98 APPLIED BIOLOGY 



current from root to leaves. This is necessary for the follow- 

 ing reasons : (1) The water carries up in solution indispen- 

 sable food materials obtained from the soil. (2) Water is 

 necessary in many parts of the plant in order to give turgidity 

 and rigidity. Without plenty of water, the plant wilts, 

 which is due to lack of water in cells and consequent loss 

 of turgidity. A Windsor bean plant grown in a pot is excel- 

 lent for showing this. Allow the soil to dry until the plant 

 wilts, then water the soil. (3) Water is necessary in order to 

 dissolve sugar (e.g., maple sap and other sweet juices of 

 plants) and other food-substances which must often be 

 transported in solution from one part of a plant to another. 

 (4) Water is needed because it is used in the chemical com- 

 bination of such food-substances as starch, sugar, and oils 

 which are made by the cells of the plant. (5) Water in large 

 quantities is needed by growing plants because such a 

 large proportion of the substance of new cells is water. 

 (6) Evaporation of water results in cooling the plant, thus 

 preventing a dangerous amount of internal heat. 



In addition to these special reasons why plants need water, 

 we must remember that all living matter requires water 

 ( 12). Without water there is no life, so far as we know. 

 Even seeds which are apparently dry contain a certain amount 

 of water (8 to 15 per cent of their weight). 



96. Food Requirements of Plants. In one of the first 

 lessons ( 16) we found that a plant is made up of water, 

 carbon, and mineral matters (in ashes), and also some carbon 

 and other elements making up the gases which were burned. 

 When chemists carefully analyze these substances from 

 plants they find the following ten chemical elements : carbon, 

 (C), hydrogen (H), oxygen (O), nitrogen (N), sulphur (S), 

 phosphorus (P), iron (Fe), potassium (K), calcium (Ca), 

 and magnesium (Mg) in every plant ; and still other elements 

 are found in many plants, but are not absolutely necessary 

 for plant life. The first four (C, H, O, N) form the chief part 



