ROOTS AND THEIR FUNCTIONS 83 



ground, and especially the leaves, are contin- 

 ually losing water through the stomata, and 

 the water in the xylem conducting cells and 

 vessels is being as continually drawn upon. 

 Thus the whole water system is in a peculiar 

 condition of considerable " tensile stress." 

 This condition may be compared to a wire 

 which is subjected to a powerful pull. This 

 comparison may appear at first sight to be 

 far fetched, but it really does illustrate fairly 

 well what is going on, especially in the water 

 conduits of tall trees. For when water is 

 enclosed in suitable tubes (and the conducting 

 tissues of the water conduits are suitable in 

 this respect) the force required to break such 

 a column of water is very great, many times 

 that of the pressure of one atmosphere. As 

 everybody knows, in an ordinary tube water 

 can only be maintained at a height of about 

 32 feet by means of atmospheric pressure 

 alone. But pure water, completely filling 

 clean tubes of appropriate structure, will 

 maintain itself at a height many times 

 32 feet, owing to its capacity of resisting 

 tensile stress. 



Although there are certain difficulties, all 

 of which have not as yet been fully met, in 

 explaining the movement of the current of 

 water up through the trunks of tall trees, 

 there is little doubt that the principle just 

 indicated is the main factor in the matter, 

 for though the column of water thus main- 

 tained is very stable as a whole, the individual 



