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, ag 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. 
