452 



THE AMERICAN NATURALIST 



\you XL 



7. Except when the temperature is rising past the vicinity of 

 0° C. the pressure is very moderate or slight. In the latter case, 

 however, it may rise to the height of from 6 to 9 kg. per sq. in. 

 in less than one and one half hours. Pressure then begins to fall 

 w^hether or not the temperature still continues to rise. 



8. The flow is in a general way coincident with, and propor- 

 tional to, the pressure. 



9. Expansion of gas in the wood can by no means account for 

 the amount of pressure exhibited by the maple; anfl such ex- 

 pansion is likewise probably incapable of accounting for the total 



10. Water expansion in the wood, wdiile it can readily account 

 for the pressure, if confined, is probably not so closely confined, 

 and at any rate is incapable of accounting for the volume of flow. 



11. The expansion of the wood can account for the pressure 

 only when the sap is confined, and such complete confinement is 

 highly improbable. It cannot account for the volume of flow. 



12. No combination of these theories can produce a sufficient 

 explanation of both pressure and flow. 



13. Freezing is not capable of accounting for the phenomenon. 



14. The only theor}^ so far advanced that can account for all 

 the observed phenomena is the living-cell theory. This seems in 

 most respects satisfactory'. 



15. Living cells could produce pressure by contraction only 

 when the outlet of the cell is unconnected with the chamber imme- 

 diately around the contracting membrane. The structure of the 

 •cell renders this improbable, and besides, the delicacy of the proto- 

 plasmic membrane precludes the formation of such high pressures 

 by this means. 



16. Osmotic phenomena seem the only resource. Only by 

 flow through the cell from one reservoir to another, due to the un- 

 equal osmotic permeability at the two ends, does it seem possible 

 to obtain pressure by this method. Osmosis in this way seems 

 sufficient to account for even more than 9 kg. pressure. 



ttie rismg temperatu 



