LEADING DIVISIONS. 
21 
Monocotyledoiious^ having one only seed 
lobe, as the wheat; 3rd, Acotyledonous, 
having no seed lobe, but mere granules of 
seed. 
As the stems of plants are those parts 
which are most frequently found in a fossil 
state, the most interesting and useful know¬ 
ledge for their investigation consists of an 
acquaintance with the minute anatomy of 
wood. With regard to this, all flower bear¬ 
ing plants are arranged in the two leading 
divisions. Exogens and Endogens. 
In the former, which we wull name the 
bark-bearing plants, there is usually a pith 
composed of cellular tissue, a zone of hard 
wood composed of vascular tissue and divi¬ 
ded by plates called medullary rays (consist¬ 
ing of compressed cellular tissue,) and an 
inner and outer bark. The new wood is 
formed on the inside of the bark, and added 
as a layer to the outside of the former wood. 
In the latter, which may be called the 
barkless plants, there is neither true pith, 
nor hard wood, nor bark. The bulk of the 
stem is composed of cellular tissue with 
vascular fibres imbedded. The new plant 
