266 VEGETABLE PHYSIOLOGY 



and the reductases, which act in the opposite direction. The 

 former have been known for some time, the latter have 

 been observed only recently. 



The katabolic processes vary a great deal in the extent 

 to which they are carried out. They may sometimes go 

 on so far as to produce such simple bodies as carbon dioxide 

 and water, which are given off from the organism. This 

 is a very marked feature of the metabolism that may be 

 observed in every living cell. Other katabolic changes, 

 proceeding side by side with this very complete decom- 

 position, are not so far-reaching, and a great accumula- 

 tion of their products remains in the plant. Prominent 

 among them we find the cell-walls of woody or corky 

 tissue. These must not be confused with what we have 

 described as reserve materials, as the latter, unlike those 

 now under discussion, are intended for ultimate consumption. 



These changes involve the manufacture of great 

 masses of material, whose construction, though ultimately 

 dependent upon anabolism, is essentially a mark of the 

 katabolic processes. The constructive processes indeed 

 are both anabolic and katabolic, the former culminating 

 in the formation of living substances, the latter marking 

 the fabrication of its products. The great extent to 

 which the constructive katabolic processes exceed such 

 decomposition of protoplasm, as is marked by the forma- 

 tion of carbon dioxide and water, finds its expression in 

 the enormous bulk which many trees and other plants 

 attain. This increase of the size of the plant-body is very 

 much facilitated by the fact that the katabolic processes 

 in question are not attended by the excretion of any- 

 thing from the body of the organism. As a rule plants 

 have no excreta except the gaseous bodies whose elimination 

 we have already described, and these result in the main 

 from the profounder decomposition of the living substance. 

 Whatever a plant absorbs from the soil, except water, it 

 nearly always retains within its tissues, so that increase of 

 weight almost inevitably accompanies continuance of vitality. 



