112 THE VEGETABLE WORLD. 
was sufficiently powerful to move a column of mercury, in addition 
to the weight of the atmosphere, this height. Hales calculated from 
this that the force which impels the sap in the vine is five times as 
great as that which impels the blood through the large arteries of 
the horse. Having reached the leaves, the sap comes in contact 
with the air by the innumerable openings or stomates which com- 
municate with the air-cells and hollow meatus in the substance of 
the parenchyma. The respiration of the plant, that is to say, the 
chemical action which the air exercises upon the liquids which 
supply the leaves, together with the exhalation of vapour which 
proceeds from the same organs, modifies the ascending sap in the 
same manner that air modifies the veinous blood in the sanguinary 
vessels and lungs of animals, changing it into arterial blood. Thus 
it is with the leaves in the phenomena of exhalation and respira- 
tion, of which they are the seat. In ascending, the sap changes 
its nature, and becomes transformed into a nourishing fluid. 
We must now inquire what is the path which this new 
descending sap takes after it has undergone this important modi- 
fication. Everything tends to the belief that it circulates in the 
bark. Let us see what are the facts which justify this opinion. 
If a stem or branch is strongly bound in such a manner as to com- 
press the bark, a cushion or excrescence is formed above the liga- 
ture, which continues to increase, and appears to proceed from the 
stoppage of the circulation, showing that the nutritious fluids 
come from above, for the parts beneath the ligature show 20 
increase. The same phenomenon is produced upon the trunk of 
the tree when annular or spiral incisions are made all round it. 
Again, the trunks of trees round which creeping or twining plants 
wind themselves, demonstrate the same physiological fact in a very 
peculiar manner. Above the natural ligature produced by the 
pressure of the parasitic plant, a natural cushion is formed, and @ 
swelling produced by stopping the sap in its supposed descent 
through the bark. 
_ In speaking of the structure of roots, we have enumerated th 
causes under the influences of which the sapascends; the causes which 
determine its descent are, we must acknowledge, imperfectly know? 
to us. It appears most probable that it is through the deep layers 
of the bark, especially through the net-like fibres of the “ier, the 
