the Branches in Shrubs and Trees . 
643 
permits a more active absorption by the cells (osmotic absorption by the 
living, and imbibition by the walls of the dead, cells) and their consequent 
swelling, thus producing a straightening and therefore an outward movement 
of the branch. The inward movement would be caused by a lesser absorp- 
tion, which would permit the loss by transpiration to exceed absorption 
and hence render the cells flaccid, permitting the branch to assume its 
natural curve. That there is a steady loss of water from the twigs during 
the winter, including even the coldest weather is, I believe, well known. 
I have myself noted that, on the coldest days in the winter on which 
measurements were made, little ice crystals stood upon the lenticels of both 
Linder a and Salix. Now this steady loss of water implies a steady, even 
though small, absorption through the winter. It is well known, however, 
that with decreasing temperature the power of osmotic absorption falls 
much more rapidly than the rate of transpiration ; hence with a falling 
temperature the loss of water from the parenchyma cells becomes in- 
creasingly great as compared with the possibility of renewing the supply 
osmotically ; the turgidity of the cells must then decrease, and the same 
effect will follow as if the stem is dried out by any other method, namely, 
its curvature is increased and hence an inward movement results. The 
lagging of the movement behind the temperature-changes, earlier men- 
tioned, is strongly in confirmation of this view. Unfortunately, my attempts 
to test this hypothesis experimentally have given very unsatisfactory 
results, so that I am unable to either confirm or disprove it, and as further 
experiment is not now possible until another winter, I must leave its com- 
pletion to a future time or to others. But I regard this as by far the most 
probable explanation of the movement. 
Turning to the question as to how a higher temperature increases the 
water-content of the stem, it is obvious that this is bound up with the still 
unsolved problem of the physics of sap-ascent. The roots of these shrubs 
extend down below the frost line in the soil, so there is no difficulty as to 
the root supply. 
The explanation here attributed to the movement obviously applies to 
both seasonal and secondary fluctuations, and would make them the result 
of the same causes. 
As to the significance of the movement to the plant, I think the 
probabilities are that the movement is a purely physical phenomenon, 
merely an incidental result of the operation of a physical agency upon the 
mechanism the plant happens to present, and that it has no ecological 
advantage. It must be noted, however, that it still remains a possibility 
that the movement may be due to a differential absorption of water, this 
occurring more actively in the cells on the inner than on the outer faces of 
the branches, in which case it might not belong under incidental or physical, 
but under irritable movements, when it would be removed from the ther- 
