OUTER EPIDERMAL WALL 



103 



contain a varying proportion of cutin. The outermost lamella of the 

 wall contains the greatest amount of cutin and constitutes the cuticle 

 (Fig. 23 c), which forms a continuous pellicle over the whole of the 

 epidermis. The cuticle is always present, and cellulose layers are 

 probably also of universal occurrence. The cutinised layers, on the 

 other hand, are often absent when the outer wall is thin, and may be 

 lacking even when it is quite thick (Fig. 23 b). When present, these 

 cutinised layers are as a rule sharply marked off from the underlying- 

 cellulose layers ; the surface of contact between the two may be 

 smooth, but is sometimes uneven, owing to the fact that the cutinised 

 layers project at a number of points in the form of minute teeth or 



es 



A 



Fig. 23. 



A. Cells from the foliar epidermis of Aloe acinacifolia. B. Cells from the foliar epidermis 

 of Allium Cepa : c, cuticle ; cs, cutinised layers ; b, cellulose layers. 



ridges of various shapes. This " interlocking " of the cellulose and 

 cutinised zones, which is particularly well illustrated by the foliar 

 epidermis of certain species of Aloe, undoubtedly helps to bind the two 

 layers firmly together. The thickness of the outer wall has a two-fold 

 significance. In the first place it serves to diminish the rate of 

 transpiration, because the cutinised layers resemble the cuticle in 

 being relatively impervious to water ; secondly, it has the effect of 

 increasing the mechanical strength of the epidermis. It will be con- 

 venient to discuss these two effects separately. 



It is an easy matter to show, by comparative investigation, that there 

 is a close correspondence between the thickness and degree of cutinisa- 

 tion of the outer epidermal wall, on the one hand, and the extent to 

 which the underlying tissues require to be protected against excessive 

 transpiration on the other. Thus the outer epidermal walls of submerged 

 plants are generally delicate and hardly thicker than the radial or inner 

 walls. Such plants accordingly wither and dry up very quickly if 

 they are removed from their native element. In their case the epidermis 

 is evidently incapable of restricting transpiration to any great extent. 



