52 Irrigation and Drainage 



epidermis of the leaf, these guard cells of the breathing pores 

 contain chlorophyll grains, and are thus able, in the sunshine, to 

 decompose carbonic acid and carry on the processes of building 

 plant-food-; but the very fact that food is being elaborated in 

 these cells causes the sap in them to become more dense, and 

 this, in its turn, causes water from the direction of the roots to 

 enter these cells more rapidly than the elaborated materials es- 

 cape, and so to distend them, and open wide the breathing pores 

 just at the time when air should be admitted to the interior of 

 the leaf. But just as soon as the stimulating effect of sunlight 

 becomes too feeble to allow work to be done in them, then both 

 on account of the elastic tension of these cell walls and because 

 of the diminished osmotic pressure toward the guard cells, more 

 fluid escapes from them than enters them in a given time ; they 

 become limp, and their concave faces flatten and approach each 

 other, thus shutting off the entrance of air to the interior of the 

 leaf and at the same time reducing the loss of water to the 

 mininum. 



Again, if the soil moisture becomes insufficient to meet the 

 demands of the plant, or if hot, drying winds take away the 

 moisture from the leaves faster than osmotic pressure can supply 

 it from the roots, then these guard cells are in the very position to 

 be most and first affected by the shortage of water, and hence are 

 where they will collapse and check the loss from the leaf surface. 

 But just as assimilation cannot go on in the absence of sunlight, 

 so it cannot go on properly in the presence of sunshine if there 

 is a great deficiency of water; and hence we see that the guard 

 cells are so conditioned that they will shut off the air from the 

 interior of the plant at just those times when, if it could be 

 changing, it would be doing an injury by wasting moisture, which 

 is so indispensable to growth, and which it is usually really dif- 

 ficult for plants to get enough of to insure their most rapid and 

 complete development. 



The mechanical principle upon which the guard cells are 

 opened and closed may be readily understood from Fig. 6. For 

 simplicity in illustrating the principles, let A, B, C, D represent 

 four views of a pair of guard cells, A being the pair with the 



