STRUCTURE 



235 



Fig. 83. — Epidermis of a leaf 

 of lily, showing stomates. 



B. Stomates. — By far the most interesting feature of 

 the epidermis is the stomates. These seem to be doing 

 something. In Figure 83 you see among the cells of the 

 epidermis numerous sausage-shaped cells which occur in 

 pairs. These are the guard-cells of the stomates. The 

 stomate itself is a slit-like opening 

 in the surface of the leaf and it lies 

 between the guard-cells. It opens 

 directly into the intercellular spaces 

 of the mesophyll ; a rather promi- 

 nent intercellular space lies just be- 

 neath it. (See Figure yg, page 220.) 



Stomates are usually very much 

 more abundant on the under than 

 on the upper side of leaves; often 

 they are not found at all on the 

 upper side. On leaves which stand erect they are about 

 equally distributed on both sides, and on leaves which lie 

 on the surface of water, like those of the water lily, they 

 occur only on the upper side. Usually, on the under sur- 

 faces of leaves, stomates are very abundant ; about sixty 

 thousand to the square inch of leaf surface is common, 

 and they have been found in such density as to indicate 

 more than four hundred thousand to the square inch. 

 Stomates are not confined to leaves. They are usually 

 found in connection with any green tissues. They occur 

 in the epidermis of young stems and fruits. 



Stomates have always been a puzzle to botanists and 

 the puzzle is by no means settled yet ; their behavior and 

 the causes which control it are not yet satisfactorily ex- 

 plained. By changes in the condition of the guard-cells, 

 the stomates are opened or closed, but the causes of such 



