102 VEGETABLE PHYSIOLOGY 



their life. The mechanical effects which follow the collapse 

 of the tissue are the consequence of the assumption of a 

 flaccid condition, and they intensify the check to the escape 

 of watery vapour from the affected organ. 



The course of events in a normal leaf during active 

 transpiration appears to be, then, the setting up of a tension 

 in the parenchymatous cells of the leaf by evaporation 

 from their surfaces, which tends to cause them to collapse 

 and become flaccid. This tendency is opposed and over- 

 come by a greater force excited by the turgescence of those 

 cells whose osmotic properties exert a traction upon the 

 water in the conduits or wood -vessels. Water is thus 

 supplied through the inner walls of the evaporating cells as 

 quickly as it is lost by evaporation from the surfaces which 

 abut upon the intercellular passages. 



Dixon ascertained that the osmotic pressure in the 

 leaves of transpiring branches of the Laburnum amounted 

 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, tran- 

 spiration, osmosis in the cells of the leaves, are largely 

 under the control of the living substance, and are particu- 

 larly influence^ 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 ex- 

 amined, 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. 



