60 Henry H. Dixon. 



unavoidable source of contamination. An excessive estimate of the 

 resistance will be the result. 



By using: much lower pressures the author has obtained much 

 smaller values for the resistance. Thus it was found that a head 

 equal to the length of stem traversed produced a flow^ in the wood 

 of Taxus baccata of 7*6 cm per hour. The maximal velocitj^ of the 

 transpiration current in the year obtained by E wart's method is 

 7 cm per hour. As we have seen this estimate is probably excessive. 



We are justified then in assuming that the resistance which 

 must be overcome in moving the transpiration current through the 

 stems of trees will not be much more than equivalent to a head of 

 water equal in length to the stem.') 



With this figure we are in a position to find out approximately 



the amount of energy required to raise the transpiration current in 



trees. Each cubic centimetre of water given off from the leaves of 



a tree 100 m high requires an expenditure of work to the extent of 



100 X 100 gramme centimetres to lift it, and, as we have seen, 



about the same quantity of work to overcome the resistance of 



the conducting tracts; this makes a total of 2 X 10* gr. cms = 



2 X 10* 

 1^0 \/ in-' ^^^- ^^ ^'^ ^^^- ^*^^' transporting in the conducting tracts 



one cubic centimetre from the roots to the leaves of a 100 m tree. 



To evaporate a cubic centimetre of water at 20 " C requires 

 592'5 Cal Therefore the work done in transporting the water from 

 the roots to the leaves of a 100 m tree will not require more than 

 the one thousanth part of the energy required for the evaporation of 

 the water. Hence to obtain the energy needed to raise the w^ater in 

 a 100 m tree the amount evaporated will only be diminished by one 

 thousanth part. Even taking the highest and certainly excessive 

 estimate of the resistance, the amount evaporated will only be reduced 

 by Vsoî if) in addition to evaporation, the energy absorbed by the 

 leaf has to do the work of transporting the water from the roots. 



We have seen that the osmotic forces of the cells of the leaves 

 are not as a rule called upon to supply any of the energy needed in 

 raising water, but the stress set up by evaporation from their cell- 

 walls is transmitted across them. This stress exists in the solvent 

 while the osmotic pressure is exerted by the dissolved substances 

 acting against the protoplasmic membranes. It is clear that, if the 

 stress continues greater than the osmotic pressure, the water contents 

 of the cells wäll be reduced, the cells will lose their turgor, and the 

 leaf will fade. As long as the leaves are turgid the osmotic pressure 

 of the cells must be as great as, or greater than, the tension in the 



H. ÏÏ. Dixon, On the Transpiration Current in Plants. Loc. cit. 



