644 Ganong . — Undescribed Thermometric Movement of Branches 
mometric towards the thermotropic category. If this should prove to be 
true it will render it probable that the movement has some ecological value. 
The inward movement of the branches might be supposed to be protective; 
decreasing slightly the leverage of winter winds upon them, and as well the 
loss of heat and loss of water, but so slight is the amount of movement 
that the advantage can hardly be appreciable. 
Since it is not the application of heat directly which determines the 
movement, but an indirect action through water absorption, it might be 
more exact to speak of the movement as an indirect thermometric 
movement. 
In summary: — 
(a) Some shrubs and small trees, and probably very many, exhibit 
a marked inward and outward movement of their naked winter branches. 
(b) Two forms of the movement occur, a primary or seasonal move- 
ment, inward during the early part of the winter and outward in spring, 
and a secondary movement which is inward with a fall and outward with 
a rise of temperature. Probably these two are due to the same causes, the 
seasonal being simply a secondary movement of large amplitude. 
(e) The movement is correlated with changes of temperature, though 
it is not caused by temperature directly, but by the larger or smaller 
quantities of water which the temperature determines in the plant. A smaller 
quantity of water, due to transpiration exceeding absorption, decreases 
turgidity and permits the natural inward spring of the branches to manifest 
itself, while a larger quantity, due to absorption exceeding transpiration, 
permits an increase of turgidity and consequent swelling, straightening, and 
outward movement of the stem. 
(d) The movement has probably no ecological significance, but is 
merely incidental to the construction of the stem, and is properly indirectly 
thermometric. 
Note. — The irregular lines near the N line on the lower half of Fig. 57 were accidentally 
introduced into the copy, and they cannot be removed from the plate. They have of course no 
meaning for the subject under consideration. 
