CHEMICAL AND PHYSIOLOGICAL STUDIES. 159 



metabolism is often quite as complex as the anabolic. The 

 oxygen required by the cell for the purposes of oxidation is 

 obtained either from the air or water surrounding it or from 

 easily reducible substances in the cell itself. Most of the oxygen 

 produced during the assimilation of COo and HjO is, as will be 

 shortly proved, evolved from the cell as free oxygen, and is not 

 utilised for the purposes of oxidation, although, in the case of 

 water-plants, some of it may be dissolved in the water and 

 re-utilised. As will be pointed out a little further on, the inter- 

 mediate reactions involved in nitrogenous katabolism occasionally 

 result in the formation of such bodies as alkaloids or glucosides ; 

 and many of the bye-products of both carbohydrate and nitro- 

 genous metabolism consist of organic acids, such as oxalic, tannic, 

 meconic, ulmic, &c., which combine with bases present in the 

 cell-sap to form definite salts which at times separate out in 

 the sap (vide raphides). These bodies — viz., the alkaloids, 

 glucosides, and organic acids — are, as a rule, removed to those 

 cells of a plant where they will have no further action ujjou 

 metabolism. 



Constructive processes in the cell are partly anabolic, and 

 partly katabolic ; thus, the building up of fresh protoplasm from 

 proteids, carbohydrates and amido-acids (see tn.fra) is an anabolic 

 process, whilst the formation of cellulose, wood, and cork are 

 instances of katabolic construction, cytoplasm being broken down 

 again in these latter. 



The Enzymes (ferments), which have been mentioned above, 

 are formed by the protoplasm by a sort of double process — viz., 

 anabolic to start with, and the substances so produced (zymo- 

 gens) are broken down again (katabolism) to form the ferment. 



Oils and fats arise in the cell during metabolism by a break- 

 ing down of the cytoplasm during oxidation ; and many of the 

 non-nitrogenous vegetable acids met with are products of kata- 

 bolism, but in a few instances they may be formed as bye- 

 products during anabolic processes (oxalic acid). 



The formation of the cell-wall by the cytoplasm has been 

 shown to be connected with the deposition of microsomata upon 

 the wall, and the conversion of these into cellulose (or pectose) 

 by a process of self-decomposition (secretion). The cell-plate 

 (vide Chap, viii.) is formed in much the san^e manner. 



In a few cases the formation of oils and fats has been shown 



