ON VEGETABLE PHYSIOLOGY. 57 



from age, by the constant pressure of the surround- 

 ing* parts many of these fibres lose their vascular 

 character, and form what has been termed the 

 heart or dead wood. 



The cellular tissue is made up of a fine transpa- 

 rent membranous substance, which, crossing 

 itself in different directions, forms cells, hexag- 

 onal or circular, according to the part of the plant. 

 Into these cells the tubes empty their fluid con- 

 tents for future elaboration. The two parts, 

 therefore, the fibrous portion and the cellular tis- 

 sue, frequently come into contact, and by running 

 into each other in most instances form a portion 

 of the same mass; though, for the sake of illus- 

 tration, we shall consider them as holding a dis- 

 tinct place in the plant. We shall now proceed 

 to the root, the first organ to be described. 



The root, which, though often differing in its 

 bulk and shape, is similar in all in its structure and 

 use (with the exception of the bulb, which, from 

 containing the rudiments of a new individual may 

 more properly be considered a variety of the 

 seed,) is that part of the vegetable which fixes the 

 plant to the ground, is its organ of nourishment, 

 and the apparatus by which, through its various 

 ramifications below the surface, it imbibes food 

 from the soil. 



In its structure, it is composed of the same 

 parts as the stem and branches, and therefore may 

 only be considered as the stem inverted; the 



