PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 309 



are full of sap, close as soon as there is a danger of too much evaporation, the 

 mechanism must he considered as excellently regulating transpiration, and as pro- 

 viding a true preventative against over-evaporation. 



This closure of the exhalent chambers in the interior of the leaf, important as it 

 is, would alone be sufficient in but a very few cases to ward oft" this threatened 

 danger. If the epidermis which stretches over the thin-walled transpiring cells of 

 the spongy parenchyma is itself thin-walled and succulent, water will be exhaled 

 from it also into the dry atmosphere; this loss of water from the epidermal cells is 

 compensated for by water drawn from the neighbouring parenchymatous cells in 

 the interior of the leaf, and ultimately the leaves would wither up if the supply of 

 water from the roots were stopped or became insufficient. Therefore the epidermal 

 cells must be adequately prevented from exhaling. When this is the case, and when 

 the stomata are closed, the spongy parenchyma, and, generally speaking, all the 

 succulent cells in the interior, are securely protected. 



The walls of the epidermal cells in the first stages of their development are 

 composed mainly of cellulose, and are uniformly thin and delicate on all sides. The 

 outer wall, which is in contact with the air, then becomes thickened and divided into 

 an inner and an outer layer. The inner retains its original character, but the outer 

 — the so-called " cuticle " — undergoes great modifications. The cellulose becomes 

 changed, and is replaced by a mixture of stearin and the glyceride of a fatty acid 

 (suberic acid), forming a tallow-like fat which is termed cutin or suberin. In 

 consequence of this metamorphosis the cell-wall becomes less and less permeable to 

 water, and when it has attained a considerable thickness it becomes at length almost 

 entirely impervious to water and aqueous vapour. Frequently, between the inner 

 cellulose and the outer corky layer, other so-called " cuticularized layers " are 

 formed, whose chief constituent is again suberin, and which often attain to a con- 

 siderable bulk. 



Aquatic plants, which are not exposed to the danger of excessive evaporation, of 

 course do not require this protection. Plants whose leaves are surrounded by air, 

 on the other hand, can never entirely dispense with it. The thickness of these 

 corky layers is extremely variable according to the condition of humidity of the air. 

 Where the air is very damp throughout the year, the outer wall of the epidermal 

 leaf-cells appears to be only slightly thicker than the inner, and the cuticle only 

 forms a thin continuous layer. On the other hand, plants which are temporarily 

 exposed to dry air possess very highly developed cuticular strata. Especially when 

 the leaves are evergreen and remain several years on the branches, as, for example, in 

 the Holly (Hex Aqwifolium, see fig. 73 2 ), and in the Oleander (Nerium Oleander, 

 fig. 73 3 ), the cuticular layers are so strongly developed that the outer wall of the 

 epidermal cells is many times thicker than the inner wall. Evergreen parasites, as, 

 for example, the Mistletoe (fig. 73 1 ), those tropical orchids and Bromehacese which 

 live epiphytically on the bark of trees and are often exposed to great dryness in the 

 hot season of the year, cactiform plants, and generally the majority of succulent 

 plants, possess epidermal cells with very strongly thickened outer walls. This is so 



