STRUCTURE.] PALMS. 223 



course of the stem for some distance, and then turns outward 

 again, finally losing itself in the cortical integument. In the 

 course of their downward descent, the woody bundles gra- 

 dually separate into threads, till at last the vascular system, 

 which for a long time formed an essential part of each of 

 them, disappears, and there is nothing left but woody tissue. 

 In this view of the growth of Endogens, the trunk of such 

 plants must consist of a series of arcs directed from above 

 inwards, and then from within outwards ; and consequently 

 the woody bundles of such plants, instead of being parallel 

 with each other, must perpetually intersect each other. If 

 Mohl's view of the structure of Endogens be correct, they 

 must after a time lose the power of growing, in consequence 

 of the whole of the lower part of their stems being choked 

 up by the multitude of descending woody bundles. The 

 lower part of their bark, too, must be much harder, that is, 

 much more filled with woody bundles, than the upper. 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 sr cause is in ope- 

 ration? 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. (Taylor'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 



