94 ACCOUNT^ OF SOME EXPERIMENTS ON THE 



transferred from the internal tubes to those near the surface, which alone 

 appear to communicate with the central tubes of the young shoots and 

 leaves. Indeed, we have frequent evidence that trees possess this power; 

 for we see that the whole sap of the stock is carried into an inserted bud 

 or graft. 



I at one time suspected that a small portion of sap, in its descent from 

 the leaves, had been carried down by the wood, through my incisions, 

 in the preceding experiments on the crab-tree, because 1 observed a very 

 small increase in size, in the lower part of the stocks ; which, I think, 

 could not have taken place without some matter derived from the leaves. 

 But subsequent observation induces me to believe, that the small quantity 

 of additional matter found in the lower part of the stock came from a 

 different source. In those experiments I paid little attention to any 

 small shoots which sprang from the trunk at some distance below the 

 incisions ; and the buds, which usually began to vegetate about mid- 

 summer, were not always rubbed off till some minute leaves appeared. 

 Through these I now believe that a small quantity of sap was thrown 

 into the bark, and carried up through its tubes by capillary attraction, 

 when the current from above was intercepted ; for the increase of size 

 in the stock always diminished, as it ascended towards the incision ; 

 which, I think, would not have been the case, had it been produced by 

 nourishment descending from the upper parts of the tree. 



Nothing has occurred in the preceding experiments to throw much 

 light on the office of the medulla, to which Linnseus and subsequent 

 writers have annexed so much importance ; but I will now endeavour to 

 point out one of its offices. In the young and succulent shoot this 

 substance is extremely full of moisture ; and, as there is an immediate 

 communication bet\veen it and the leaf, through the central tubes, I 

 conclude it forms a reservoir, to supply the leaf with moisture, whenever 

 an excess of perspiration puts that in a state to require it. Some reser- 

 voir of this kind appears to me to be necessary to plants, for their young 

 leaves are excessively tender, and they perspire much ; and cannot, like 

 animals, fly to the shade and the brook. In the mature annual branches, 

 and in those of more than one year old, the medulla is dry, and, I think, 

 it is evidently lifeless ; but the space it occupies is never filled with wood, 

 as some naturalists have imagined. 



The heart or coloured wood, distinguished from the alburnum, seems 

 to execute an office somewhat similar to the bone in the animal economy. 

 The rigid texture of the vegetable fibre, has rendered this substance 

 unnecessary in the young subject ; but, as the powers of destruction, 



