370 
NATURAL HISTORY. 
entirely on its being freed from the pressure of the bark and 
wood. But we cannot accede to this opinion : for although 
in the beginning, the succus proprius Hows Horn both 
margins of the incision, in a little while it is dischaiged 
from the superior margin only. . . 
Hence it appears clearly, that the course of this juice in 
its vessels is never from the roots towards the branches, but 
always in the contrary direction. 
Besides the vessels of the succus proprius, and those con- 
veying the sap, a third kind are found in vegetables, named 
air-vessels, or vasa aeria. These are chiefly situated in the 
wood, leaves, and petals, but arc wanting in the bark of 
trees, and in the herbaceous plants. They are formed by a 
number of small filaments, spirally rolled up, so as to form a 
cavitv in the middle. The name of vasa aeria has been given 
them because they are empty of liquor, and because a great 
quantity of air is certainly found in the wood of plants, 
where these vessels are chiefly placed, and where there is no 
peculiar organization. They are supposed to be the insti ll- 
ments of respiration in vegetables; but in what manner this 
function is performed, is not clearly understood. 
Dr. Hill has demonstrated, that the cuticle of plants is an 
organized substance, containing vessels. In trees and shrubs 
these vessels have an external opening ; but in the herbaceous 
plants this is wanting. Trees and shrubs only are possessed 
ofuasaaertfl; and, when a plantisplaced under the exhausted 
receiver of an air-pump, the air enters through the cuticle, 
and only issues from the wood, in which the vasa aeria are 
situated. From these circumstances taken together, and 
considered attentively, we have reason to conclude, that the 
air’s proper entrance to the vasa aeria is through these cuti- 
cular vessels. Thus, in the early part of the spring, the 
gentle heat expands the mouths of these vessels, before con- 
tracted bv the winter’s cold. Into these orifices the external 
air rushes and presses down to the roots. To these it gives 
energy, as it does to the moving fibres of animals ; and by 
its {Treasure it may assist in propelling the juice upwards. 
An additional quantity of air is evolved by the internal 
motions of the plant, and the whole passes oil with the 
perspirable matter. In this way, there seems to be a circu- 
lation of air through plants, assisting and assisted by the 
powers which move the juices. 
On this account, trees overgrown with moss have few 
leaves, weak shoots, and no fruit. The practice of gar- 
deners is therefore to be commended, who, in the spring, 
strip the moss from the bark of aged trees, and thus 
