igS TRANSPIRATION 



Transpiration-current. — The very extensive loss of water from 

 plants by transpiration would soon end in flagging and death if 

 more water were not absorbed to take the place of that which 

 is given off. The necessary absorption takes place at the root 

 in the manner previously explained and between the root- 

 hairs, where the water enters, and the leaves, where the bulk of 

 it escapes into the air, there is a continuous upward movement 

 of a stream of water through the root and stem of a growing 

 plant. This current of water is termed the traiispiration-current. 



By its means the necessary turgidity of the living cells in all 

 parts of the plant is maintained, and it is concerned with the 

 conveyance of a constant supply of dissolved food-materials 

 from the soil. 



The water absorbed by the root contains dissolved in it 

 various substances which are essential for the nutrition of the 

 plant, and these substances are carried to the cells of the leaves 

 and other organs where they are left and utilized, only pure 

 water escaping in the transpiration-process. Moreover, it may 

 be noted that the conditions which bring about active transpira- 

 tion and rapid movement of water, namely, a high temperature 

 and exposure to bright daylight, are just the conditions which 

 are essential for the rapid formation of organic substance from 

 the food-materials and for the utilisation of the food in the 

 nutritive processes carried on by the plant. 



The movement of water in all parts of plants from cell to cell 

 by simple osmosis, is much too slow to be of use in maintaining 

 an adequate supply to the upper parts of plants where rapid 

 loss is occurring. The transpiration-current travels more rapidly : 

 in certain herbaceous plants it has been found to move at the 

 rate of 5 or 6 feet per hour, when the conditions for trans- 

 piration have been favourable; probably it is slower than this 

 in most trees. 



The path along which the water is conducted is the wood of 

 the plant. That it is not conveyed by the pith of a tree is clear 



