164 INTERNAL STRUCTURE OP LEAVES. 



247. Having now noticed the chief varieties of leaves as 

 regards their external form, we shall proceed to consider their 

 internal structure; and this exhibits a degree of complexity 

 which would scarcely be anticipated. The internal structure ^f 

 the leaf cannot be well examined, without a high magnifying 

 power. It is necessary to cut the leaf across with a sharp knife, 

 and then to pare off an excessively thin slice from the cut edge ; 

 so that when a section, exhibiting the thickness of the leaf, from 

 one surface to the other, is placed under the Microscope, the 

 light may be sent through it. A portion of such a section of the 

 leaf of the Lily, which may be regarded as sufficiently charac- 

 teristic of leaves in general, is shown in the accompanying figure. 



o The colourless cells 

 cf the cuticle cover- 

 ing the upper sur- 

 e face of the leaf, are 

 seen above ; and 

 g those of the under 

 c c c cuticle, below. Be- 



FIG. 72 SECTION OF THE LEAF OF'THE LILY; a, cells com- f ween these ire 

 posing cuticle of upper surface; ft," cells of cuticle of lower 



surface ; c, c, stomata ; d, upper closely-set layer of paren- seen a large 



cbyma ; e,.f, lower rows of cells more loosely arranged. , 



which represent the coloured parenchyma or fleshy portion of the 

 leaf. These are arranged with considerable regularity, and are 

 packed very closely together beneath the upper surface ; and 

 there are scarcely any spaces between them in. that part. Below, 

 however, it is seen that the cells are of less regular form, and 

 that they do not come into nearly such close contact, so that 

 there are many spaces amongst them ; which, communicating 

 freely with each other, form what are termed the intercellular 

 passages and spaces of the leaf. 



248. The stomata are chiefly to be found in the lower sur- 

 face ; and they always open into the vacant spaces beneath the 

 cuticle, and not against the cells in contact with it. It is the 

 large proportion of these vacant spaces, which usually contain a 

 considerable quantity of air, that occasions the colour of the 

 under side of the leaf to be usually much lighter than that of the 



