PLANT PHYSIOLOGY 133 



of complex but unknown nature cause them to combine within 

 the cell protoplasm into new substances. 



These new substances are of many varieties. The most im- 

 portant among them is the class of compounds which we have 

 already learned to call proteids. Proteids contain chiefly the 

 elements carbon, oxygen, hydrogen, and nitrogen, and are built 

 out of the nitrates and other minerals absorbed from the soil, 

 in combination with the sugars brought to them from the leaves. 

 Proteids are not the only substances manufactured in the plant 

 cells. Fats are produced which may be stored away in the plants 

 or used for other purposes. Wood is also made and deposited 

 around the protoplasm, forming the walls of the wood cells. 

 Numerous other substances are produced which we need not 

 mention, for the end result is the growth of all parts of the 

 plant which increases in size as these new substances are formed. 

 In all cases, however, the starch made by the leaves is the foun- 

 dation of the new substances made. Starch is always used up 

 and the plant can grow only so long as it has starch at hand in 

 abundance. This process of using starch and making other 

 substances is known as metastasis. 



One of the results of the use of starch for any of these purposes 

 is a combination of part of its carbon with oxygen, forming CO 2 . 

 This is a process similar to the respiration of animals, and the 

 CC>2 is in plants, as in animals, a waste product which must be 

 excreted. It is thus seen that plants carry on two opposite 

 processes. By photosynthesis CO 2 is utilized, starch is formed 

 and is set free; by metastasis O is used, starch is destroyed and 

 C0 2 is set free. During the ordinary life of a plant in daylight, 

 although both processes are going on simultaneously, photo- 

 synthesis is much more vigorous than metastasis, and much 

 more starch is made by the plant than is used, so that oxygen 

 is constantly eliminated. Photosynthesis, since it takes place 

 only in sunlight, can occur only in the daytime, while metastasis, 

 requiring no sunlight, can go on in the night. The process of 

 metastasis goes on fully as well, and certain phases go on better, 



