Nutrition of Multicellular Plants - 255 



hydrogen-oxygen parts of proteins and other 

 essential compounds (about 10 percent). In- 

 evitably, therefore, plant growth represents 

 a net increase of organic matter in which the 

 stores of useful energy are derived entirely 

 from the sun. 



Again it must be emphasized that the syn- 

 thetic powers of plants are amazingly com- 

 plex and that plant metabolism is still a 

 very active field of research. For example, 

 the role of the trace elements (boron, man- 

 ganese, zinc, etc.), which are needed in ex- 

 tremely small amounts to maintain the 

 growth of many plants, is now receiving an 

 intensive investigation (see Fig. 13-20), and 

 the relation between these elements and spe- 



cific metabolic enzymes is undergoing clarifi- 

 cation. 



RESPIRATION 1 IN HIGHER PLANTS 



Each organ of the plant takes in oxygen 

 and gives off carbon dioxide mainly on a 

 local basis. All the cells use oxygen and pro- 

 duce carbon dioxide as oxidative metabol- 

 ism proceeds, and consequently respiration 

 must occur in all the organs. 



The leaves differ from the stem and roots 



1 In a broader sense the term "respiration" em- 

 braces all processes of oxidative metabolism. Here, 

 however, it merely designates the taking in of oxygen 

 and the giving off of carbon dioxide by the organism. 



Fig. 13-20. Radioactive zinc (Zn 65 ) accumulated in the seeds of the fruit of a tomato plant grown in a culture 

 solution containing only 0.1 part of radioactive zinc per million parts of solution. Although the amount of zinc 

 present in one seed is only three-billionths of one gram, without this small amount no seed would be formed. This 

 photograph, called a radioautograph, was made by placing thin slices of the young fruit in contact with a 

 photographic plate. Thus an exposure of the film is effected only in local areas where the radiations from the 

 accumulated Zn 05 are sufficiently intense. Such experiments show that small amounts of zinc are essential to 

 many plants, and that the plant does not differentiate between ordinary zinc and the radioactive isotope (Zn 6r '). 

 (Courtesy of Perry R. Stout, Division of Plant Nutrition, University of California.) 



