228 ANATOMY AND PHYSIOLOGY OF PLANTS. 



accident; but if it does not separate spontaneously, it is not renewed 

 when once destroyed. 



31. The last sentence intimates, that it is not usual in 

 all plants for the epidermis to separate spontaneously, 

 but in every case it is constantly renewed, or in other 

 words, it is continually accumulating. 



Where it remains, the old cuticle cracks as the diameter of the 

 stem or branch increases; it is then gradually pushed outwards, and 

 the accumulation of successive layers in this manner, forms the rug- 

 ged coats which are peculiar to many trees, as the elm, the oak, 

 and others. 



32. The vegetable epidermis is capable of extension, 

 but not to that extent which has been supposed. 



As there is a constant renewal of cuticle, there must be a propor- 

 tional growth of its parts, so that it is not simple extension which 

 enables it to cover the whole surface of the plant, but a new cuticle 

 is added to produce this effect. 



33. The use of the epidermis is, to defend the inte- 

 rior parts from the action of the air, and to regulate the 

 perspiration and absorption of the plant. 



34. The powers of the cuticle in regulating these 

 functions is fixed according to the nature of the plant. 



In succulent plants, which require much moisture to be retained 

 in their leaves, the cuticle is so constructed as to assist absorption, 

 and rather to prevent transpiration, as we have already instanced 

 with the American aloe; on the other hand, in plants which do not 

 require much moisture, the epidermis is accordingly constructed. 



35. Probably the most important intention of tiie 



