LEAVES AND THEIR FUNCTIONS 



133 



Photosynthesis. — The process by which starch is formed in 

 green leaves in sunlight is called photosynthesis. This process is 

 the formation of carbohydrate from the carbon dioxide absorbed 

 from the air and from the water present in the cells of the leaf. 



Chemical Action in Starch Making. — In the process of starch making in 

 a leaf, water (H2O) and carbon dioxide (CO2) are combined in such a way 

 as to make starch, the molecule of which is expressed by the formula 

 CeHioOs. This combination is expressed as follows : 5 HoO + 6 CO2 = 

 CeHioOs + 120. The starch thus formed is either stored in the leaf or 

 changed by digestion to some form which can pass by osmosis from cell to 

 cell ; that is, a soluble material like grape sugar. The oxygen is passed off 

 through the stomata of the leaf.^ 



Diagram (after Stevens) to illustrate the chemical processes which take place in the cells of 



a green leaf in the sunlight, 



Proteid Formation in the Plant. — Proteid material is a food which is 

 necessary to form protoplasm. Proteid food is present in the leaf, and 

 is found in the stem and root as well . Proteids can seemingly be manufactured 

 in any plant cells, irrespective of their poi^ition in the plant body The pres- 

 ence of light does not seem to be a necessary factor. The mineials brought 

 up in the soil water form part of its composition, and starch or grape sugar 

 give three elements. The element nitrogen is taken up by the roots as a 

 nitrate (nitrogen in combination with lime or potash), for plants are unable 



^ It seems probable that food material is first made in the form of a mtgar, then 

 changed to starch ; when transported from one part of the plant to axiother it is 

 changed back to sugar. 



