No. 474] 



SAP FLOW IN MAPLE 



427 



the walls of the vessels are not so thick as those of the fibers, it is 

 very likely that the large tubes would become to a slight extent 

 collapsed. If they were completely filled with sap, the pressure 

 would be transmitted directly to the pressure gauge. Later in 

 the day, the temperature having penetrated to the inner layers of 

 the tree, the pressure would disappear. Such a theory could also 

 account for the very slight pressure observed in twigs which have 

 a small diameter and are therefore quickly heated throughout. 



Again, from another standpoint,^ it is known that wood expands 

 more across grain than longitudinally. Probably, like the swelling 

 of wood cells, this is dependent upon the micellar structure of the 

 walls themselves. As in the case of swelling therefore, it may be 

 that the wood is free to expand tangentially, but is retarded radially 

 by the massive pith-rays, the cells of which lie upon their side so 

 to speak, and therefore expand less in the radial direction. The 

 wood may in this way, independent of the contrast between inner 

 and outer temperature develop an internal pressure, which would 

 tend to compress the vessels as in the previous case. Pressure 

 produced by this method, however, would not tend to disappear 

 with the penetration of the heat. 



As in the water-expansion theory, here, too, the same relation 

 must be supposed to exist between the wood fibers and the vessels, 

 and the same impenetrability of the walls must be assumed. Con- 

 sequently the objection must again l)e brought forward to assuming 

 such a high degree of impermeability. The expansion of wood is 

 even less than that of water and hence a still higher inipenneability 

 must be assumed. Granted tliis im])ernieabiliry, however, an 

 almost unlimited pressure could be theoretically obtained. 



Suction could be accounted for, as in the water-expansion theory, 

 either by supposing a normal two or three pound suction in the 

 tree, or by supposing that some fluid had passed into the wood 

 fibers and was retarded in its return. 



The occurrence of maximum pressure so early in the day could 

 be accounted for under the first method of pressure-origin by 

 assuming that after a short time the tein{)erature in the outer and 



' Roth, F. "Timber." U. S. Dept. Agnc, Dept. l-mrdnj, Lull. 10, p. 32. 



