CHAP. n. 
ENDOGENOUS STEMS. 
81 
§ 2, Of the Endogenous Structure, 
41 
Plants of an arborescent habit having this structure being 
almost exclusively extra-European, and most of them being 
natives only of the tropics, botanists have had fewer opppor- 
tunities of examining them, and, consequently, our knowledge 
concerning them is far more limited. It is, therefore, pro- 
bable that in this department of the subject there will be 
hereafter much to add and correct. 
In Endogenous plants the vascular 
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, 
having no 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- 
