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 
