410 



THE AMERICAN NATURALIST 



[Vol. XL 



into two very definite groups as regards the bleeding phenomena, 

 with a different source, and perhaps different cause, for the bleeding 

 in the two cases. Those which, like the maple, bleed early in the 

 spring and are dependent upon temperature, constitute one class, 

 while the late-bleeding ones, like birch and grape, which are not 

 intimately dependent upon the temperature, constitute a class by 

 themselves. This paper is concerned entirely with the first of 

 these two groups, and only a few words will be said about the other 

 group at the close, as a matter of comparison. 



Branches cut from certain trees at a low temperature and brought 

 into the warm laboratory often show bleeding from the cut sur- 

 face. Clark tested sugar maple, white birch, elm, hickory, button- 

 wood, chestnut, and willow in this respect. The maple soon began 

 to bleed at the rate of 24 drops per minute, while the buttonwood 

 bled 11 drops, and the hickory exuded a little very sweet sap, 

 precisely as in sj)ring. The birch, chestnut, elm, and willow did 

 not flow at all, and were not even moist on the cut surface. I 

 have often repeated this experiment with maple branches. By 

 passing the branch between the flues of a radiator a very vigorous 

 exudation can frequently be obtained. The trees that bleed in 

 this way are usually those in which the vessels are comparatively 

 saturated with sap. According to Clark, a mercurial gauge at- 

 tached to the end of a frozen branch of sugar maple indicated 

 pressure and suction when the temperature was raised and lowered 

 precisely as it would have done upon a maple tree during the 

 ordinary alternations of day and night in the spring of the year 

 when the sap is flowing. 



Late Winter Flow in ^Maple and Other Trees 



Considering its importance from an economic standpoint, 

 subject of maple-sap flow has received corni)aratively little atten- 

 tion. In 1874 and 1875 Clark^ published the results of several 

 years of detailed work upon the maple. These two pajoers really 



