326 PAKTS THKOUGH WHICH SAP KISES. [BOOK n. 



f 



it enters the roots, and does not partake of the deleterious 

 qualities of the descending sap of the bark. Observations of 

 the same nature have also proved that it descends through 

 the bark and liber. But the sap is also diffused laterally 

 through the cellular tissue, and this with great rapidity ; as 

 will be apparent upon placing a branch in a coloured infusion, 

 which will ascend and descend in the manner just stated, and 

 will also disperse itself laterally, in all directions, round the 

 principal channels of its upward and downward route. In 

 trees this lateral transmission takes place chiefly through the 

 medullary rays, which keep up a communication between the 

 bark and the heart-wood, and convey to the latter the 

 secretions which the former may have received from the 

 leaves. 



It is, however, by no means in the alburnum or outer layers 

 of wood, that the sap rises exclusively. It certainly passes 

 up any part of the wood which is permeable to fluids, and may 

 even refuse the alburnum. This was proved by an experi- 

 ment made by me some years ago in charging the wood of a 

 Sycamore tree, in the month of August, with pyrolignite of 

 iron by Boucherie's process. The solution of iron rose freely 

 near the centre of the wood, even to the extremities ; but 

 absolutely refused to pass by the alburnum. 



With regard to the vessels through which this universal 

 diffusion of the sap takes place, it has already been stated 

 that its upward course is always through the woody tissue, 

 and partially also through the articulated bothrenchym ; and 

 that it passes downwards through the parenchym, and woody 

 tissue of the bark, and through the vessels of the latex. But 

 there can be no reasonable doubt that it is also dispersed 

 through the whole system, by means of some permeable 

 quality of the membranes of the cellular tissue, invisible to 

 our eyes, even aided by the most powerful glasses. It 

 has also been suggested that the sap finds its way upwards, 

 downwards, and laterally, through the intercellular passages. 

 That such a channel of communicating the sap is employed 

 by nature to a certain extent I do not doubt, especially in 

 those plants in which the intercellular passages are very large ; 

 but whether this is a universal law, or has only a partial 



