CHAPTER IV 



THE INFLUENCE OF WATER ON LAND-PLANTS XERO- 

 PHYTESXEROPHYTIC FACTORS AND CHARACTERS 



WE considered in the last chapter the transpiration- or 

 water-current, and we pointed out how important it is 

 that this current should be maintained in sufficient 

 motion and strength to satisfy the needs of the plant 

 during the various phases of its existence. The needs 

 of the plant, of course, vary with the seasons. In spring 

 the demand upon the water-current is greatest, because 

 growth is then most vigorous ; in winter the demand 

 sinks to a minimum, and the transpiration-current 

 becomes almost stationary. This stream of water starts 

 at the roots, where it is absorbed, and after dividing into 

 countless tributaries, ends at the leaf-surfaces, where 

 it is transpired. It is clear, therefore, that anything 

 which tends either to diminish the amount of water 

 absorbed by the roots or to increase the quantity of water 

 transpired through the stomata, must weaken the strength 

 and flow of the transpiration-current, and in either case 

 the plant may suffer through lack of water. In the 

 summer this is serious, for if the deficiency becomes too 

 pronounced, the plant may dry up and perish. Even 

 when means are present, whereby the loss by transpira- 

 tion is so regulated that it does not exceed the absorption, 

 the current runs slow, and as the materials necessary in 

 the construction of food are contained in this current, 

 growth is checked, and the plant suffers from lack of 

 nutrition. To a small extent, however, every plant has 

 control over its transpiration. The guard-cells of the 

 stomata are self-regulating, and they adapt the width 

 of the apertures to the state of the weather. This is 

 manifestly advantageous to the plant. During the day 



32 



