482 Forestry Quarterly 



wood elements is to be more logically explained from the require- 

 ments of his theory than from those of any of the theories hereto- 

 fore advanced. The presence of so many cross walls offering, as 

 they do, enormous resistances to flow, is incomprehensible in any 

 view which regards water as being forced through the stem. 

 From the standpoint of his theory, however, they become neces- 

 sary to confer stability on the tensilely stressed transpiration 

 stream. The various thickenings on the walls of the conducting 

 tubes seem suited to resist crushing forces, but no such forces are 

 called for in the previous theories, and they seem needlessly strong. 

 But for tensile stresses these thickenings are essential and the 

 strength of the tubes may be severely tested in times of excessive 

 evaporation when high tensions develop in the sap. This line of 

 reasoning, however, might lead one into tangled paths. For 

 example, suppose we had two trees of the same size and leaf area, 

 but with great differences in the thickenings and reenforcements of 

 the wood cells, as is the case in an oak compared with a poplar; 

 shall we say that the tensile stresses developed by performing 

 the same amount of work vary in magnitude, or shall we say 

 that nature blundered and made the ceUs too strong in one case 

 or too weak in the other. 



The principal objection brought against Dixon's theory in 

 explanation of the ascent of water in trees, is that it would require 

 continuous columns of water unbroken by gas bubbles to support 

 and transmit the required stresses, and that such unbroken water 

 columns do not exist in a tree. Dixon meets the objection with 

 evidence that the conducting tubes in the leaves rarely, if ever, 

 contain gas bubbles, and that probably the same condition exists 

 in the conducting elements of the newer wood through which most 

 of the transpiration stream passes. In addition, he holds that the 

 presence of some gas bubbles in the columns would not militate 

 against his theory. A gas bubble in a tube would spread until 

 it met the side walls, then if the tension was sufficient, it would 

 spread longitudinally until it occupied the entire tube, but 

 owing to imbibitional forces the retreating water would be held 

 with great tenacity by the walls of the tubes. The gas could pass 

 only with extreme difficulty through the wet membrane. So the 

 bubble would become practically rigid and the water would pass 

 around it as it does in the case of an island in a river. One-half 

 of the tracheal tubes, Dixon asserts, might thus be occupied by 



