268 The Physiology of Plants BOOK in 



observation, also, that a rise in the temperature of the 

 air surrounding a woody plant checks absorption by its 

 roots, a fact which he explained by showing that the 

 gases existing in the vessels at a pressure below that of 

 the air will expand or contract as the temperature rises or 

 falls, so that the negative pressure must vary in amount. 



Pappenheim, in 1892, ascertained that the gaseous tension 

 in the tracheides of certain Conifers often amounts to not 

 more than one-fourth of an atmosphere. Schwendener, in 

 the same year, found this to be about the same in the 

 vessels of woody stems. Haberlandt found that a similar 

 negative pressure may be detected in the elongated cells 

 of the central cylinder of Mosses. 



Von Hohnel not only proved the existence of negative 

 pressure in the experiments described, but he ascertained 

 that it is dependent on transpiration by showing that it 

 gradually disappears as transpiration ceases. Strasburger, 

 in 1891, and Pappenheim, in 1892, showed that it is practi- 

 cally the same throughout the whole length of a branch. 



When, however, we have attributed these two functions 

 to the process of transpiration, the latter has not received 

 the fullest explanation that can be given. Experiments 

 made at the close of the century by Brown and Escombe 

 suggest a further purpose of the highest importance, so 

 important indeed that it appears strange that emphasis 

 was not laid upon it much earlier. Their views may be 

 expressed in the following quotation from an address given 

 by Dr. Brown at the Dover meeting of the British Associa- 

 tion in 1899 : 



' The principal factor which determines the amount of 

 transpiration in a plant must be the amount of radiation 

 falling on it. It is essential that the water-bearing mechan- 

 ism should be able to keep up a good supply of water to 

 the leaf lamina in order to prevent the temperature rising 

 to a dangerously high point. This " safety valve " func- 



