52 Science of Plant Life 



The balance between transpiration and absorption. The 

 amount of water in the cells of the plant as a whole is de- 

 termined largely by two processes : (i) the rate of absorption 

 — the taking of water from the soil ; and (2) the rate of 

 transpiration. The relation between these two rates de- 

 termines the water balance inside the plant. If the tran- 

 spiration is rapid and absorption is slow, internal drought 

 results and the plant may wilt. If the transpiration is slow 

 and the water intake is rapid, the cells will be filled to their 

 utmost capacity. 



Importance of the water balance. Of all the factors that 

 influence the growth of plants and modify the form, size, and 

 structure of leaves, the water content of the cells is the most 

 important. Abundant water permits a plant to grow to its 

 greatest height, and permits the leaves to attain their largest 

 size and number. Long-continued internal drought may 

 cause the plant to be dwarfed and the leaves to be small and 

 few in number. In the river bottom the bur oak may develop 

 into a magnificent tree 100 feet in height, while on the dry 

 uplands it may attain only a stunted growth of less than 15 

 feet. An average leaf on a large tree will have twice the area 

 of a leaf on a stunted one, and the number of leaves on the 

 larger tree will be many times the number on the smaller. 



In the summer, when the soil is dry and the air is hot, 

 transpiration may cause the leaves to lose water so rapidly 

 that they droop, and we say that the plant is wilted. Water 

 has passed out of the cells of the leaf faster than the water- 

 conducting tissue has brought in water to replace it, and the 

 cells are no longer distended and firm. They are like a foot- 

 ball that is only partly inflated. After a heavy shower the 

 plants quickly recover, because the water available in the 



