86 
ORGANOGRAPHY. 
BOOK I. 
§ 1. Of the Exogenous Structure, 
The cellular system in an Exogenous stem chiefly occupies 
the centre and the circumference, which are connected by thin 
Jig. 34. vertical plates of the same nature 
as themselves. The central part 
{a^fig- 34.) is the pzY/i, that of the 
circumference (d) is the harh, and 
the connecting vertical plates (c) are 
medullary rays. 
The pith is a cylindrical or an- 
gular column of cellular tissue, 
arising at the point of separation 
between the root and stem and ter- 
minating at the leaf-buds, with all of which, whether they are 
lateral or terminal, it is in direct communication. Its tissue, 
when cut through, almost always exhibits an hexagonal cha- 
racter, and is frequently larger than in any other part. When 
newly formed, it is green, and filled with fluid ; but its colour 
gradually disappears as it dries up, and it finally becomes 
colourless. After this it undergoes no further change, unless 
by the deposition in it, in course of time, of some of the 
peculiar secretions of the species to which it belongs. It has 
been contended, indeed, by some physiologists, that it is 
gradually pressed upon by the surrounding part of the vas- 
