Ko. 474] 



SAP FLOW IN MAPLE 



411 



laid the foundation for our scientific knowledge of the bleetHng 

 in these trees. Not until 1903, after 28 years, did the next im- 

 portant contribution to the problem appear. This was the bulletin 

 from the Vermont Experiment Station,* and here in one hundred 

 and forty-one pages, are extensive records of experiments and 

 observations of the most painstaking sort, representing the work 

 of several men during a number of sugar seasons. It is the latest 

 and most important contribution yet made to our knowledge of 

 the subject. The last thirty pages of this bulletin are devoted 

 exclusively to tables recording the results of typical experiments 

 and determinations along the various lines of research. 



In the succeeding pages I shall first attempt to present in con- 

 densed form the main facts connected with maple-sap flow as 

 determined by these investigators and then turn to a consideration 

 of the various theories in detail. In this review my own observa- 

 tions are added only when they are at variance with the others. 



Both Clark and the ^>rmont workers found slight suction ob- 

 taining in the maple tree all through the growing season. This 

 negative pressure, averaging 2.25 kg. per sq. in. fluctuated some- 

 what during the season, and to a slighter extent it also showed a 

 daily periodicity. The latter, however, was not marked. Suction 

 continued in most cases throughout midwinter until February or 

 March. Under certain weather conditions, however, the suction 

 frequently lessened until the zero point was reached and positive 

 pressure resulted. During most of the time until March the 

 tension of any sort was almost Jiil. Then the great oscillations 

 which are characteristic of the sugar season, and which are closely 

 related to the phenomena of sap flow, set in to continue until the 

 buds began to swell. After the buds swelled, the pressure quickly 

 disappeared. Sugar has been made from the maple, according 

 to Clark, in all the winter and spring months from October to May, 

 but, except in the spring, always in small quantities. The flow 

 is said to be better in October and November than later, and rarely 

 occurs in December, January, and early Februar}- except on very 

 warm days. During the warm December of 1905 on several 



> Jones, C. H., Edson, A. W., Morse, W. J. "The Maple Sap Flow." Ver- 

 mont Agrk. Exp. Sta., bull. 103, Dec. 1903. 



