30 TROWBRIDGE: THE THERMOMETRIC MOVEMENTS 
The matter was again strikingly called to mind during an exceed- 
ingly cold period in the early part of February, 1912. As the 
phenomenon seemed not to be generally known among botanists, — 
a series of measurements was made to ascertain the extent of the 
movements, and, if possible, to learn something of their origin. 
During the course of the work, two or three papers on the 
subject came to light, two of which anticipated the observations 
of the writer with respect to the general phenomena of the thermo- 
metric movements of branches of large trees. Since, however, 
the field of the investigation has been extended in several respects, 
and because the matter is so little understood, the observations — 
made are published in full in the present paper. 
The general facts relating to thermometric movements of 
branches of large trees as based on the writer’s observations are 
as follows: It appears in the case of certain species, particularly 
the European linden (J%ilia europaea), that when the temperature 
of the air reaches the freezing point of water, 32° F. (0° C.), the 
branches of the tree, large and small, begin to bend and continue 
this movement with further reduction of temperature. At the 
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lowest temperature measured, about 0° F. (— 17.5° C.), the ends 4 
of the large branches of a particular linden under observation 
showed depressions as great as from three to five feet below their 
position at 32° F. The movements of the branches were found 
in general to correspond closely to the changes in temperature — 
of the surrounding air, as will be shown. The observations made 
consisted of daily and hourly records of. the height of branches of 
several species of trees with corresponding meteorological data. 
The work has been supplemented by photographs. The relation 
of the air temperature and the relative humidity to the observed 
changes in the branches will be presented in the form of curves. 
The subject is an important as well as an interesting one, 
since frost cracks of the linden have been found by the writer to 
accompany in no accidental manner the thermometric movements 
of the branches. Frost cracks have much to do with the injury 
to various trees by insects and by decay, a matter well known, 
and at present a subject of investigation by the Bureau of Plant 
Industry of the Department of Agriculture of the United States. 
