224 LEAVES 



is through dead cells. Those conductive vessels of the wood 

 through which water chiefly ascends are dead. In this 

 region osmosis ceases to be the controlling force. Here 

 solutes cease to be independent of the solvent as to the 

 rate and direction of their movement. They are carried 

 along in the mass movement of the solvent. Water and the 

 solutes in it move together, and far more quickly than when 

 the movement is by diffusion and osmosis. Now if- there 

 were no transpiration, no loss of water at the top, evidently 

 there could be no such mass movement of water and solutes 

 up through the conductive vessels of the wood. Without this 

 mass movement the upper parts of the plant would cease 

 to get the supplies they need. Except in the lowliest 

 plants and in those submerged in water, the journey of 

 the solutes from where they enter to where they are used 

 is far too long for the slow movements of osmosis alone 

 to attend to it. Were it not for the mass movement 

 through the wood vessels, plants could never have attained 

 the stature which they now possess; without this mass 

 movement all the living cells in the upper part of the plant 

 would be without the raw materials for doing their work. 



Thus we see that transpiration, though attended by 

 obvious dangers, is also attended by obvious benefits. 

 Whether its advantages are as great as its disadvantages 

 is an unsettled question. 



B. General Relation of Water to the Plant. — We can 

 hardly consider transpiration without considering the 

 general relation of water to the plant. If you can form a 

 true picture of the way water moves through the plant and 

 passes from it, it will help you greatly in understanding 

 all other features of plant life. Evidently the role of 



