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 Exoojens. 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 EUmens (tab. xix.), and De Candolle in his 

 OrgaTwgraphie (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 



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