Ch. Ill, 4] ANATOMY OF LEAVES 33 



means mere gaps in the epidermis, for each is flanked by two 

 special cells called the guard cells, which close and open 

 the stomatal slit in ways, and under conditions, later to be 

 noted. 



The picture of our typical leaf (Fig. 8) shows that the 

 stoma opens into a specially large air space. This space 

 is continuous with others, and with passages in a con- 

 tinuous but irregular system which ramifies everywhere 

 through the chlorenchyma, extending even in thin vertical 

 passages (not clear in our figure, though shown by suitable 

 sections) amongst the densely packed cells of the upper, or 

 palisade, chlorenchyma. Thus every cell of the chloren- 

 chyma is reached by the air system, and therefore can re- 

 ceive carbon dioxide from the air; and by the same route 

 the waste product oxygen is returned to the atmosphere. 

 The air system is not constructed of cells, but is inter-cel- 

 lular, being formed by a splitting and separation of the cell 

 walls in the course of their development. 



The leaf of our picture happens to possess a smooth 

 epidermis, but where trichomes are present the epidermal 

 cells can be seen to extend into one-celled, several-celled, 

 or many-celled hairs, scales, or prickles. Sometimes the 

 chlorenchyma also has part, as with many prickles, in which 

 case the structures are called emergences. Some of the 

 ceils inside the leaf, as shown by a single example in our 

 picture (Fig. 8), contain crystals, which are excretions, or 

 matters useless to the leaf and thus disposed of; and such 

 single specialized cells are called idioblasts. 



The mechanism of the leaf as a photosynthetic organ for 

 the production of food sugar from carbon dioxide and water 

 is sufficiently well known to permit its representation by a 

 diagrammatic plan, as given herewith (Fig. 11). The student 

 should now understand the process so well that with a good 

 section of leaf before him, perhaps aided by our diagram, he 

 can see it proceeding as clearly in imagination as he could 

 with the physical eye were he sufficiently small to wander 



