637 



ENDOGENS. 



ENDOGENS. 



638 



and gives the type from which all the analagous structures must be 

 deduced. The separate vascular bundles, especially so far as they 

 form the arc, by no means always run in one and the same vertical 

 plane, their emergence deviating frequently about 50 and more of 

 the circumference of the stem, laterally from the vertical of their 

 starting point, as may be easily observed for instance in Yucca 

 gloriosa. The XantTwrhcea Australia appears to me to differ most 

 strikingly from the simple type of the stem. Here the fascicles of 

 the vascular bundles emerging into the leaves, evidently have a three- 

 fold origin from three different zones of the stem. Quite in the 

 interior another plexus of vascular bundles appears, the course of 

 which however I could not make out, as the piece in my possession 

 was not sufficiently large for me to have it. Still it appeared to me 

 that the vascular bundles had not quite reached the middle of the 

 stem. It will at least suffice to draw the attention of more favoured 

 observers to this striking structure. Perhaps the history of the 

 development of Aletrii fragrant will afford some conclusions on the 

 point. An old stem of about 4 '25 Paris inches in diameter consists 

 of two parts ; the primary stem about 7 lines in diameter, in which 

 the vascular bundles exhibit the usual arc-like course and an external 

 much more solid zone, gradually formed by the cambium-layer. The 

 vascular bundles passing from within to the leaf-cicatrices permeate 

 this external layer in a perfectly horizontal direction. The external 

 layer becomes however divided again into four zones, which produce 

 the appearance of annual rings when seen in the transverse section. 

 The three external ones are when taken together of about the same 

 thickness as the fourth internal one ; they differ in this, that in the 

 external ones the fibres do not ascend vertically but obliquely, con- 

 sequently in a spiral round the axis, and wind towards the left ; in 

 the second, in like manner, but winding towards the right ; in the 

 third again, turned towards the left ; and finally becoming gradually 

 horizontal in the fourth. I may remark here that whilst the paren- 

 chyma is arranged in vertical rows in tlje primary stem, it appears to 

 be in horizontal rows between the external vascular, in the manner 

 of the medullary rays. 



" An essential difference presents itself here, according as the forma- 

 tive layer is limited to the terminal bud, or whether there is a con- 

 tinuous layer in the whole circumference of the stem below the rind, 

 which is there bounded internally by it. The hitter occurs in the 

 case of normally branching stems, as for instance in the Dracaena, 

 Alomect, and Aroidcie, the former in normally simple stems, as for 

 instance in the Tvlipaceas and Palms, with undeveloped internodes. 

 Beautiful investigations on this subject may be found accompanied 

 by the carefully selected results of earlier observations in linger, 

 (see his ' Bau und Wachsthum des Dikotyledonenstammes,' Peters- 

 burg, 1840, page 34). I must finally make mention of the singular 

 stem-formation in the tropical Orehidacaz. A large portion of these, 

 such for instance as are commonly described as having tubers, have 

 not very thick stems (generally branched) with abbreviated internodes. 

 Those branches however which come to blossom produce a peculiar 

 form, which has hitherto been known as tuber (knolle). Either one 

 of the more central internodes of the blossom-bearing branch 

 swells into a disproportionate mass of very varying shape, or all the 

 lower internodes of the branch form a longer or shorter, more or less 

 thick fleshy mass. In both, as for instance in Epidatdrum cochlea- 

 turn, and BUtia Tankemllia, the regular course of the vascular 

 bundles may be distinctly observed, but in the case of the last-named 

 plant (I know not whether the same holds good for all similarly 

 formed) there is a peculiar vascular system intended for the new 

 lateral buds. Little branches pass from the external vascular 

 bundles, and run together in a horizontal direction below the rind 

 from both sides up to the buds. On cutting vertically through one 

 of these stems we find a transversely -severed strikingly-large group 

 of vascular bundles below the rind, corresponding to each interuode. 

 It unfortunately happens with the Orcltidacea as with the Cacti that 

 it is a matter of difficulty to obtain a sufficient quantity of material 

 to ascertain its anatomy or its history of development." 



