38 Wisconsin Academy of Sciences, Arts, and Letters. 
elaborated foods present in different regions of trunks is not 
primarily responsible for the distribution of radial growth, for 
on such an assumption the greatest growth would always occur 
on the stem just below the branches, while as a matter of fact it 
usually occurs within two meters of the ground. In fact it is 
claimed that both the distribution of metabolized food and radial 
growth are regulated by the wind-pressure-and-weight stimuli. 
The wind effects are thought to induce the transfer of most of 
the food elaborated in the leaves of a recently isolated tree to the 
lower part of the trunk where increased radial growth is caused 
by the increase of the mechanical wind-stimulation. Attention 
is called to the fact that in case of excentric annual rings the ex- 
centrieity is chiefly due to an excessive production of the so- 
called summer wood, thus upholding the view that swaying and 
weight stimuli are especially effective during the latter part of 
the period of radial growth. The data seemed also to show that 
after trees with excentric rings are perhaps about 73 years old 
or have begun to decline in their rate of growth the new rings 
decrease markedly in excentricity and in conformity with that 
it is noted that late season growth is less in trees which have 
reached the age of decline in growth rate. 
Schweinfurth 72 reported that about the Red Sea tree trunks 
all have a greater radius on the south side owing to the occur¬ 
rence there of a continued and strong north wind during the 
summer. The presence of reduced branches on the north side is 
thought to have caused the reduced growth on that side. 
A more detailed application to Schwendener’s mechanical 
principles of plant structure to excentric radial growth in 
branches was made by Ursprung. 73 He maintained that the dis¬ 
tribution of radial growth of both stem and branches is deter¬ 
mined by the compression-strain stimulus resulting from the 
weight of the structure and the action of the wind. Non-verti¬ 
cal stems and branches were usually found to have an elliptical 
cross section with the longer diameter in the direction of gravity. 
This is said to increase the carrying capacity of the wood be¬ 
cause the force required to bend such a branch in a vertical plane 
is proportional to the third power of the vertical diameter and to 
72 Schweinfurth. Sitzungsber. Ges. Naturfor. Freunde. Berlin 1887. 
p. 4. 
73 Ursprung, A. Beitrag zur Erklarung des excentrischen Dicken- 
wachstum. Ber. Deut. Bot. Ges. 19:313-26. 1901. 
