THE EPIDERMIS 101 



In the case of submerged water-plants the dermal system does not 

 need to cope with so many requirements, mainly because such plants 

 run no risk of drying up under normal conditions. But a new factor 

 is introduced owing to the fact that water-plants possess a highly 

 developed ventilating system, which comprises numerous large air- 

 passages in addition to the usual labyrinth of small intercellular 

 spaces ; evidently a continuous dermal tissue is required if these large 

 spaces are to be kept water-tight. In the case of plants which grow 

 in running water an additional element of danger arises from the 

 shearing effect of the current, which might readily bring about a 

 gradual disintegration of the tissues, if these were not enveloped in a 

 continuous skin. 



The demands which are made upon the dermal system of subter- 

 ranean structures vary in accordance with the water-content of the soil ; 

 such organs, therefore, in this respect hold a position which is inter- 

 mediate between that of aerial and aquatic organs, approaching more 

 nearly to one or to the other according to the amount of water 

 that is available. 



The detailed discussion of the different tissues included in the 

 dermal system will afford some indication as to the extent to which 

 these tissues are actually able to protect the various parts of the 

 plant-body against hostile external conditions. 



77. THE EPIDERMIS. 



The epidermis represents the first stage in the evolution of a dermal 

 system. Consisting in the majority of cases of a single layer of cells, 

 it separates the underlying tissues from the external medium and 

 preserves them from the dangers which have been enumerated above. 

 This delimitation and protection jointly constitute the principal 

 function of the epidermis. From the anatomico-physiological point of 

 view, therefore, a superficial layer can only be regarded as epidermal if 

 its histological features are really correlated with the performance of 

 this [protective] function. It is by no means the case that every 

 superficial layer is intended to serve as a protective epidermis. The 

 outermost layer of a young root, for example the so-called piliferous 

 layer or rhizodermis 46a is mainly concerned with the absorption of 

 water and nutrient salts, and is hence not an epidermis in the anatomico- 

 physiological sense. Similarly it would not be consistent with 

 anatomico-physiological principles to regard the guard-cells of stomata 

 as part of the epidermis ; these structures must on the contrary be 

 assigned to the system which is concerned with ventilation. In the 

 same way the " epidermal " hydathodes which occur on certain leaves, 



