INFLUENCE OF WATER ON LAND-PLANTS 33 



the stomata are generally open, but if the weather is 

 very dry and hot, as it often is at midday, the guard-cells 

 lose their turgidity and close. This safeguards the plant 

 against the risks of excessive transpiration during the 

 heat of the day, but it is at the expense of assimilation, 

 for when the pores are closed, no carbonic acid gas can 

 enter the leaf. The stomata also close at night, probably 

 owing to the withdrawal of light, but this, again, serves 

 the plant well, for the ground cools at night, and the 

 roots then absorb less water. 



The amount of water retained in the plant may be 

 reduced in two ways : 



(a) By those causes which diminish the absorption 

 of water by the roots. 



(6) By those causes which increase transpiration. 



Causes which reduce Absorption. 



1. Cold. Just as a rise in temperature increases the 

 activity of all the vital functions, so a fall in temperature 

 decreases it. For this reason, the power of the roots 

 to absorb water declines as the soil becomes cold, and 

 this happens, even though the actual amount of water 

 present may increase. As freezing-point is approached, 

 the roots become extremely inactive, and when the 

 ground is frozen no water is absorbed at all. 



2. A Sour or Salt Soil produces a similar effect. The 

 water absorbed by roots is really a very dilute solution 

 of mineral salts, the amount of solids dissolved in the 

 water rarely reaching 1 per cent. As this proportion 

 of solids is exceeded, the activity of the roots declines, 

 and when the amount reaches 3 to 5 per cent., the roots 

 cease absorbing altogether ; in fact, a very strong solution 

 presented to the root-hairs will actually withdraw water 

 from the plant, and the cells will collapse. Salt water 

 is therefore as good as no water at all, for none is absorbed. 

 Just as with The Ancient Mariner, there may be 



" Water, water everywhere, 

 Nor any drop to drink." 



We conclude from this that, as far as plants are con- 

 cerned, there are several conditions which produce a 

 state of dryness besides drought. Plenty of water may 



3 



