104 
ORGANOGRAPHY. 
BOOK I-, 
bundles. Is this the case ? The lower part of their bark, too, 
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 ? These inquiries 
have been partially answered by Mr. George Gardner, from 
observations made by him in Brazil. He made a vertical 
section of a Palm tree four inches in circumference, and he 
was able plainly to trace woody bundles proceeding from the 
base of the leaves to the centre of the stem, at an angle of 
18°; they then turned downwards and outwards to within a 
few lines of the external cortical part of the stem, running 
parallel with its axis. The distance between the ends of the 
arcs was about two and a half feet. He adds, that the wood 
of Palm trees is much harder at the bottom than in any other 
part of the stem, the inhabitants of tropical climates using 
only this part for economical purposes. (Tailor’s Magazine, 
xi. 553.) 
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 
