CHAPTER IV 
THE INFLUENCE OF WATER ON LAND-PLANTS—XERO- 
PHYTES—XEROPHYTIC 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 Jeaf-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 
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