CHAPTER III 



THE INFLUENCE OF WATER ON PLANT-LIFE 



Water is tlie most important factor in ecology. In con- 

 junction with heat, it determines climate. It plays a 

 dominant part in the hfe and well-being of every individual 

 plant, and decides the form and character of the vegeta- 

 tion everywhere. Where water fails, there is no vegeta- 

 tion ; where it is most abundant, there the vegetation is 

 most luxuriant and varied. 



The Role of Water in Plants. 



1. Water is the medium which conveys to aU parts of 

 the plant-body the materials required for its nutrition 

 and growth. These must be in solution, for solid particles 

 cannot pass through the tissues of plants. These sub- 

 stances are, for the green plant : 



(a) Mineral salts, absorbed by the roots from the. soil, 

 and destined to be employed in the elaboration of 

 food ; and 



(6) The food itseK (carbohydrates, proteins, etc.), con- 

 veyed in a watery sap to every living part of the plant 

 — to the points of growth, to the centres of work, or to 

 the seats of storage. 



2. The body of the plant is nearly aU water. In a 

 living cell which has reached its fuU size, the hving 

 substance, or protoplasm, is little more than a thin skin 

 lining the wall. The rest is sap, a watery liquid con- 

 taining nutriment and other substances in solution. 



3. Moreover, the living cell can only keep in health 

 and perform its functions successfully while it is turgid, 

 or stretched with water. As soon as the cells lose their 

 turgidity, the leaves and shoots become limp and droop, 



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