124 



LEAVES AND THEIR WORK 



giving the energy needed to make the food. This whole process 

 is a very dehcate one, and will take place only when external con- 

 ditions are favorable. For example, too much heat or too little 

 heat stops starch-making ; the presence of stored food in the leaf, 

 or of too much carbon dioxide in the atmosphere, may stop its 

 work. This building up of food and the release of oxygen by 

 the plant in the presence of sunlight is called photosynthesis. 



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 

 CjHioOe. This combination is expressed as follows : 5 H2O -I- 6 CO2 = 

 CeHioOs + 12 O. 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.' 



Proteid-Making and its Re- 

 lation to the Making of Living 

 Matter. — Proteid material is 

 a food which is necessary to 

 form protoplasm. Proteid 

 food is present in the leaf, and 

 is found in the stem or root as 

 well. Proteids can apparently 

 be manufactured in any plant 

 cells, the presence of light not 

 seeming to be a necessary 

 factor. How it is manufac- 

 tured is a matter of conjecture. 

 The minerals 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 



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

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

 changed back to sugar. 



An example of how a tree may exert 

 energy. This rock has been spht by the 

 growing tree. 



