CHEMICAL AND PHYSIOLOGICAL STUDIES. 15) 



The vital processes involved in the building up during deoxi- 

 dation and subsequent breaking down of the protojalasm during 

 oxidation are included under the comprehensive term meta- 

 bolism, the building up process being known as anabolism, 

 and the breaking down katabolism. Thus, it is usual to speak 

 of the nitrogenovis and carbohydrate metabolism of a cell, these 

 two being, in fact, the main vital phenomena in assimilation. 

 Moreover, wherever metabolic activity is proceeding, water must 

 always be present, since every chemical reaction in the cell 

 involves this substance. In this respect, then, it would be quite 

 as correct to use the term assimilation in connection with water 

 as it is in the case of the formation of carbon-compounds from 

 the CO., derived from the medium surrounding a plant, or the 

 manufacture of proteids from the nitrogenous raw materials 

 supplied ; for in both these latter cases water is assimilated quite 

 as much as carbon and nitrogen. 



For growth, to proceed satisfactorily in a green plant there are 

 certain elements, in addition to those already mentioned (see 

 p. 7), which have been found to be absolutely indispensable. 

 These are Potassium, Phosphorus, Calcium, Magnesium, and Iron. 

 Sodium does not appear to have the same importance as Potassium 

 in metabolism, and most plants can do without it ; in Fungi, on the 

 other hand, Calcium may be dispensed with, but Iron is necessary 

 to the normal growth of these plants. Certain elements — viz., 

 Ca, Mg, K, and Fe — are always found in the ash produced by the 

 combustion of protoplasm, but it is probable that salts of these 

 metals exist in the living substance not in any chemical combina- 

 tion, but rather as substances which it is extremely difficult to 

 get rid of during analysis. They are thus termed metaplasm. 

 The ash of plants contains in its composition many more 

 elements than the four mentioned above, but, as in the case of 

 Iodine and Bromine in sea-plants, Silica in cereals, and, at times. 

 Aluminium, such elements are not absolutely essential to growth. 

 Nearly every element, including some of the rarer ones (rubidium, 

 thallium, &c.), has been found in the ash of various plants, but it 

 appears that only Potassium, Magnesium, Calcium, Iron, Sulphur, 

 Nitrogen, Phosphorus, Oxygen, Carbon, and Hydrogen (with, 

 perhaps, sodium and silicon) have any real metabolic value. 

 As will be shortly seen. Iron is essential for the formation of 

 chlorophyll. 



