vi TENSION REQUIRED TO RAISE THE SAP 117 



view, and misquotes Berthelot as stating that air-free water 

 may support a tension of more than 200 atmospheres. 



It is needless to criticise Ewart's calculations of the 

 resistance based on Poiseuille's formula, which he himself 

 admits is quite inapplicable to the case, owing to the 

 presence of transverse partitions and irregularities in the 

 cross-sections of the tracheal tubes. It may be noticed, 

 however, that when care was taken that discontinuities 

 were not present in the water columns of the wood experi- 

 mented upon, the flow observed was of the same order as 

 that calculated by the formula. In an experiment on a 

 piece of yew wood the approximation was very remark- 

 able. The actual amount transmitted through a length 

 of 15 cm. was 4'2 c.c. per hour, while the calculated amount 

 was 9*8 c.c. The average distance from one another of 

 the cross-partitions in the fine tubes composing the wood 

 is - 25 cm. Therefore, about 60 partitions must be 

 traversed in passing through a length of 15 cm. This 

 indicates that the resistance offered by the walls, or 

 rather by the pits in the walls, to the passage of water is 

 very slight. 



First method. Ewart also endeavours to find experi- 

 mentally the resistance offered to the transpiration current. 

 His first method, that of Janse and Strasburger, was as 

 follows : " Leafy branches 4 to 8 feet in length were cut 

 under water and kept in darkness for half an hour. 

 Clean ends were then cut under water, placed in freshly 

 filtered eosin solution, and at once exposed in the open 

 on bright, cloudless, breezy June days between 10 a.m. 

 and 1 p.m., with a shade temperature averaging 18 to 

 22 C. The conditions for transpiration were, therefore, 

 optimal. 



" After a timed period the stem was removed, and 

 rapidly sectionised from apex downwards until the eosin 

 solution was visible in the wood, the length of the remain- 

 ing portion of the stem giving the rate of flow during the 