Grasses are Endogens with hollow stems strengthened by transverse 

 platen at the nodes. This is seen in the bamboo, whose joints are 

 used as cases to hold rolls, or in any of our indigenous species. In 

 this case the deviation from habitual structure is owing to the circum- 

 ference growing faster than the centre, the consequence of which is 

 the tearing the latter into a fistular passage, except at the nodes, 

 where the arcs of ligneous tissue originating in the leaves cross over 

 from one side of the stem to the other, and by their entanglement 

 and extensibility prevent the possibility of any rupture taking place. 

 That this is so is proved by the fact that the stems of all Grasses are 

 olid, or nearly so, as long as they grow slowly ; and that it is when 

 the rapidity of their development is much accelerated that they 

 assume their habitual fistular character. Independently of that 

 circumstance their organisation is quite normal. 



Xanthorhfea haitilit has been shown by Do Candolle to have an 

 anomalous aspect. When cut through transversely the section 

 exhibits an appearance of medullary rays proceeding with consider- 

 able regularity from near the centre to the very circumference. 

 ('Organographie Ve'ge'tale/ t. 7.) But such horizontal rays are not 

 constructed of muriform cellular tissue like real medullary processes, 

 but are composed of ligneous cords lying across the other woody 



tissue : they are in fact the upper ends of the woody arcs pulled from 

 a vertical into a horizontal direction by the growth of the stem and 

 the thrusting of the leaves to which they belong from the centre to 

 the circumference. Such a case throws great light upon the real 

 nature of the more regular forms of endogenous wood. 



Other appearances are owing to imperfect development, as in some 

 of the aquatic species of this class. Lemna, for example, has its stem 

 and leaves fused together into a small lenticular cavernous body ; and 

 in Zannichellia and others a few tubes of lengthened cellular tissue 

 constitute almost all the axis ; but the examination of such cases is 

 comparatively unimportant, and would lead too much into details of 

 subordinate interest. 



By far the most striking kind of anomaly in the stem of Endogens 

 is that which occurs in Barbacenia. In an unpublished species of 

 Barbacenia, from Rio Janeiro, allied to B. purpwea, the stems appear 

 externally like those of any other rough-barked plant, only that their 

 surface is unusually fibrous and ragged when old, and closely coated 

 by the remains of sheathing leaves when young. Upon examining a 

 transverse section of it the stem is found to consist of a small firm 

 pale central circle, having the ordinary endogenous organisation, and 

 of a large number of smaller and very irregular oval spaces, pressed 

 closely together but having no organic connection ; between these are 

 traces of a chaffy ragged kind of tissue which seems as if principally 

 absorbed and destroyed (fig. 2, A). 



Fig. 2. A 



A vertical section of the thickest part of this stem exhibits, in 

 addition to a pale central endogenous column, woody bundles crossing 

 each other or lying parallel, after the manner of the ordinary ligneous 

 tissue of a palm stem (fig. 2, B), only the bundles do not adhere to 

 each other, and are not embodied as usual in a cellular substance. 

 These bundles may be readily traced to the central column, particu- 

 larly in the younger branches (fig. 2, C), and are plainly the roots of 

 the stem, of exactly the same nature as those aerial roots which serve 

 to stay the stem of a Screw-Pine (Pandanw). When they reach the 

 earth the woody bundles become more apparently roots, dividing at 

 their points into fine segments, and entirely resembling on a small 

 scale the roots of the palm-tree. The central column is much smaller 

 at the base of the stem than near the upper extremity. 



The age of endogenous trees has been little studied. When the 

 circumference of their stem is limited specifically it is obvious that 

 their lives will be limited also ; and hence we find the longevity of 

 palms inconsiderable when compared with that of exogenous trees. 

 Two or three hundred years are estimated to form the extreme extent 

 of life in a Date-Palm and in many others. But where, as in Draccma, 

 the degree to which the stem will grow in diameter is indefinite the 

 age seems, as in Exogens, to be indefinite also ; thus a famous Dragon- 

 Tree, Dracama Draco, of Oratava, in Teneriffe, was an object of great 

 antiquity so long ago as A.M. 1402, and is still alive. 



