STRUCTURE. 538 
filled with a viscous or semi-fluid matter called 
protoplasm. The investing walls are formed of a 
substance called cellulose, intermixed with a 
watery fluid absorbed into the plant by the 
medium of its roots. 
An embryo plant, then, consists of a system of 
cells and their contents; and the convenient ex- 
pression tisswe is used to imply the entire sub- 
stance of which it is composed. There are 
differing kinds of tissue. Cellular tissue, as its 
name implies, consists of little cells or bags— 
which are mostly roundish or oval, in shape—and 
their enclosing protoplasm. It makes up the softer 
parts and forms the major portion of the sub- 
stance of most plants. It indeed is very much 
the same to the plant as the flesh is to an animal 
body. Wood tissue consists of tubes or cells 
longer one way than another. These cells form, 
lying together and compressed, what we call wood. 
The walls of this kind of tissue become thickened 
as the plant grows; and it is by the thickening 
process that the qualities of hardness and firmness 
which we distinguish in wood are originated. 
Vascular tissue consists also of elongated cells 
