196 ON FACILITATING THE EMISSION 



I have cited, in a former communication*, a part of the evidence upon 

 which I have inferred that the sap of trees descends from their leaves 

 through the bark ; and I shall here only observe, in support of that 

 opinion, that if a piece of bark be everywhere detached from the tree, 

 except at its upper end, it will deposit, under proper management, as 

 much, or nearly as much wood, upon its interior surface, as it will if it 

 retain its natural position ; and that the sap which generates the wood, 

 deposited in the preceding circumstances, must descend through the 

 bark, as it cannot be derived from any other source. 



When a layer is prepared, and deposited in the ground, the progress 

 of the sap, in its descent towards the original roots, is intercepted upon 

 the side where the partially detached part, or tongue, of the layer is 

 divided from the branch ; and this intercepted sap is, in consequence, 

 generally soon employed in the formation of new roots. But there are 

 many species of trees which do not readily emit roots by this mode of 

 treatment ; and I suspected that, wherever roots are not emitted by 

 layers, the sap, which descends from the leaves, must escape almost wholly 

 through the remaining portion of bark, which connects the layer with the 

 parent plant. I therefore attempted, in the last and the preceding 

 spring, to accelerate the emission of roots by layers of trees of different 

 species which do not really emit roots, by the following means, having 

 detached the tongue of the layers from the branches in the usual 

 manner. 



Soon after Midsummer, when the leaves upon the layers had acquired 

 their full growth, and were, according to my hypothesis, in the act of 

 generating the true sap of the plant, the layers \vere taken out of the 

 soil, and I found that those of several species of trees did not indicate 

 any disposition to generate roots, a small portion of cellular bark only 

 having issued from the interior surface of the bark in the wounded parts. 

 I therefore took measures to prevent the return of the sap through the 

 bark, from the layers to the parent trees, by making, round each branch, 

 two circular incisions through the bark, immediately above the space 

 where the tongue of the layer had been detached ; and the bark between 

 these incisions, which were about twice the diameter of the branch apart, 

 was taken off. The surface of the decorticated spaces was then scraped 

 with a knife, to prevent the reproduction of the bark, and the layers were 

 recommitted to the soil ; and at the end of a month I had the pleasure 

 to observe that roots had been abundantly emitted by every one. In 

 other instances, I obtained the same results by simply scraping off, at the 



* Page 190. 



