No. 474] 



SAP FLOW IN MAPLE 



449 



temperature at which pure water is at its greatest density suggests 

 that a re-arrangement of molecules in the water might be the source 

 of the stimulus. The objection to this is that only pure water 

 has this point of maximum density while in solutions of but slight 

 concentration the point rapidly approaches the freezing point, 

 and soon these two are identical. 



It must be borne in mind that the idea that pressure is due to 

 unequal permeability as above outlined is theory, not demon- 

 strated fact. Its usefulness should lie in directing future in- 

 vestigation. 



Trees that Bleed Late in Spring 



Trees of this second group show quite a different behavior in 

 regard to the environmental factors, especially temperature. In 

 general the seasonal flow steadily increases from its inception 

 until the maximum is reached and then as gradually declines. The 

 composition of the sap of the different species differs according 

 to the date of flow, and especially the time of beginning. There 

 is little similarity in the composition of the sap in the different 

 species. That of birch contains a large percent (6 f^) of sugar, 

 l)Ut this is olucose, not cane sugar, while that of the grape is almost 



down, the stump sin-face will continue to bleed, in the birch and 

 grape very freely, while the cut sin-face of the trunk will soon 

 become dry. 



According to Clark, tiie black l)irch begins to bleed about 

 April 1, attains its maximum the last of April, and stops about 

 the middle of May. The wild grape commences about :\Iay 1st, 

 arrives at its maximum of flow and pressure about ]May 30th, and 

 ceases early in June. The pressure and flow in both of these plants 

 fluctuates very little as compared with the maple, and depends 



perature affect the pressure slightly, hut only aftt-r several hours. 

 Nearly the whole fluctuation (•()nsi>ts in a rcuular diurnal [trriodi- 



