38 Wisconsin Academy of Sciences, Arts, and Letters. 



rial KM air<l foods present iu 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 pf the mechanical wind-stimulation. Attention 

 is called to the fact that in case of excentric annual rings the ex- 

 centricity 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 Schv.-einfurth. Sitzungsber. Ges. Naturfor. Freunde. Berlin 1867. 

 p. 4. 



- 1 Urspruner, A. Beitras: zur Erklarung des excentrischen Dicken- 

 wachstum. Ber. Deut. Bot. Ges. 19:313-26. 1901. 



