102 
ORGANOGRAPHY. 
BOOK I. 
§ 2. Of the Endogenous Structure, 
43 
Plants of an arborescent habit having this structure being 
almost exclusively extra-European, and most of them natives 
only of the tropics, botanists have had much fewer oppor- 
tunities of examining them, and, consequently, our knowledge 
concerning them is more limited. Nevertheless, the investi- 
gations of Mohl and others have thrown great light upon 
their real organisation. 
In Endogenous plants the vascular 44 
and cellular systems are as distinct as 
in Exogenous, but they are differently 
arranged. The cellular system, instead 
of being distinguishable into pith,. bark, 
and medullary rays, is a uniform mass, 
in which the vascular system lies im- 
bedded in the form of thick fibres, 
seldom having any tendency to collect into zones or wedges 
resembling wood. The fibrous bundles consist of woody 
tissue, enclosing spiral or other vessels. 
The following is an explanation of the opinions generally 
entertained concerning the formation of an Endogenous stem. 
Its diameter is supposed to be increased by the constant addition 
of fibrous bundles to the centre, whence the name; those bundles 
displace such as are previously formed, pushing them out- 
