108 VEGETABLE PHYSIOLOGY 
to between six and eight atmospheres, a force which is 
capable of raising a column of water to a height of more 
than 200 feet. 
Careful consideration of the facts recorded in this chapter 
shows us that although we cannot fully explain the ascent 
of the transpiration current, we can see that it ultimately 
depends upon the behaviour of the protoplasm. All the 
factors which aid its progress, root-pressure, transpiration, 
osmosis in the cells of the leaves, are largely under the 
control of the living substance, and are particularly influenced 
by the power it possesses of allowing more or less water to 
pass through it, according to its condition. Moreover all 
the external influences which we have examined, which are 
brought to bear upon these factors, are mainly efficient in 
as far as they affect the protoplasm in the exercise of this 
power. 
