EPIDERMIS 93 



where absorption of water takes place over the whole surface. 

 Most young organs, before they have attained their full size, 

 possess but a very thin cuticle and exhibit considerable cuticular 

 transpiration ; hence the necessity for other devices for the 

 reduction of transpiration (e.g. hairs, folding of leaves, etc.). 

 Even a very thick cuticle is, however, not completely imperme- 

 able, and its efficiency in reducing transpiration is sometimes 

 augmented by a covering of wax secreted by the epidermal cells ; 

 when present in any considerable quantity (e.g. leaves of the 

 Sea Holly, fruit of the Plum, etc.), this gives the surface a bluish 

 tinge. Such coverings of wax also prevent the collection of 

 moisture on the surface of the plant. They are readily rubbed 

 off, but may be subsequently renewed. 



Not uncommonly the epidermis can be easily stripped off 

 and so examined in surface view, when it will again be seen to 

 form a continuous layer (Fig. 44, B-D) without intercellular 

 spaces, the only gaps being constituted by the stomata to be 

 described below. The shape of the cells in such surface sections 

 is very varied. In stems (Fig. 44, D) and in the leaves of most 

 Monocotyledons (Fig. 44, C) they are usually considerably 

 elongated parallel to the longitudinal axis, whilst in Dicotyle- 

 donous leaves they are in most cases roughly isodiametric (Fig. 

 44, B). Where the lamina is thin the lateral walls of the epi- 

 dermal cells are often undulated (Fig. 44, B). Through this 

 wavy outline the cells interlock, so that the surface of contact 

 between them is increased, and the risk of tearing by the wind 

 is proportionally diminished. It is significant that the lower 

 epidermis, which has numerous points of weakness constituted 

 by the stomatal perforations, generally exhibits this undulation 

 to a more marked degree than the upper. This feature is also 

 more pronounced in the leaves of certain aquatics than in those 

 of the corresponding land forms. 



The epidermis keeps pace with the increasing size of the 

 underlying tissues by the division of its cells, but usually all the 

 septa are at right-angles to the surface, so that it remains a single 

 layer. This subdivision is plainly . recognisable in the stem of 

 the Runner Bean, where the spindle-shaped epidermal cells, 

 after reaching a certain size, become divided by transverse septa 

 (Fig. 44, D). 



