8 4 
THE PLANT WORLD. 
F. Ganong*, on the willow, mountain ash, paper mulberry, flower¬ 
ing dogwood, spice bush, etc., show that in winter the ascending 
branches have a tendency to move inward, nearer and nearer to 
the main stem, while on the approach of spring, these same 
branches move outward, separating more and more from the 
main stem. A branch of the willow, Salix laurifolia, for example, 
moved inward 12.7 cm. or about five inches, between October, 
when the leaves were all on, and January, when the tree was in 
full winter condition. From January to June the branches moved 
gradually outward 44.5 cm., or nearly eighteen inches. 
The seasonal movements accompanying leaf fall and leaf 
formation are obviously due to the removal of the weight of the 
leaves in the one case, and the addition of weight in the other. 
The further continued movements during each season, irrespec¬ 
tive of the leaves, Professor Ganong found to be correlated with 
changes in temperature, a higher temperature resulting in an out¬ 
ward, and a lower temperature in an inward movement, these 
secondary movements having been observed, also, within a single 
day and night. 
The explanation offered for this movement of the branches is 
as follows. With a fall in temperature the amount of water 
gained by osmosis is below that lost by transpiration; the cells 
become flaccid, and an inward movement of the twigs results, the 
inward position being the natural one for dry tissues. On the 
other hand, with a rise in temperature, absorption exceeds trans¬ 
piration ; there is an increase of turgidity in the cells, and a con¬ 
sequent swelling, straightening, and outward movement of the 
twigs. In brief, the movement is connected with the amount of 
water in the stem, the amount of water being dependent upon 
temperature, and the seasonal movement is simply a temperature 
movement on a larger scale. M. M. B. 
Mosses, Trees and Points of the Compass. —A recent num¬ 
ber of del et Terre (December 16, 1904) contains a note on the 
orientation of moss growths on trees. It has been stated that 
mosses grow so much more frequently on the north sides of trees 
that a traveler who has lost his way in a forest can by this means 
