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PLANT PHYSIOLOGY 



The indispensable ash elements may be divided into two 

 distinct groups: the nonmetals, such as sulphur and phosphorus; 

 and the metals, potassium, calcium, magnesium, and iron. 

 Sulphur and phosphorus are found combined with the most 

 important organic substances of the protoplasm in the proteins 

 and nucleoproteins and should therefore be referred to the group 

 of organogens, like the four elements that are volatile on burning 

 (C, N, H, and 0). As they possess a number of similar prop- 



FiG. 73. — Water culture of tobacco in nutrient solution and with deficiencies 

 of various elements. 1, without N; 2, without P; 3, Avithout K; 4, without Ca; 

 5, without Mg; 6, normal nutrient solution; 7, without B; 8, without S; 9, with- 

 out Mn; 10, without Fe. 



erties, S and P may be compared with nitrogen, which is also 

 indispensable for the building of the protein molecule. The 

 metals, on the contrary, are present in the plant mostly as free 

 ions. This is especially the case with potassium, up to 98 per 

 cent of which is found in the plant in ionic form and may be 

 extracted from the dry plant substance by water, combustion 

 not being necessary. Metallic elements acting upon the colloidal 

 substance of the cell play the role of regulators of vital processes 

 stimulating certain of them and inhibiting others. 



This division, however, is not quite distinct. Some organic 

 compounds of metals are present in the most important parts of 



