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. 



