IV.] VARIOUS THEORIES, &c. 141 



According to this, we must look upon the bordered 

 pits as partly filters to let water pass from one column 

 to another, and partly as valves which, so long as they 

 are wet, permit no air to enter, functions they seem 

 adapted to fulfil. 



While it is perhaps still impossible to explain in all 

 its details the ascent of water in tall trees, then, we 

 must regard it as most probably depending on long 

 capillary columns of water being maintained in the 

 vessels, which are pulled by the osmotic draught in 

 the cells of the leaves, to replace the water lost by 

 transpiration ; since we are assured by De Vries and 

 others who have measured the force of the osmotic 

 draught, that it amounts to the equivalent of the 

 pressure of many atmospheres, and by Dixon and Joly 

 that the columns are capable of withstanding the 

 strain. This amounts in great measure to a 

 compromise between the capillary and the osmotic 

 theories of previous speculators on the subject, and is 

 not at variance with most of the observed facts. ^ 



Much, however, remains to be done before it can be 



fully accepted, 



^ Those who wish to go further into these matters should read Mr. 

 F. Darwin's paper, On the Ascent of Water in Trees, at the Liverpool 

 Meeting of the British Association, 1896; Annals of Botany^ December, 

 1896, p. 630. 



