1918} CURRENT LITERATURE 487 
Fibers of tension.—JACCARD” has investigated anew the already frequently 
studied differences in the structure and composition of the wood on the upper 
and lower sides of dorsiventral branches of dicotyledonous trees, along with 
the stimuli producing these differences. In the upper side of such branches he 
frequently finds what he has termed “wood of tension” and “fibers of tension, 
while in the lower side he finds ‘‘wood of compression” and “fibers of com- 
pression.” The fibers of tension are produced by the tension stimulus acting 
upon the cambium region. This stimulus may result from the weight of the 
branch or from bending due to other causes, as negative geotropism or torsion. 
Hence the fibers may occasionally appear on the lower side of the branch or 
even on vertical branches. They can also be produced by the mechanical 
bending of upright stems. The duration and intensity of the stimuli are 
important as in tropisms. There is also a summation of stimuli as in tropisms. 
He speaks of the formation of tension fibers as a purely physiological response, 
which has no hereditary or phylogenetic significance. 
The wood of tension differs from the wood of compression in the following 
ways: more compact grouping of wood fibers with a corresponding reduction 
of vessels; more considerable development of medullary rays with their 
reserves; more regular grouping of the wood fibers; longer fibers with smaller 
lumina. The microchemical study indicates that the fibers of tension are made 
up of a combination of hemicellulose, pectin, and lignin. Fibers of tension are 
more general in summer than in autumn wood. Of the indigenous trees of 
France, Tilia only lacked fibers of tension, and of the introduced forms Lirioden- 
dron Tulipifera lacked them. Rhus typhina lacked while R. cotinus bore them. 
They are generally absent in such shrubs as Lonicera, Ribes, Ligustrum, Vibur- 
num, and Corylus.—Wm. CROCKER. 
Ecological anatomy of leaves.—The variations in transpiration and in 
structure exhibited by the leaves of various forest trees have been studied by 
ANSON, using material from isolated trees growing in the open. Light, 
evaporating power of the air, temperature, humidity, and wind velocity were 
measured at the south periphery and at the center of the crown of the same 
tree, the transpiration of leaves from these two positions determined by the 
use of peismtec ss ci sigh and green eee S - hes ye nye areas soaps = 
finally 
in cross-sections. All the environmental factors showed wide differences, 
which may be illustrated by taking those obtaining within and without 
branches of Acer saccharum, one of the ro tree species studied. Here the con- 
ditions within the crown compared with those at its south periphery were for 
#7 JaccarD, P., Bois de tension et bois de Sa a dans les branches doriven- 
trales des sons Rev. Gen. Bot. 19: 225-242. 
N, HERBERT C., Leaf structure as gat to environment. Amer. Jour. 
Bot. 4: eA figs. 21. 1917 
