HOW PLANTS GIVE OFF WATER 101 
the cells inside the leaf. Two cells, each shaped like a 
crescent, 1f you take a surface view, fit in such a way 
around the opening that they stand guard over it. We 
call them guard cells. 
During the day the guard cells are filled tightly 
with water and press back against the other cells and 
keep the little holes (stomates) open. At night they 
lose some of their water, so they are not so tight. 
They then collapse a little, so that 
their inner edges come together and 
close the opening. The water vapour 
cannot escape so fast when the stomates 
are closed. 
On very sunny days during dry 

weather, if the roots cannot give the 
Fic. 136. The stomate 
closed; the air spaces in 
lose some ot their water, SO that they the leaf can also be seen. 
plant enough water, the guard cells 
close up and prevent such a large escape of water as 
would take place should they remain open. Is this not 
a good arrangement which the leaf has to prevent the 
loss of too much water during dry weather? Some- 
times, however, the ground gets so dry that the roots 
cannot get enough water for the plant. The plants 
then wilt, and sometimes die. 
Leaves help to lift water in the plant. As the water 
evaporates or transpires from the surface of the leaves 
more water is drawn up into the leaf to take its place. 
