CHAPTER TWELVE 



THE WATER BALANCE IN PLANTS 



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 obvious. 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 

 during the growing period 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 loo 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. 



The balance between transpiration and absorption. The 

 amount of water in the cells of the plant as a whole is determined 

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

 water from the soil ; and (2) the rate of transpiration. The rela- 

 tion between these two rates determines the water balance of the 

 plant. If the transpiration is rapid and absorption is slow, in- 

 ternal drought results and the plant wilts. If the transpiration 

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

 utmost capacity. 



In 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 dis- 

 tended and firm. They are like a football that is only partly 

 inflated. After a heavy shower the plants quickly recover, 

 because the water available in the soil has been increased and 

 more water is taken into the plant. The shower has also covered 

 the leaves with a film of water and made the air moist around 



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