2l8 
R. B. HARVEY 
the precipitation of proteins, which has been shown by the author to occur 
on freezing (7, p. 103). This decrease of H+ concentration favors the 
activity of catalase and oxidase. The author has shown (7) that in hardened 
cabbage leaves there is no production of intumescences from frozen spots. 
The reason ascribed for this was that there was no permanent combination 
of protein and H+ and hence no H+ change or protein precipitation in this 
case. There may be attendant upon the protein precipitation, which 
could occur especially at the outer boundary of the protoplast, an increased 
Fig. 2. Intumescences on Bryophyllum leaf induced by freezing. Notice occurrence 
of intumescences along the veinlets and the brown color along the veins in the light-colored 
areas. Light-colored areas are due to death of tissue because too great a percentage of the 
area was frozen. Local freezing occurred in these areas along the veins only, and brown 
color developed there. Subsequently the cells in the light-colored areas died. Renewed 
growth is not due to isolation, for three islets of normal tissue occur in the upper portion of 
the leaf and these show no renewed growth. 
permeability of the membranes for oxygen, so that the oxygen concentration 
within the cell is increased. The next attendant condition would be the 
formation of increased quantities of organic peroxide. The oxidation pro- 
ceeds to the formation of colored compounds, finally producing substances 
such as purpurogallin from pyrogallol or melanins from tyrosine. The 
oxydo-reductases are prevented from normally reducing these colored 
compounds by the increased concentration of oxygen in the cells (15). 
