No; 474] 



SAP FLOW IN MAPLE 



419 



of physiologists to this substance as a source of pressure, and until 

 recently it has seemed to many a very probable explanation. 



In 1767, DuHameP and Dalibard noticed that saturated wood 

 became lighter when heated in hot water because of the loss of a 

 portion of the contained fluid, and regained nearly its original 

 weight when cooled, because of reabsorption. This was true 

 except when the water in the wood froze, in which case the wood 

 became again lighter in proportion to the frost. 



Hartig,^ in 1853, experimented with normal living wood using 

 shoots of poplar, birch, etc. He found that, if these were taken 

 before sap flow had normally begun and warmed over a lamp or 

 in the hand, a small amount of sap would appear on the lower 

 surface. If the temperature was near that at which sap flow 

 normally appeared and the twig was normal, the excretion would 

 occur to a slight extent simply by the application of the finger to 

 the bark. 



In 1860 appeared the exhaustive paper of Sachs ^ who was the 

 first to put the gas-expansion theory on a firm scientific basis. A 

 cylindrical stick of Rhamnus frangida 20 cm. long and 1.5 cm. 

 thick, after having lain for 4 weeks in water of from 4° to 8° R. 

 was treated and weighed as follows: — 



6 hrs. in water at 20° R. weighed 51.4 gms. Water inside 28.6 

 16 " " " " 3° " 52.5 " " " 29.7 



1 " " " " 30° " 51.4 " " " 28.6 



2 " " " " 25° " 51.7 " " " 28.9 

 1 " " " " 40° " 51.2 " " " 28.4 

 1 " " " " 2° " 54.1 " " " 31.3 



16 " " " " 4° " 54.2 " " " 31.4 



1 " " " " 30° " 52.6 " " " 29.S 



2 " " " " 4° " 54.3 " " 31.5 



Although, as seen from weights No. 2 and No. 7 which are at 

 nearly the same temperature, there was a progressive increase in 



' Du Hamel. Du transport, de la conservation et de la force dt's bois. Paris. 

 1767. 



^ Hartig. Th. "Uehcr die endoslnotischen Eigeiischaften der Pflanzen- 

 hiiute." Bot. Zeit., vol. 11, n. 313. 18.53. 



