20 
DIFFERENCES IN STRUCTURE. 
of results ; these appear to be the contrasts 
exemplified in all branches of natural 
history. 
The differences which exist amongst the 
numerous trees and plants which adorn 
the earth, are grounded on differences in 
their minute organization. Thus, one pro¬ 
vince of vegetation is characterized by the 
absence of what is termed vascular tissue, 
and on the inhabitants of this province 
being classified together they are found to 
exhibit many other striking evidences of 
alliance. Those which are of a vascular 
structure, are again divisible into such as 
grow by additions to the centre ; and such 
as increase by additions at the circumference; 
the former being called Exogens, and the 
latter Endogens. These are the three lead¬ 
ing divisions of the vegetable kingdom ; but 
as it happens that structural differences do 
not alone characterize plants, so some other 
variations are nearly as universal, and one 
especially—the form of the seed, is used to 
express the leading divisions equally with 
structure and mode of growth. Thus all 
plants are divided into 1st, Dicotyledonous, 
having tw’o seed lobes, as the bean; 2nd, 
