266 THE MOVEMENTS OF WATER 



warming the pots in which they were growing for twelve hours to 37 C. 

 to 39 C. When once induced, the exudation of water may persist under 

 the original conditions. Some plants were induced to bleed by keeping 

 the roots in saline solutions for some time, and then replacing them in 

 water or in a normal nutrient solution. Moreover, Scheit and Wieler 

 were also able to cause an escape of water from certain plants by creating 

 a vacuum over the cut stem. 



Under normal conditions of growth, and especially when transpiration is 

 active, the relationships are usually such that no positive exudation-pressure 

 can be produced. This is commonly the case in trees in summer time, 

 whereas in herbs the necessary accumulation of water frequently takes place, 

 especially during the night. As the leaves unfold on a tree, and transpira- 

 tion becomes increasingly active, the bleeding-pressure gradually disappears, 

 and as the process of foliation continues, the daily variations indicated by 

 a manometer attached to the tree become more and more marked l . 



Both the pressure and the rate of flow show more or less marked 

 variations during short intervals of time, and these may occur in decapitated 

 stems or roots, even when the external conditions are kept perfectly constant 

 and no transpiration is permitted. These are apparently partly due to the 

 daily periodicity, and partly to oscillations of short duration ('automatic 

 variations '), such as usually accompany any vital manifestation, and are 

 especially well shown in the processes of growth and of movement. 

 A certain genetic relationship probably exists between all such variations, 

 for they are simply the external signs of a changed internal activity. The 

 connexion is, however, obscure as yet, and since our knowledge of the causes 

 which induce a periodicity in the power of bleeding is insufficient to render 

 any satisfactory explanation possible, it will be better to discuss the general 

 problems of periodicity in plant life in connexion with the phenomena of 

 growth and movement. It is not even known in a single case whether the 

 pressure and the amount of flow vary correspondingly, and although it is 

 probable that the rate of flow increases as the pressure rises, frequently 

 no precise relationship appears to exist between the two 2 . 



The daily periodicity was first recognized by Hofmeister, and was more closely 

 studied by Baranetzky, Detmer, Brosig, and Wieler 3 . According to these authors 



1 Cf. Hales, Statics, 1748, pp. 68, 73, &c. ; Briicke, Ann. d. Physik u. Chemie, 1844, Bd. LXIII, 

 p. 193; Hofmeister, Flora, 1858, p. 6 ; Th. Hartig, Bot. Zeitung, 1861, p. 17. Brucke found that 

 during prolonged experiments on a vine the daily variations decreased, probably because the wood 

 became less readily permeable, and hence allowed differences of pressure to be manifested only 

 slowly and with difficulty. 



2 Wieler, 1. c., p. 146 ; Hofmeister, Flora, 1862, p. 114. Pitra (I.e.) was unable to detect any 

 daily variation in cut twigs. 



3 Hofmeister, Flora, 1862, p. 106 ; Baranetzky, Unters. iiber d. Period, d. Blutens, in Abhand- 

 hmgcn d. Naturf.-Ges. zu Halle, 1873, Bd. xm, p. 3 ; Detmer, Beitrage z. Theorie d. "Wurzeldruckes, 



