140 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



investigation of such columns in wood, and partly, no 

 doubt, owing to the prevalent impression that the 

 columns in the tree were of the nature of Jamin's chains. 



In spite of incompleteness in detail, then, we have 

 to suppose some such theory as the following. 



When a column of water in the vessels is once 

 formed, it can be maintained so long as no air or 

 solid particles enter, because a capillary column of 

 clean water is not easily broken even by pulls 

 equivalent to the pressure of several atmospheres. 

 As the leaves evaporate water above, the osmotic 

 draught of the transpiring cells increases, and they 

 draw in water from these columns with a force equal 

 to the pressure of many atmospheres. Great as the 

 tensile strain on the columns of water is, their tenacity 

 is such that they do not break, and so the traction is 

 continued down to the soil. 



In course of time air slowly passes in or separates 

 as the negative pressure increases in the tubes, and this 

 breaks the older columns, and the wood in which 

 this occurs is no longer in play, but passes over to 

 heart-wood : on the other hand, new columns of water, 

 raised into position by osmotic forces, are built up by 

 the cambium, and so the supply is kept up, each 

 column being active so long as no air filters in, or, 

 more probably, until a certain maximum quantity 

 finds its way into the strained column. 



