OF TREE BRANCHES AT FREEZING TEMPERATURE 51 
wards move upwards. If the water in the branches was freezing 
as pure water, then the branches would continue to descend while 
the temperature remained below freezing, since ice would continue 
to form at any temperature below freezing. On the other hand, 
sap from the standpoint of physics is a solution, composed of 
salts of metals and of dissolved organic substances, and, as such, 
must freeze fractionally as the temperature is reduced. The sap 
thus becomes more concentrated as the freezing process continues. 
On a rise of temperature a fractional melting occurs. 
In general, in the case of a solution composed of some salt, a 
point is reached where the solution will freeze at a definite tem- 
perature (in a manner similar to pure water); namely, when the 
solution reaches a certain degree of concentration. The indica- 
tions are thus that it is the sap content of the branch rather than 
the water content which plays the important réle in the thermo- 
metric movements of branches. Geleznow studied the water 
content of branches with little success in so far as determining 
the cause of the thermometric movement, and in the light of the 
explanation given above, negative results should be expected from 
a study of the water content. The mechanism bringing about the 
branch movements may well be a process in the living cells where 
enormous tension is produced in some way from the gradual 
separation of the ice crystals from the sap, the latter concentrat- 
ing as the fractional freezing process advances. If this partial 
explanation is the true one, it should be expected that at some 
low degree of cold, certain trees would cease to show a thermo- 
metric movement of the branches, the sap having reached the 
so-called cryohydrate concentration where the solution would 
become solidified. Since the sap is a complex solution, the tem- 
perature of complete solidification would probably not be a 
definite one. 
The question of the freezing of the tissues of trees appears 
not to be a proven question; yet there seems to be evidence that 
at least a partial freezing of trees at temperatures not far below 
freezing occurs. 
In a paper entitled, ‘The Maple Sap Flow,’’* experiments on 
*C. H. Jones, A. W. Edson and W. J. Morse, Vermont Agric. Exp. Sta., 
Bull. 103: 41-184. pl. 1-17 +f. I-5. 1903. 
