NATURAL HISTORY. 
568 
substance, in which are placed two kinds of organs, the 
vessels peculiar to the plants, anti the longitudinal fibres. 
Of the use of these nothing can be said at present. 
2. On removing the bark, the wood appears. Its sub- 
stance is denser than the bark, and its structure more 
difficult to be demonstrated. But it has been discovered 
likewise to contain rasa propria, and longitudinal fibres, and 
besides these, large vessels with spiral coats, which run 
from one end of the tree to the other, and are denominated 
rasa cera, or air vessels. Between the wood and the pith 
lies a green coloured substance, first accurately described 
by Dr. John Hill, and by him affirmed to contain all the 
parts of the plant in embryo; he gave it the name of 
Corona. 
3. In the centre of the tree resides the pith, which, in 
young plants is very abundant. As they approach to matu- 
rity it grows drier, and appears in a smaller quantity; and 
in very aged trees it is entirely obliterated. Its substance is 
cellular, and, according to the author just mentioned, is of 
a similar structure in all plants. These are the solid parts 
of vegetables. 
But there are likewise fluids, or juices in vegetables ; and 
these are of two kinds. The one is of the same nature in all 
the variety of vegetables; the other varies according to the 
different plants in which it exists. The former, wliich is 
called the succus communis, when collected early in the 
spring, from an incision made in the birch, or vine, differs 
little from common water. The latter, which is named the 
succus proprius, possesses various properties in various 
plants, and gives to each its sensible qualities. These two 
juices never mingle with each other in the tree; and the 
latter is found in the rasa propria only. 
It is not yet ascertained, whether the juices of plants are 
transmitted through vessels, or a cellular substance. Each 
side of the question has had its advocates, who have sup- 
ported their respective opinions with probable arguments; 
but it is to be regretted that, on so interesting a subject, no 
conclusion can be formed from the actual direction of vege- 
tables. It, however, seems most probable, that all the fluids 
of plants are transmitted through vessels. 
Of the Course of the Succus Communis, or Sap- 
Botanists have made many experiments to ascertain the 
course of the sap. Early in the spring, when the sap begins 
to flow, incisions have been made in the trunk and branches 
of trees, as far as the pith ; and, in such cases it has been 
