362 



Scientific Proceedings, Royal Dublin Society. 



Table T. 

 Acer pseudojjlatanus. 



Three measurements on photographs of low magnification of the medullary 

 rays (lumina and walls included) alone gave 21"4 per cent., 14"0 per cent., 

 and 7'1 per cent, respectively. 



The average area occupied by the lumina of the cells amounts to about 

 28"2 per cent, of the whole. 



The determination of the " leak,' or downward percolation of water in a 

 piece of stem of Acer pseudoplataniis under a head of water equal to its length, 

 was made on a cylinder of wood having an average cross-section of 3'70 sq. 

 cm. and a length of 10'4 cm. The " leak " was found to be 0'398 c.c. per 

 minute. 



The average cross-section of the upward streaming protoplasm in this 

 stem would be 0'522 sq. cm. The minimum velocity of streaming required 

 merely to overcome the " leak " observed would be 0'763 cm. per minute, or 

 about 25 times that observed by Janse. Furthermore, to give rise to an 

 upward movement of water in the tracheae of, say, 10 cm. per hour, would 

 require that the streaming of protoplasm in the cells should be at least 30 

 times as fast as that observed by that investigator. 



The foregoing determination of the rate of percolation was made with a 

 piece of stem which had been partially injected with water, as it seemed that 

 the supposed secretion of water upward by the cells would have to overcome 

 the leak through all the tracheae downward ; however, in case it might be 

 urged that the presence of air-bubbles in the tracheae was a necessary link in 

 the mechanism, another determination was made on a piece of wood cut 

 directly from the tree and without any injection of water. As the experiment 



