GROWTH. 181 



absence of transpiration. When the leaves are formed 

 and transpiration sets in actively, the root-pressure is 

 relieved and the stems will no longer bleed immediately 

 on being wounded. In some plants the excessive root- 

 pressure even causes drops of water to exude from the 

 leaves. 



3 1 6. We may observe, then, three distinct movements of 

 water in the plant: (1) the rapid movement to make up 

 for the loss by transpiration, (2) the slow movement to 

 supply the growing cells with requisite moisture, and (3) 

 the movement due to root-pressure. 



317. Growth. Growth has already been referred to 

 as consisting in the formation and subsequent enlargement 

 of new cells, accompanied in many cases by change of 

 form. It has also been mentioned that the enlargement 

 is the result of the introduction of new particles of 

 vegetable material into the spaces between the molecules 

 of the parts already formed a process known as intussus- 

 ception. It is now generally admitted that each of the 

 molecules of which the plant-body is made up is enveloped 

 in a sheath of water. We know that the presence of 

 water is essential to growth ; when it is absorbed by a 

 growing cell the immediate effect is to stretch the cell, as 

 it were, to its utmost capacity ; in other words, to separate 

 the molecules as far as possible and so increase the amount 

 of water between them, thus making it possible to inter- 

 pose new molecules of solid matter. The use of the water, 

 also, as a vehicle for conveying ihe new material is obvious. 

 This new material, the presence of which is essential to 

 growth, is commonly supplied to the growing points from 

 older parts which serve the purpose of storehouses, as 



and tubers, or of manufactories, as the leaves. 



