THE LEAF 123 



with water deprived of carbonic acid. We shall not see 

 any bubbles of gas. We infer that leaves give off gas, 

 but only in water containing carbonic acid. 



We are astonished at first to notice that bubbles are 

 given off only on the lower surface of the leaf ; but this 

 phenomenon will be explained as soon as we become 

 acquainted with the microscopic structure of the leaf. 

 We notice in every leaf, or rather in its lamina, two 

 different parts, which are readily distinguishable : 

 the nerves or veins, and in between them the ground 

 tissue of the leaf. On closer examination we find that 

 the upper and lower surfaces are covered with a layer of 

 tissue, the epidermis, which is easily peeled off ; and 

 if we prepare the leaf, by macerating it well in water, we 

 can separate it into three layers, the upper epidermis, 

 the middle part, and the lower epidermis. From this 

 middle part, consisting of the ground tissue and the veins, 

 we can remove the former by carefully beating the leaf 

 with a soft brush and get a beautiful thin transparent 

 network of ribs and veins like a cobweb. We shall 

 consider later the significance of this part of the leaf ; 

 meanwhile let us confine our attention to the ground 

 tissue and the epidermis. The epidermis consists of 

 one layer of cells, distributed in one plane ; the ground 

 tissue is porous and spongy, with spaces of considerable 

 size filled with air. Because of these air spaces leaves 

 float in water ; but if we pump all the air out of them 

 under the water, they will sink, becoming at the same 

 time darker in colour and more transparent ; this is 

 because water has taken the place of air in between the 

 cells. After these preliminaiy explanations we shall 

 now understand this diagram which shows in a magnified 

 and somewhat diagrammatic form a little square of 

 leafy tissue cut out somewhere between the veins 

 (fig- 35)- Here sections in both the transverse and the 

 longitudinal planes are shown, as well as the lower 

 surface of the leaf. The ground tissue consists of two 



