No. 474] 



SAP FLOW IN MAPLE 



451 



Owing to the difference in structure between roots and stems 

 the exact mechanism having to do with the root exudation pheno- 

 mena in all these trees is a different problem, and not to be dis- 

 cussed here. 



Molisch * has recently come to the conclusion that many cases 

 of bleeding in trees through tap-holes or other wounds are local, 

 and exist only after the incision is made. The wound here acts 

 as a stimulus starting repair phenomena, with increased turgidity 

 of the neighboring cells. He is inclined to believe, however, that 

 the spring flows in maple, birch, and grape are general phenomena 

 and of another category. To me it also seems that the conditions 

 outlined at the beginning of this paper rather preclude the con- 

 sideration of the maple phenomena as local. But I cannot see 

 that the fact that they were local would in any -way preclude the 

 action of the pith rays as here outlined. 



Summary 



1. The source of pressure for the bleeding of trees in spring 

 seems to be localized in different portions of the plant in different 



2. Trees may be rouulily classified into two groups in this re- 

 gard: (a) those in which ihr source is mainly in the trunk and 

 branches. These, as for exanii)le the maple, bleed early in the 

 season, {b) Those in which the source is in the root only. These 

 bleed later in the spring, as for example the birch and grape. 



3. Only the problem of the cause of pressure in the maple is 

 considered in this paper. 



4. There seems to be an undoubted relation between the oc- 

 currence of pressure in the maple and the fluctuations in temper- 



5. In con.ieetion with no other fluctuating factor of the en- 

 vironment can such rehition he shown. 



G. Pressure exists ..nly when the temperature is rising. When 

 it falls or remains constant, suction occurs. 



