THE CUTICLE OF ENDOGENOUS STEMS. 



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on the outer side of the previously-formed wood, but pass down in irregular masses 

 near to the centre of the stem. Such stems, like exogenous stems, are composed of a 

 woof and warp, each of which holds the same relation to the other in both great 

 divisions of trees, and they differ only in their relative proportions and mode of 

 arrangement. Thus the cellular or 

 horizontal warp is proportionally in- 

 creased in endogenous rather than in 

 exogenous stems; and this, together 

 with the arrangement of the woody 

 fibre into bundles, gives a more open 

 character to the section of the stem. 



A section of an endogenous stem 

 (Fig. 144) exhibits the following struc- 

 tures: First, an external inclosing 

 layer or bark, x ; secondly, a series of 

 circular lines, which represent the cut 

 surfaces of vascular tissue, y; and 

 thirdly, the mesh- work intervening be- 

 tween the bundles, which is the cellu- 

 lar tissue or pith, z. "We shall consider 

 each of these separately, 



Cuticle. The epidermis, cuticle, 

 or bark, of endogenous stems, differs 

 materially from the analogous structure 

 in exogenous plants. It cannot, in any 

 lormal instances, be separated from the stem, as may be readily seen by attempting to 

 peel a cane. It does not naturally crack and separate as does the bark of our forest 

 trees; but is hard, dense, smooth (usually), non-corrugated, inelastic, but slightly 

 extensible, and is a permanent unchanging structure. Thus, the diameter of such 

 stems is necessarily greatly restricted ; and it is in length only that endogenous plants 

 can be greatly developed. It does not consist of a series of layers, which may be 

 detached from each other, and distinguished by various names, but is simply formed of 

 one or two layers a mass of flattened cells, with bundles of woody fibre intermixed, 

 and connecting it with the internal parts of the stem. The non-extensibility of these 

 layers is not evident until the tree has attained to somewhat of its natural diameter ; 

 for the bamboo may appear at first as large as the finger only, and subsequently exceed 

 in circumference a man's thigh. Moreover, a few plants, as the Dracaena or dragon's 

 blood tree, referred to at page 1, has attained to a circumference of forty feet. Thus, 

 whilst it is true that the width of endogenous stems, as compared with their height, is 

 much less than in exogenous trees, we must admit that the cuticle is extensible, and 

 must infer that its further development is prevented by a degree of expansibility which 

 does not proceed beyond a certain point ; or that its further non-development is simply 

 a part of the general law which governs the growth of these plants. It is difficult to 

 agree with the common opinion, that the limited power of expansion, which the cuticle 

 is said to possess, is the cause of the limited diameter of the stem ; and it seems more 

 philosophical to assume, that it is only a part of these occurrences which accompany 

 the normal development of these structures. 



That the size of the stem remains the same at all periods after its full development 



d t fc * 



Fig. 144, showing, by a horizontal and perpendicular 



section, the structure of endogenous stems, 

 a, 2, cellular woof of the stem. 

 b d, bundles of vascular tissue, 

 y, their cut ends. 

 x, the so-called cuticle or bark. 



