ENDO 



KNDOOF.yS. 



636 



anything like parallel direction, but curve outwards as they pass 

 downward*, lotting their extremities in the root*, or in the cellular 

 integument oo the ouUide of the firet circle of cords (Jig. 1) ; at the 

 wne time the Moood leaf puibe* the fint leaf a little from the centre 

 toward* the circumference of the plane or cone of growth ; the con- 

 sequence of which U that the ligneous oordi next the base of the first 

 leaf are drawn a little outwards, and form descending axes which 

 henceforward* are found at first to curve inwards towards the centre 

 of the young stem, and afterwards outwards towards its circumfer- 

 ence. In this manner leaf after leaf is developed, the horizontal 

 cellular system enlarging all the time, and every successive leaf, as it 

 forms at the growing point, emitting more woody bundles curving 

 downwards and outwards, and consequently intersecting the older 

 arcs at some place or other ; the result of this is that the first formed 

 leaf will have the upper end of the arcs which belong to, it longest, 

 and much stretched outwardly, while the youngest will have the arcs 

 the straighteat ; and the appearance produced in the stem will be that 

 of a confused entanglement of woody bundles in the midst of a quan- 

 tity of cellular tissue. A* the stem extends its cellular tissue longi- 

 tudinally while this is going on, the woody arcs are consequently in 

 proportion long, and in fact usually appear to the eye as if almost 

 parallel, excepting here and there, where two arcs abruptly intersect 

 each other. As in all cases the greater number of area curve outwards 

 as they descend, and eventually break up their ends into a multitude 

 of fine divisions next the circumference, where they form a cortical 

 integument, it will follow that the greater part of the woody matter 

 of the stem will be collected near the circumference, while the centre 

 is kept comparatively open, and will consist chiefly of cellular tissue ; 

 and when, as in many palms, the stem has a limited circumference, 

 beyond which it is its specific nature not to distend, the density of 

 the circumference must, it is obvious, be proportionally augmented. 

 It is, however, a mistake to suppose that the great hardness of the 

 circumference of old palm -wood is owing merely to the presence of 

 augmenting matter upon a fixed circumference ; this will account but 

 little for the phenomena. We find that the woody bundles next the 

 circumference are larger and harder than they originally were, and 

 consequently we must suppose that they have the power of increasing 

 their own diameter subsequent to their fint formation, and that they 

 also act as reservoirs of secretions of a hard and solid nature, after 

 the manner of the heartwood of Exogens. 



When the growth of the stem of an Endogen goes on in this 

 regular manner, with no power of extending horizontally beyond a 

 specifically limited diameter, a stem is formed, the transverse section 

 of which presents the appearance shown in the following cut. 



Fig. 1. 



There are a number of curved spots crowded together in a confused 

 way, most thick and numerous at the circumference, comparatively 

 mall and thinly placed at the centre ; and the only regular structure 

 that is observable with the naked eye is that the curves always 

 preetnt their convexity to the circumference. 



When there is no limited circumference assigned by nature to an 

 , then the curved spot*, which are sections of the woody arcs, 

 more equally arranged, and are lees crowded at the circutn- 

 Never is there any distinct column of pith, or medullary 

 rays, or concentric arrangement of the woody arcs ; nor does the 

 cortical integument of the surface of endogenous stems assume the 

 character of bark, separating from the wood below it; on the con- 

 trary, as the cortical integument consists very much of the finely 

 divided extremities of the woody arcs, they necessarily hold it fast to 

 the wood of which they are themselves prolongations, and the cortical 

 integument can only be stripped off by tearing it away from the whole 

 surface of the wood, from which it does not separate without leaving 

 myriads of little broken threads brhin.l. 



This account of the structure of the Endogenous Stem is identical 

 with that given by Dr. Lindley in his ' Vegetable Kingdom.' Schlei- 

 den's view of the structure of the stem, as given 1>y Dr. Lindley, is 

 somewhat different The following passage expounds this view : 



" In all plant*, the woody bandies, whose development 

 always proceeds from the interior to the exterior, are either limited 

 or unlimited in their growth. Commonly every woody bundle 



consists of three different physiological parts ; firstly, of a tissue of 

 extreme delicacy, capable of rapid development, in which new cells 

 are continually generated, and deposited in various ways, in two 

 different directions, namely, next the circumference in the shape of a 

 peculiar kind of lengthened cellular tissue with very thick walls, the 

 liber, and next the centre in the form of annular, spiral, reticulate, 

 and porous vessels ; secondly of woody cells, which are either uni- 

 form in appearance or different, and form wood, properly so called. 

