104 THE WORK OF PLANTS 
This condition of things sometimes happens at night 
when the soil is warm and the air damp and cool, so 
that the green leaves cannot transpire rapidly. We say, 
then, that root pressure exceeds transpiration. When a 
plant wilts on a hot, dry day, it 1s transpiring faster than 
the roots can lift up water, because there is so little water 
in the dry sou. Transpiration now exceeds root pressure. 
