CHAPTER II 



ASCENT OF SAP IN STEMS. CRITICISM OF PHYSICAL 



THEORIES 



Early writers. Since the ringing experiment of 

 Hales (1727) 1 and the experiments of Magnol and De la 

 Baisse, who about the same time supplied cut branches of 

 plants with coloured fluids and thus mapped out the con- 

 ducting tracts, physiologists have been agreed that the 

 upward movement of water from the roots takes place in 

 the woody tissues of plants. In contrast to this unanimity 

 concerning the path of the upward current are the very 

 divergent views which are held as to the nature of the 

 process by which the water is raised. 



Of the views of the earlier writers it is hard to obtain 

 a clear conception ; their point of view was so utterly 

 different from that of the present day. Much of their 

 work is vitiated by the fact that they constrained them- 

 selves to see in plants a circulation of fluids similar to 

 the circulation of the blood of animals. Little attention 

 was paid to the forces causing this circulation. 



It is true that Christian Wolff (1723) believed that the 

 forces involved were the expansion of air and capillarity ; 



1 Ic is remarkable that this classic experiment was not devised by Hales 

 to trace the upward path of the water current, but to prove that there is no 

 circulation of sap in trees comparable to that of the blood of animals. 

 Indeed, from Hales's own account, it appears that he thought the upward 

 movement of water was slightly interfered with by the ringing.. 



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