68 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



Hence, at the time when most water is ascending 

 through the wood, the cavities of the elements contain 

 much air (and no doubt vapour). The determination 

 of the specific gravity, shows that the stibstance of the 

 wood cell-walls is heavier than water in the ratio of 

 1-56 to i ; and of course it was easy to determine how 

 much water a given piece of wood contained, and it 

 was found by this method also that the water could 

 not have been held in the walls : it also showed that 

 some of the water was in the cavities. 



Sachs then, by the ingenious method of finding how 

 much water is absorbed when a piece of dry wood is 

 suspended in vapour, determined that the elements 

 imbibe by means of their lignified cell-walls, to the 

 extent of half the volume of the latter. 



Based on a long series of such investigations, 1 

 Sachs finally came to the conclusion that the lignified 

 walls of the wood-elements have certain remarkable 

 molecular properties : that they absorb relatively but 

 little water, but that this ivater is wonderfully mobile. 

 On this assumption he gave to the world his daring 

 hypothesis, which is that the water in the molecular 

 interstices of the walls of the tracheides, vessels, &c. 

 moves upwards to compensate that lost by transpira- 



1 See Sachs's Uber die Porositat des ffolzes, Wiirzburg Arbeiten, ii. 

 1879. 



