ro 4 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



tend to exfiltrate water sooner than those at higher 

 levels in the tree, and this being so we have to con- 

 sider not only the absorbing or sucking action of the 

 parenchyma, but also its exfiltration or forcing action. 

 Employing the pith of Helianthus^ Westermaier 

 found that in summer the lower parts are filled with 

 liquid only at the periphery, whereas in the upper 

 parts of the stem the whole of the pith is full of 

 sap. A cylinder of this pith more than 50 cm. 

 long, at first flaccid, became turgescent in twenty- 

 four hours when exposed to moisture with its end 

 dipped in water. 



In such experiments, it may happen that parts 

 here and there, above the level of the water in which 

 the lower end is placed, become turgescent and 

 stiff, while parts lower down are flaccid pointing to 

 a stronger osmotic draught in the turgid cells. 



Applying this to the case under discussion, we 

 must note the numerous points of contact and com- 

 munication between the tracheal system on the one 

 hand, and the living, osmotically active cells of the 

 wood-parenchyma and medullary rays, on the other. 



The hypothesis which follows depends, firstly, on 

 the keeping up of the chapelet de Jamin^ by supplies 

 of water from the roots. Secondly it must be 

 assumed that the individual columns of the chaplet 



