The Green Plant 499 



The important physiological functions of leaves are photo- 

 synthesis, transpiration, respiration, digestion and assimilation. 



Photosynthesis, or Food-making. The very great service 

 rendered by leaves is the making of carbohydrate food. This 

 is done only by the green parts of plants, principally the leaves, 

 in the light of the sun. Although apparently the first visible food 

 product made is starch, experts have demonstrated that the very 

 first product manufactured is sugar, which in most cases is quickly 

 changed to starch and. stored in various parts of the plant. This 

 very interesting fact should always be remembered. All the sugar 

 in the world is made by green leaves. 



Food Materials. The materials from which carbohydrate 

 food is made are water and carbon dioxide. Water is obtained 

 from the soil, and carbon dioxide from the air. Other mate- 

 rials needed for the manufacture of other kinds of food 

 necessary to the plant are brought into the plant in solution in 

 water through the medium of the root hairs, tiny threadlike 

 branches of the roots. In this way the plant obtains compounds 

 that contain nitrogen and sulphur. These are combined in the 

 cells with the carbohydrates, and thus the protein food needed 

 for growth and development is provided. Fats are formed by 

 chemical changes in the starches and sugars in plants. They are 

 found stored mainly in nuts, seeds, and grains. 



The Main Requirements for the Manufacture of Carbo- 

 hydrates. The five main requirements for the manufacture of 

 carbohydrates are : a living green plant, the presence of carbon 

 dioxide, some degree of heat, the presence of sunlight shining on 

 the plant, the presence of water, and certain chemical changes. 

 The process takes place not only in the leaves but also in any green 

 part of the plant. The air contains the carbon dioxide. The rays of 

 the sun furnish the energy. The chemical changes take place in the 

 green cells when the two compounds, water and carbon dioxide, 

 are broken up into the elements of which they are composed and 

 are recombined into new substances by a series of steps not fully 

 understood. In the process oxygen is released. The immediate 

 product of this process is grape sugar. In most plants this is 

 changed to starch in the leaves very soon after its formation. 



