258 The Physiology of Plants BOOKIII 



Wieler in 1893, wherein he discussed the phenomena of 

 both daily and annual periodicity, the latter of which he 

 was the first to study. His experiments were based upon 

 plants kept in pots throughout the year under constant 

 external conditions, and they enabled him to show that 

 while the pressure rises to a maximum in spring, it is 

 absent altogether from many trees for some weeks during 

 the winter, being thus clearly associated with the vital 

 activities of the roots. 



At the commencement of the period under review, the 

 conception of transpiration as a vital process was only very 

 imperfectly realized. Hales had shown that the exhalation 

 of vapour from the surface of a leaf is less than that from 

 an equal area of free water, so that it was evident the pro- 

 cess is not one of ordinary evaporation. A more detailed 

 determination of the fact was made by Unger in 1855, but 

 it was reserved for Sachs to point out clearly that the 

 transpiratory process must be included among the vital 

 phenomena of the organism. He carried out a series of 

 researches into the subject in 1860, taking up the question 

 of the difference observed at the outset by Hales, and 

 showing that such difference is much greater than had been 

 supposed. He pointed out that as cuticular transpiration 

 is but small, and as the greater part of the actual evapora- 

 tion takes place into the intercellular spaces of the leaf, 

 a comparison must be made between the area of the 

 boundaries of these and that of a free surface of water, 

 and not between the latter and the area of the superficies 

 of the leaf. The ratio so arrived at is at least ten times 

 as great as that based upon the latter. Sachs pointed out 

 that the difference can only be attributed to the living 

 substance of the leaf acting as a powerful restrainer of the 

 evaporation of the water. He showed that if a dead mem- 

 brane is substituted for a living one, the evaporation 

 becomes greater than that from a free surface of water. 



