FORMATION OF OVERGROWTHS 
219 
If the cell has been injured to such an extent as to be killed, all the oxygen 
acceptors are changed to the final state of equilibrium with the air. In case 
the cell recovers, an increased rate of oxidation has been established in it 
above the rate of oxidation in the cells in areas not frozen, owing to the 
greater concentration of organic peroxide and decreased H+ concentration. 
Child (16) has shown that dominance of growth is conditioned by a 
higher rate of metabolism in the growing region than in the surrounding 
area. When this increased rate is once established, it then opens the way 
for further growth at the expense of the surrounding tissue. In case too 
great a percentage of the leaf cells are frozen, there is no great concentration 
and transformation of the oxygen acceptors in any one area, as shown by 
little development of brown color. Hence there is little growth produced, 
because no one frozen area dominates a sufficiently large area of normal 
tissue which supplies , it. This is actually the case. The most rapidly 
growing areas are small and somewhat scattered. Loeb (17) has suggested 
that the quantity of growth in Bryophyllum calycinum is conditioned by 
the quantity of some growth-promoting substances within the leaf. It is 
entirely possible that the usual sources of energy, the carbohydrates, are 
not the only substances concerned, but also that there are chromogenic 
substances of equal importance. Oxidase determinations indicate that 
there may be such substances produced or accumulated in plant tumors 
caused by Bacterium tumefaciens. 
Loeb (21) also suggests that the dominance of a growing apex is due to 
the production within it of inhibiting substances which hold in check the 
neighboring buds. The difficulty with this assumption is that if the in- 
hibiting substance is produced in the dominant apex it should depress 
growth there also since it would be in greatest concentration there. This 
ought to be a general objection to such assumptions. It seems to the 
author more plausible to assume that in correlation of growth the dominance 
of a growing area is conditioned not by the production of an inhibitor for 
the area around it, but by the removal from the surrounding area of growth- 
stimulating substances and their accumulation in the dominant area. 
These growth-stimulating substances, such as the chromogens of Palladin 
(23), are produced by all the cells and are diffusible. We have seen in a 
case cited above (Bryophyllum) how they accumulate in a frozen area 
and are associated with a renewal of cell growth. 
In the case of inoculation of tissues with Bacterium tumefaciens, the 
presence of this bacterium favors the action of the respiratory enzymes by 
locally decreasing the H+ concentration. This condition is continuous, and 
hence the growth process should be continuous. The bacteria in such tumor 
tissue evidently do not produce substances detrimental to cell multiplica- 
tion, but may produce substances which favor oxidation within the tissue. 
