554 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
shown that CO2 is ehminated at a much higher rate at 30° C. than at low 
temperatures. 
With stored fruits which contain considerable quantities of acids, the 
acids seem to be utilized in respiration, especially at the low temperatures, 
though there may be some decrease in carbohydrate content as well. The 
acids as a rule break down more rapidly at high temperatures than at low. 
This has been shown to be the case with apples by Bigelow, Gore, and 
Howard and by others. It is also true of grapefruit within limits. The 
sugar content of apples decreases very slowly at low temperatures, while 
with grapefruit^^ there seems to be a decrease of acidity at the low tempera- 
tures, and at high temperatures over long periods the acidity apparently 
increases and the sugars decrease. This, of course, could be explained as 
Miiller-Thurgau explained the carbohydrate changes in potatoes, or on the 
theory that the grapefruit uses acids in respiration at low temperatures and 
carbohydrates at high temperatures with possibly the formation of acids. 
If space permitted, numerous instances could be cited illustrating this 
retarding of chemical processes in plants by lowering the temperature. The 
chemical reactions in plants apparently follow the same rules as chemical 
reactions in vitro, though it is, of course, impossible at present to duplicate, 
outside the plant itself, all the complex interrelated reactions which go to 
make up the so-called vital activities of the organism. 
With a sufficiently low temperature the water, which, of course, is the 
main constituent of fruits and vegetables, freezes, and most of these re- 
actions cease. Some few reactions may proceed slowly after the fruit or 
vegetable is solidly frozen, but most of them will take place only in liquid 
solution. When the organism is thawed and the water or solute is again 
liquid, many of the chemical reactions which were going on before freezing 
are resumed. In the case of most plant parts, however, there is a dis- 
organization of the chemical and physical equilibria, and the processes are 
not checked and balanced as in the living plant. This, of course, leads to a 
breaking down and a decomposition of the tissue. The changes in the plant 
tissue brought about by crystallizing out of the water are discussed by 
Harvey, Miiller-Thurgau,^^ and others who have worked on this particular 
phase of the problem. In the work on freezing of fruits and vegetables 
carried on in our laboratories, freezing-point determinations, that is, deter- 
minations of the temperature at which the water crystallizes in the plant, 
have been made on a large number of varieties of some twenty or thirty of 
^0 Bigelow, W. D., Gore, H. C, and Howard, B, J. Studies on apples. U. S. Dept. 
Agr. Bur. Chem. Bull. 94. 
Hawkins, L. A. A physiological study of grapefruit ripening and storage. Jour. 
Agr. Res. 22: 263-279. 192 1. 
^2 Harvey, R. B. Hardening process in plants and developments from frost injury. 
Jour. Agr. Res. 15: 83-1 11. 1918. 
1^ Miiller-Thurgau, H. Ueber das Gefrieren und Erfrieren der Pflanzen. Landw. 
Jahrb. 9: 133-189. 1880. 
