104 DERMAL SYSTEM 



As a matter of fact the delicate cuticle is quite pervious to water ; 

 under normal conditions its functions are principally mechanical, while 

 it may also serve to prevent Bacteria and other parasitic micro-organisms 

 from gaining access to the interior of the plant. Among water-plants 

 the epidermis is often thin-walled even in the case of organs which are 

 not submerged ; in the floating species of Lcmna, for example, the 

 outer epidermal wall is no thicker on the upper side of the fronds, 

 which is exposed to air, than it is on the lower surface, which is 

 constantly wetted. 



The opposite extreme, as regards the structure of the outer epidermal 

 wall, is illustrated by the plants which grow in arid regions ; here the 

 outer wall is greatly thickened and strongly cutinised, and the plants 

 are thus enabled to reduce their cuticular transpiration enormously. 

 This type of epidermis prevails, for example, among the plants inhabiting 

 the deserts of Asia and Africa, in the Australian Grass-trees (Xantho- 

 rrhoea), Proteaceae and Epacridaceae, and in many Arctic xerophytes. 

 Even in a humid climate edaphic conditions may render the restriction 

 of transpiration imperative. This circumstance explains the occurrence 

 of a thick-walled epidermis in many epiphytes of the tropical rain- 

 forest, and in halophytes. Many alpine plants similarly rely upon a 

 heavily cutinised epidermis to counteract the tendency towards exces- 

 sive transpiration induced by low atmospheric pressure and intense 

 insolation. 



The outer epidermal walls often display varying degrees of thickening 

 and cutinisation in different organs of the same plant. It is, of course, 

 quite evident that certain parts of a plant may stand in less urgent 

 need of protection against loss of water than others. Floral organs, 

 for example, are comparatively short-lived structures, and are, moreover, 

 usually put forth at the time when there is least risk of desiccation. 

 Hence they do not require to be protected against desiccation to the same 

 extent as foliage-leaves which remain active throughout the vegetative 

 season, and which are therefore more exposed to changes of weather. 

 The epidermis is accordingly almost always thin-walled in the case 

 of petals, stamens, stigmatic surfaces, and the like. Again, there is 

 nothing surprising in the fact that the upper epidermis of a leaf is 

 often thicker-walled than the lower. In Daphne chrysantha the outer 

 wall is 4'2/x in thickness in the ease of the lower epidermis of the 

 leaf, while it is more than twice as thick (8'6/u) in the case of the 

 upper epidermis. In Vinca minor the corresponding measurements 

 are 3'1/a and 5*4//. Of the two leaf-surfaces it is obviously the upper 

 that receives more light and heat, and that hence requires more 

 effective protection against evaporation. 



Experiments have repeatedly been performed for the purpose of 



