112 THE PHYSIOLOGY OF PLANTS 



lating organs in such a way as to display the greatest super- 

 ficial area. 



This principle is carried*out, in the first place, in the shape 

 .of the chlorophyll-containing bodies themselves, which are 

 divided up into numerous small granules. Besides this, the 

 assimilating cells are united so as to form thin expanded sur- 

 faces, the leaf-blades. 



In these it will be noticed that the assimilating cells (Pig. 15), 

 the actual green tissue (mesophyll) (Fig. i S, w), is protected on 

 either surface by a layer of cells, which can often be stripped 

 off as a colourless skin. This protective covering (epidermis) 

 consists of closely fitting tabular cells, with a thicker outer 

 wall, and generally devoid of contents (0 and 00). These cells 

 are covered in on their outer surface by a continuous layer of 

 a substance which exhibits at times the character of cork, at 

 times that of wax, and which renders the cells more or less 

 impervious to gases and liquids. This layer is termed the 

 cuticle ; the more fatty matter this cuticular layer contains, 

 the less liable are the leaves to wetting. The bluish or 

 whitish bloom of many stalks, leaves, and fruits is due to a 

 vegetable wax contained in or covering the cuticle. 



The cuticularised epidermis does not, however, entirely 

 close up the leaf. This would render the activity of the green 

 mesophyll entirely impossible. For the chlorophyll-containing 

 cells have the function of decomposing the carbonic acid when 

 they are in the light ; there must therefore be means by which 

 the carbonic acid gas of the air reaches the interior of the 

 leaf. This means exists in the form of a system of pores, 

 provided with a mechanism for opening and closing. 



Between the cells of the leaf-tissue or mesophyll are 

 irregular spaces connected one with the other, which are 

 termed intercellular spaces (i), and these are filled with air. 

 Every green cell of the leaf can therefore take in gases from 

 the intercellular spaces, and give off gases into them. The 

 air contained within the leaf undergoes changes by the activity 

 of the cells, and collects in large and definite spaces or 

 respiratory chambers, which lie immediately beneath the 

 epidermis, and above which the latter is interrupted. The 

 openings of the epidermis are termed breathing pores (stomata, 



