THE WATER-RELATION 91 



than by night. This explains how under day conditions the suctorial 

 factor is chiefly effective in moving the Transpiration-Stream upwards. 



The stoma is such an important organ in the life of aerial plants that its 

 action deserves special attention. There is a good deal of variety of detail 

 of structure in different cases, but the chief features are these. The two 

 guard-cells, attached at their ends, are usually curved, the wall facing the 

 pore being shorter than that in contact with the adjoining cells. The inner 

 and outer slopes of the face next the pore bear each a projecting ridge. The 



FIG. 68. 



Diagram representing a stoma in median section, and also in surface-view. The 

 continuous lines show the position of its guard cells in the open state, of turgor ; 

 the dotted lines show their position in the closed state. 



action depends upon increase and decrease of the turgor of the guard-ceJls, as 

 compared with that of their neighbours. The open stoma, with its tense cells, 

 requires more room than the closed stoma. That room has to be gained by 

 forcing the adjoining cells aside. Where the cell-walls are thick, special thin 

 areas of cell-wall are found which are effective as joints or hinges, allowing the 

 cells to adjust themselves mutually when the pore opens. Chlorophyll is 

 present regularly in the guard-cells, while it is deficient or absent in most plants 

 from the rest of the epidermis. Thus the guard-cells on the incidence of light 

 increase their supply of osmotic substance more than these, and so increase 

 their turgor. It is this increase of. turgor, beyond that of the surrounding 

 cells, acting on cells of the form and structure of the guard-cells, that makes 

 the pore open (Fig. 68). The internal pressure being equal over the whole 



