Chap, i Processes of Absorption of Water 277 



Vesque in 1883, in the course of which he saw the water 

 passing along the vessels between the air-bubbles and the 

 walls. More recently Copeland has repeated Vesque 's 

 observations on this point. Strasburger saw the same thing 

 in the tracheides of certain Conifers in 1891. 



It is impossible to put forward here the details of the 

 controversy, which extended up to and beyond the end 

 of the century, especially as it has so far not led to a satis- 

 factory elucidation of the phenomena nor to even a general 

 agreement on the part of botanists. Mention may be made 

 of the names of Russow, Nernst, Darwin, Vines, Ewart, as 

 contributing to the discussion, which is still proceeding. 



The rate at which the water moves in the wood was 

 first satisfactorily examined by McNab in 1871. He caused 

 plants to take up dilute solutions of a salt of lithium, the 

 salt of which he ascertained to travel at the same rate 

 as the water, and by subsequently making spectroscopic 

 examinations of the wood at different levels he determined 

 the rate at which it penetrated the tissue. Pfitzer, in 1877, 

 and Sachs, in 1878, carried out further and more accurate 

 researches on the same lines. 



