66 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



understood, I shall defer them till such time as they 

 crop up naturally in the argument. 



As to the explanation of the ascent of the water 

 from the soil to the leaves, two conflicting hypotheses 

 were in the field neglecting older views, which either 

 simply shelved the question by speaking of a process 

 of suction or diffusion, or tried to explain the 

 phenomenon as due to root-pressure, or capillarity, 

 or to the molecular movements in a thin film of 

 water overspreading the inner surface of the elements. 1 

 The two prominent hypotheses were (i) Sachs's view 

 that the water travels as water of imbibition in the 

 molecular interstices of the lignified walls of the 

 vessels and tracheides, and (2) the view, at the time 

 most strenuously advocated by Boehm, that the 

 water ascends in the cavities of the tracheides and 

 vessels. 



We will consider Sachs's hypothesis first. 2 Taking 

 his stand on the facts already conceded, Sachs points 

 out that we have to explain a movement by which a 

 particle of water must travel at a rate of from fifty to 

 two hundred centimetres per hour in the wood, and 

 by which the leaves are supplied so copiously, that 



1 For a summary and criticism of older views see Sachs's Text-Book, 

 second English edition. 



2 See his Lectures on the Physiology of Plants, English edition, p. 225. 



