THE LEAF 113 



Sp). They are formed by the splitting apart of two semilunar 

 epidermal cells, which are formed specially for this purpose 

 (Fig. 16, Sp, scJi). These semilunar, or rather kidney-shaped 

 cells, have their concave sides applied so that the ends of the 

 cells alone are in contact, and a space, the actual pore, lies 

 between them (*S^, c). If these kidney-shaped cells are flaccid, 

 and are drawn out lengthways by the surrounding epidermis 

 cells, the bend of each cell is lessened, and the intervening 

 slit or pore is closed. Seen from above, the stomata will then 

 have the appearance of a coffee-bean (Sp). If, however, the 

 kidney-shaped guard cells become turgid through the absorption 



.se^ 



Fig. 16.— Mesophyll Cells and Stomata of the Barley-Leap. 



a stoma ; sch, guard cells ; c, aperture of stomata leading into the respiratory cavity of 

 the leaf ; M, parenchymatous cells of a leaf, taken from near a vascular l)undle ; Z, 

 nucleus ; chl, chlorophyll graius. 



of water, the line (c) in the above figure widens out to form 

 an oblong canal. Through this canal a relatively large amount 

 of air can enter the respiratory cavity or escape from it. If 

 we consider for a moment that the leaf of an apple-tree has 

 over 100,000 such pores to every square inch of leaf surface, 

 we can form an idea of the ease with which an interchange 

 of gases can be effected between the atmosphere and the 

 inside of the leaf. 



The structure of a leaf will become still more clear to us if 

 we examine the transverse section of a leaf (Fig. 1 7). We 

 will take as an example the leaf of the india-rubber plant 

 {Ficus elastica), which is suffei'ing from a disease very prevalent 

 in plants grown in dwelling-rooms, and termed intinnesrentia. 

 This disease consists in the appearance of numerous small 



II 



