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 

 r ti©a-fiieHtioned above; though^arger-thanTretressaTy fur Lire 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 



7 " Wieler, A. Ueber den Antheil des secundaren Holzes der dicotyle- 

 donen Gewachse an der Saftleitung und Tiber die Bedeutung der Anas- 

 tomosen fur die Wasser-versorgung der transpirirenden Flachen. Jahrb. 

 Wiss. Bot. 19: 82-137. 1888. 