 Up to a certain period the development of the vascular system in 

 Monocotyledons and Dicotyledons proceeds on the same plan ; but in 

 Monocotyledons (Endogens) the active, thin, solid, delicate, cellular 

 tissue suddenly changes ; the partitions of its cells become thicker, 

 their generating power ceases, and when all the surrounding cells are 

 fully developed, they assume a peculiar form, ceasing to convey gum, 

 mucilage, and other kinds of thick formative sap." From this cause 

 all further development of vascular bundles is rendered impossible, 

 and therefore Schleiden calls the woody bundles of such plants 

 ' limited.' In Dicotyledons (Exogens), on the contrary, this tissue 

 retains during the whole lifetime of the plant its vital power of forma- 

 tion, continues to develop new cells, and so increases the mass, cease- 

 lessly augmenting both the exterior (liber) and the interior faces (wood), 

 for which reason Schleiden calls such woody bundles ' unlimited.' 

 " This," he continues, " happens according to the climate and nature 

 of the plant either pretty continuously.** in Cactactee, or by abrupt peri- 

 odical advances and cessations, as occurs in forest-trees of Europe. In 

 the latter, the stem forms an uninterrupted tissue from the pith to the 

 bark during every period of life, and the bark is never organically 

 separated from the stem ; what is considered their natural separation 

 in the spring is only a rent, produced by tearing the delicate tissue 

 already spoken of, which is present even during winter, and con- 

 stitutes the foundation of new annual zones, although compressed, 

 and filled with gum, starch, and other secretions. In the spring, 

 being expanded and swollen by the new amount of sap, it is deprived 

 of its contents by their solution." (Lindley, ' Vegetable Kingdom.') 



Scbleiden's account of the structure of the Endogenous or Monoco- 

 tyledonous stem, as given in his 'Principles of Scientific Botany,' 

 U as follows : 



" The most simple plants of this division have no vascular bundles, 

 as for instance Wolffia. Those nearest allied amongst the Ltmnaccre 

 first exhibit definite indications of these ; in Spirodela we even find 

 them combined with spiral vessels, but distributed in a plane surface 

 as the necessary accompaniment of a flat stalk. Many of the 

 Naiadtrt, as for instance Naiai, Zanichellia, Kuppia, have only a 

 central vascular bundle. In the remainder we meet with the follow- 

 ing modifications: 



"1. Developed Inter-nodes. The stalks and stems have always 

 several rings of vascular bundles, which occasionally inclose a 

 pith, where a circle of vascular bundles are connected by a 

 ring of thickened parenchyma. This is often the most external 

 (usually), often a more internal one, as in Polhos. A portion of the 

 vascular bundle passes through the nodes into the leaf, whilst a part 

 rises into the next internode. Small twigs branch off from all the 

 vascular bundles that pass through the nodes, forming a confused 

 plexus in the node, which, for the most part, merges into the axillary 

 bud. The innermost vascular bundles in the nodes supply the lowest 

 leaves, the external bundles the upper ones, as in Grasses, the Cane- 

 stemmed Palms, and the Commeiinacete. There are many groups that 

 have not yet been examined. The whole of the vascular bundles in 

 the same internode are simultaneously formed and developed, and thu 

 internode itoelf, when perennial, does not continue to increase in thick- 

 ness, whether the plant becomes branched or not The primary axes, 

 like the secondary, only grow upwards ; in fact they are devoid of a 

 cambium-layer. 



" 2. Undeveloped Internode*, The stalks (in Piftia obovata for 

 instance), and the stems of Palms, herbaceous Liliace<r, bulbs of 

 Allium, Lilium, Ac., have a conical terminal bud, sometimes shorter, 

 in accordance with which the vascular bundles run from below and 

 the exterior, upwards and towards the interior, and then from thence 

 upwards and externally, to pass into a leaf. The arc, which is convex 

 towards the interior, is longer or shorter according to the terminal 

 bud ; and the vascular bundle likewise passes through a longer or 

 shorter portion of the whole axis, according to the same conditions. 

 In the full-grown stems of the Palms, the vascular bundles connected 

 with the upper leaves do not reach the base of the stem, notwith- 

 standing the length of the arc. In the simplest case the vascular 

 bundles are wholly isolated ; they are however more frequently con- 

 nected by intermediate branches, seldom from within externally, but 

 often laterally with one another. From this cause, as well as from a 

 more or less extended vertical course of the vascular bundles before 

 they form the arc, the external part of the stem is composed of a 

 thicker cylinder of vascular bundles, whilst the inner portion, com- 

 posed only of arcs, becoming more and more isolated towards the 

 centre, and cellular tissue increasing in quantity in the inverse pro- 

 portion, appears much looser. 



" However simple we may consider the course of the vascular 

 bundles, in the Monocotyledons, hi judging of them according to 

 H. Mohl's researches, it is in fact but seldom so ; nevertheless H. 

 Mohl's representation affords the simplest and clearest delineation, 



