of a node of the sugar-cane, SaccJiarum officinarum, showing 

 wax incrustation in the form of small rods, (x 540. After STRASBUROER.) 



respect they contrast with the epidermal cells of the more fugitive 

 petals and of submerged and subterranean parts. This holds especially 

 for roots in which the epidermis has very different functions, such as 

 the absorption of water and salts. The thickening of the outer walls 

 results from the apposition of cellulose layers, the outer of which 

 usually, but not always, become more or less strongly cutinised 

 (Fig. 190). 



The outer walls of the epidermis, whether thickened or not, except 

 in the case of those forming the surface of subterranean organs and 

 especially roots, are covered by a thin continuous cutinised film called 

 the CUTICLE. This is formed on the primary walls of the epidermal 

 cells. The cuticle is often somewhat folded and in surface view appears 

 striated. The cuticle and the cutinised layers of the wall are only with 

 difficulty permeable to water and gases, and prevent the injurious loss of 

 water by evaporation. The thickening also increases the mechanical 



