CHAP. II. 
ENDOGENOUS STEMS. 
83 
must be much harder, that is, much more filled with woody 
bundles, than the upper. Is that the fact? The hardness of 
the exterior of Palm stems cannot be owing to the pressure 
of new matter from within outwards, but to some cause 
analogous to the formation of heartwood in Exogens. Is there 
any proof that such a cause is in operation ? I mention these 
things, not so much from distrust of Mohl's views, as from a 
desire to see the difficulties which seem to lie in the way of 
an ingenious theory satisfactorily removed. 
The epidermis of an Endogenous stem seems capable 
of very little distension. In many plants of this kind the 
diameter of the stem is the same, or not very widely different, 
at the period when it is first formed, and when it has arrived 
at its greatest age : Palms are, in particular, an instance of this ; 
whence the cylindrical form that is so common in them. That 
the increase in their diameter is really inconsiderable, is 
proved in a curious, and at the same time very conclusive, 
manner, by the circumstance of gigantic woody climbing 
plants sometimes coiling round such stems, and retaining 
them in their embrace for many years, without the stem thus 
tightly wound round indicating in the slightest manner, by 
swelling or otherwise, that such ligatures inconvenience it. 
A specimen^ illustrative of this is preserved in the Museum 
of Natural History at Paris, and has been figured, both by 
Mirbel in his Elemens (tab. xix.), and De Candolle in his 
Organographie (tab. iv.). We know, from the effect of the 
common Bindweed upon the Exogens of our hedges, that the 
embrace of a twining plant is, in a single year, destructive of 
the life of every thing that increases in diameter ; or at least 
produces, above the strangled part, extensive swellings, that 
always end in death. 
It is, however, certain that other Endogens do increase 
extensively in diameter up to a certain point; but this is 
effected with great rapidity ; and the horizontal growth once 
stopped appears never to be renewed : thus, in the Bamboo, 
stems are sometimes found as much as two feet in circum- 
ference, which were originally not more than half an inch in 
diameter. Others would seem to have an unlimited power of 
distension : in the Dracaenas, called in French colonies in 
G 2 
