CHAP, i Processes of Absorption of Water 269 



tion of the transpiration current is not always sufficiently 

 borne in mind, and we are too apt to think that the plant 

 requires these enormous amounts of water in order to 

 supply itself with the requisite mineral salts. The absolute 

 necessity for the supply as a dissipator of energy will become 

 evident by taking one or two facts into consideration. 

 A square metre of the lamina of the leaf of a sunflower 

 weighs about 250 grammes, and its specific heat is about 

 0-9. The hourly transpiration in bright sunshine may be 

 as much as 275 c.c. per square metre, requiring the 

 expenditure of 162,800 calories, and it therefore follows 

 that if the loss of water were stopped, the temperature of 

 the leaf would rise at the rate of more than 12 C. per 

 minute. In making our experiments in glazed cases, it has 

 sometimes been very interesting to watch the result of any 

 accidental stoppage of the water-current in the leaf-stalk, 

 and the almost instantaneous effect this has in destroying 

 the leaf when the insolation is of high intensity.' 



Brown and Escombe strongly supported their view 

 that the process of transpiration has its greatest bearing 

 on the regulation of the temperature of the leaves by 

 investigations into the proportion of the incident radiant 

 energy which is applied to the constructive processes, to 

 which attention will be directed in a subsequent chapter. 



The early ideas of the forces which co-operate in causing 

 the movement of the ascending sap, and of the pathways 

 by which it travels, were purely speculative and failed to 

 stand even the simplest test of experiment. From the time 

 of Hales to about the year 1860, the water was thought to 

 move as in capillary tubes, the vessels and the intercellular 

 spaces being spoken of as transmitting it. Quincke, in 1863, 

 put forward the view that it travels up the vessels as a thin 

 film on the inner side of their walls, a view which was 

 accepted by Sachs in the first edition of his Lehrbuch in 

 1868. Boehm advanced the hypothesis that the pressure 

 of the air contributes to its ascent. The discoveries of 

 Hartig, at the beginning of our period, that the histological 



