CHAPTER X. 



THE EPIDERMIS AND STOMATA. 



Properties of the epidermis Its protective functions Stomata 

 Communication with the atmosphere and with the inter- 

 cellular spaces Numbers and distribution Guard-cells 

 Development Mechanism of opening and shutting Sizes of 

 stomata Functions Water-pores. 



ALL that was stated in Volume I. pp. 81 91 concerning 

 the epidermis as the bearer of hairs, prickles, wax and so 

 forth, applies to the leaf as to the shoot, and we need only 

 concern ourselves here with the epidermis as the regulating 

 mechanism of the evaporation of the water- vapour which 

 accumulates in the intercellular spaces. 



We have already seen that the epidermis stretches as 

 a continuous skin over the whole surface of the leaf. This 

 skin consists of usually one layer of flattened or tabular 

 cells, fitting closely and without intercellular spaces except 

 at certain points to be referred to shortly; and two peculi- 

 arities are usually to be noted, especially in the epidermis 

 of the upper surface of the leaf. One is that the epidermis- 

 cells contain none of the green chlorophyll-corpuscles, so 

 conspicuous in the mesophyll-cells beneath, though in 

 other respects they resemble the latter in possessing the 

 protoplasm, cell-sap, nucleus, &c., of a typical cell. The 

 other peculiarity is that the outer walls of the epidermal 

 w. n. 7 



