62 MOVEMENT OF MATERIALS IN PLANTS 



and untler certain conditions it may be very high. It 

 tends to be so at night when evaporation from the leaves 

 slows down or ceases, while the absorption of water by 

 the roots continues. Thus water accumulates in the tis- 

 sues of the plant. Much of what we call dew on the 

 grass is a consequence of the squeezing-out of water in 

 this way. This is termed guttation. Bleeding from 

 wounds is deliberately brought about by man in his tap- 

 ping of maple trees in the spring to make syrup 

 and sugar; and of certain cacti and similar desert 

 plants from which water and saps are collected which are 

 used either with or without fermentation. Thus we see 

 that the processes of absorption, secretion, and excretion 

 are due to the kinetic energy of the molecules of dis- 

 solved substances and of their common solvent, water. 



