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. 
(b) 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 
