432 



THE AMERICAN NATURALIST 



[Vol. XL 



The Freezing Theory 



Professor Clark ^ and others were inclined to believe the pressure 

 in the maple due to the expansion caused by the freezing of the 

 water within the tissue. "The sap is separated from the cellulose 

 of the wood bv the cold and under ordinary conditions reabsorbed. 

 The bleeding is, therefore, a sort of leakage from the wood, but 

 this is doubtless increased by the elastic forces of the gases in the 

 tree which are compressed by the liberated sap, and the expansive 

 power must be intensified by the increase in temperature which 

 always accompanies a flow. 



" This theory explains the fluctuation of the gauges, and accounts 

 for the singular fact that the upper one shows the most pressure 

 and the greatest variations in as much as the branches and twigs 

 would of course be most quickly and powerfully affected by the 

 heat of the sun and the temperature of the atmosphere. The pres- 

 sure of the expanded gases in a tree in a normal condition would 

 facilitate the reabsorption by the wood of the liberated sap. Their 

 contraction ])y cold would also cause the cessation of flow from 

 a tree which was running, and produce the remarkable phenom- 

 enon of suction exhibited by the gauges at night or during frosty 

 weather." 



That the water would be drawn from the walls, our present 

 knowledge of the Freezing process shows to be true. First, the 



imbibition in the wall- would flow. If at the start the vessels are 

 nearly or c|uite saturated, tlien the extra water from the walls, to- 

 gether with the expansion of the forming ice, would very naturally 

 cause great pressure. In this wise the walls of the vessels or 

 other chambers containing ice would be forced apart in proportion 

 to their elasticity so that on thawing, the water would be under 

 great pressure until sufficient time had elapsed for it to return 

 again to the cells and walls from which it came. 



It is difficult to compute the amount of flow that could be 

 expected from such a source of pressure, but it might be consider- 

 able depending upon the amount of elastic expansion of the tracheal 



' Clark. Observations on the Phenomena of Plant-life, p. 62. 



