XI] FUNCTIONS OF CUTICLE 83 



of the cell-walls composing it are said to be cuticularised. 

 Owing to the resistance of the cutin to even strong sul- 

 phuric acid, which rapidly dissolves the rest of the cell- 

 walls, the cuticle can be stripped off as a thin, continuous 

 and very elastic, tough membrane if the epidermis and 

 other parts of the leaf are treated with this reagent. 



That the functions of the cuticle are protective, and 

 especially so against the loss of water from the underlying 

 cells, is evident on comparing various plants or parts 

 of plants with reference to their conditions of life. Thus, 

 we find the cuticle especially thick and well-developed 

 on plants like Cactuses, Aloes, Yuccas, Cactus-like Eu- 

 phorbias, and other forms adapted to life in arid regions 

 or other situations where it is advantageous that the water 

 in the tissues should not escape too easily. In many 

 such cases the very thick outer walls of the epidermis 

 not only show a well-developed cuticle, but the layers of 

 cell -wall immediately beneath are also altered subse- 

 quently from the condition of cellulose. These layers are 

 not cuticularised, however, and in some thin cell- walls they 

 are converted into more or less slimy or mucilaginous 

 substances, and in some cases this goes so far that the 

 swollen viscid substances burst the cuticle and flood the 

 part concerned as with an ooze. This is particularly the 

 case with certain buds and inflorescences, where the 

 viscid layers serve to protect the young organs from 

 insects, or, in some cases, against over-evaporation, or 

 water, or changes of temperature as stated on p. 57. 



The cuticle of many plants is rendered still more 

 effective as a protection by the deposition of wax : this 

 is very commonly the case where protection against 

 soakage by water is an advantage. The " bloom " on 

 certain shoots and twigs, e.g. Raspberry, Salix daphnoides, 

 and on Plums and Grapes, Cabbage and Lotus-leaves, &c. 



62 



