40 Wisconsin Academy of Sciences, Arts, and Letters. 
of radial growth, and besides, he holds that the distribution of 
growth on a tree-trunk having concentric rings could not 
conceivably be dependent upon wind action. From the meas¬ 
urements and calculations it is concluded, however, that tree- 
trunks are shafts of equal water conductance throughout. From 
insufficient data and non-convincing arguments it is concluded 
tion mentioned above, though larger than necessary for the wind- 
that the diameter of tree trunks above and below the 5 to 9 m. por¬ 
tion mentioned above, though larger than necessary for the wind- 
gravity hypothesis are of just the size required of a shaft of 
equal water conductance throughout. The morph'ogenic power 
of the water current is thought to be proportional to the rate of 
metabolism and transpiration. The rate of cambial division is 
held to depend upon and be controlled by turgidity, and the in¬ 
fluence of the environment is thought to affect radial growth 
chiefly through the transpiration stream. In the calculation up¬ 
on which this hypothesis is founded it was assumed that the 
water conduction is confined to the outermost ring or wood 
sheath. 
This hypothesis has some defects in common with the one it is 
supposed to supplant in that the distribution of radial growth is 
assumed to be controlled chiefly by one factor, other factors be¬ 
ing effective only in so far as the basic one is influenced. Jac- 
card has many difficult problems to solve before his hypothesis 
to account for the actual distribution of radial growth in trees 
can be considered a theory. The relation of the first radial 
growth and its distribution in trees to the transpiration stream 
in cases where such growth precedes actual unfolding of the 
leaves will need to be explained in the promised detailed study 
he is to publish in a future paper. Nor is it permissible to as¬ 
sume as a fact that the water current is confined to the outer¬ 
most ring of wood, especially when it is recalled that in certain 
portions of trunks radial growth may be wholly omitted during 
a number of successive years, and that many cases of girdling are 
also on record in which trees operated on vegetated and fruited 
normally during several years. 
Wieler 76 concluded that practically all water is conducted in 
76 Wieler, A. Ueber den Antheil des secundaren Holzes der dicotyle- 
donen Gewachse an der Saftleitung nnd iiber die Bedeutung der Anas- 
tomosen fiir die Wasser-versorgung der transpirirenden Flachen. Jahrb. 
Wiss. Bot. 19: 82-137. 1888. 
