126 DERMAL SYSTEM 



however, quite conceivable that they act primarily as buttresses which 

 add to the stiffness of the hairs; for such oblique hairs, especially if 

 they are stiff and bristly, often assist in preventing small animals from 

 crawling over the surface of the organs on which they occur. The 

 cells which immediately surround the base of a hair generally differ in 

 form, and often also in the character of their walls, from the ordinary 

 epidermal elements ; these are the subsidiary cells, which are so 

 frequently described as forming a ring or rosette around the base 

 of a hair. 



According to the special nature of their functions hairs contain 

 living protoplasts or consist of dead cells ; the character of their 

 walls is also naturally altogether dependent on their physiological 

 properties. 



As has already been pointed out, hairs are utilised for a great 

 variety of purposes, many of which have nothing in common with 

 the proper functions of the epidermis. In support of this statement 

 reference may be made to water-absorbing and water-secreting hairs, 

 to the extraordinarily varied types of glandular hairs, to the feathery 

 hairs of which the " parachutes " of many seeds and fruits are composed 

 and, lastly, to the remarkable trichomes that assist in the perception 

 of certain stimuli. Obviously only those trichomes which assist the 

 epidermis in its protective capacity can be regarded as epidermal 

 appendages in an anatomico-physiological sense ; the following dis- 

 cussion is therefore confined to hairs which fulfil this condition. 



The value of a hairy covering [in relation to the dermal system] 

 depends mainly upon the fact that it causes a reduction of trans- 

 piratory activity, and hence diminishes the risk of desiccation. A 

 coat of densely packed dry hairs must obviously act as a screen, which 

 protects the hairy organ from direct insolation and from the increased 

 rate of transpiration that results therefrom ; V2 it must also hinder the 

 renewal of the stratum of air overlying the transpiring surface and 

 thus further diminish transpiration. Care must always be taken to 

 distinguish between these two factors in estimating the reduction of 

 transpiratory activity which is effected by a dense covering of hairs. 

 Thus the dense coat of hairs characteristic of numerous steppe- and 

 desert-plants, as well as of many representatives of the Mediterranean 

 and alpine floras, which extends even to the directly insolated faces of 

 the leaves, must be primarily regarded as a light-screen. Where on 

 the other hand, in dorsi-ventral leaves, stomata are confined to the 

 shaded side which usually corresponds to the abaxial surface the 

 hairy covering is evidently in the first instance designed to impede 

 surface-ventilation by producing a labyrinth of spaces filled with 

 stationary air. A similar purpose is served by the hairy coats which 



