THE STEM-— rrs MODE OF GEOWTH. 



39 



Growth, or the manner in which the Fibrous system is 

 deposited in the Cellular. 



219. Exogenous Structure. — Fig. 9 represents a 

 transverse section from a stem of the Oak. It is com- 

 posed of distinct concentric zones, or circular layers, one 

 of these being formed every year from the vital nutriment 

 in the Cambium Layer (202), and deposited between the 

 Liber and Alburnum, or the outer wood and inner bark. 

 A new layer of wood is thus formed on the outside of the 

 older tissues, which are pressed inward, and become 

 greatly solidified, so that the tubes almost lose their 

 calibre, as you have already seen. This mode of growth 

 is favorable to increase in diameter, and a wide lateral 

 development; and consequently we see, very generally, 

 stout and thick stems in our forest trees, with an um- 

 brageous expansion of the branches. It is also favorable 

 to longevity ; for as all the vital forces are renewed annu- 

 ally, and the woody accumulation in the tissues serves 

 for mechanical support, it would seem that there could 

 be no necessary limits to the life of such a tree. The 

 general structure of an Exogenous stem has been very 

 clearly defined in the last chapter, in the Anatomy of the 

 Wood. 



220. Endogenous Structure. — Here there are no 

 distinct layers, but the woody tubes are deposited among 

 the cellular tissue, without any apparent order. The 

 new wood in growth is deposited toward the centre, and 

 becomes indistinguishably mingled with the old, which 

 is pushed outward. Thus the growth can only take 

 place by a gradual distension of the stem; and that no 

 longer than while it remains sufficiently flexible to yield 

 to the pressure. The vital action being thus checked 

 horizontally, takes an upward direction ; and hence 

 Palms, and other endogenous trees, are distinguished by 

 those lofty and slender forms which are at once so majes- 

 tic and so graceful. In fig. 6 may be seen a section of 

 a Palm stem, with the bundles of woody fibre curved first 

 toward the centre, and then toward the circumference, 

 and thus interlacing each other. In the transverse sec- 

 tion at the summit, may be seen the divided ends of the 

 fibre, appearing like dots. But in the arborescent 

 Yuccas the stem remains soft, and thus capable of un- 

 limited distension — a feature which is more remarkably 

 shown in the gigantic Dragon-tree of the African Islands, 



DeHno Shrub, Under-sbrub, Herb. On what principle is tlie nest great 

 division? What two great classes? Howmarked? With what other division 

 does each correspond ? Instances of each. Describe the Exogenous structure. 

 Endogenous. Where is the new wood deposited in E.^ogens ?— in Endogens ? 

 In what direction do Exogens incline to expand?— Endogens? What dis- 

 tinctions nearly or quite lost in the endogenous stem ? To what may its bun- 

 dles of Bbro be traced ? What parts inseparable or Indistinguishable In Endo- 



which appears like an enormous herb, and is, in fact, a 

 land of Lily. In Palms the wood of the lower parts of 

 the stem is more solid and compact than that of the upper. 



221. The stem of an Endogen afibrds no definite dis- 

 tmction of pith, wood, and bark, nor is it traversed by 

 medullary rays. It consists of threads, or bundles of 

 woody fibre, surrounding a group of ducts. These bun- 

 dles may be traced directly from the base of the leaves 

 and in some instances clear to the roots, in young plants. 

 At other times, curving outwardly, as seen in the figure, 

 they lose themselves in the cortical integument of the 

 rind, which is thus interwrought with the woody system, 

 and becomes inseparable from it — or is not to be sepa- 

 rated as a distinct layer. 



222. As Palms generally do not form lateral buds, 

 if the terminal bud is destroyed, they, having no means 

 of reproducing it, decay and perish. The leaves are 

 borne in a cluster at the summit ; and the diameter of the 

 trunk, never being contracted by the protrusion of branches, 

 which, in dividing the substance must diminish the bulk, 

 is of nearly equal thickness through its whole extent. 



223. But sometimes two or more lateral buds devel- 

 ope, and the stem branches. This is seen in the Screw 

 Pine, fig. 3, Plate IX. In this and all such cases, the 

 branches preserve the Palm-like character, and are cylin- 

 drical, producing leaves only at their extremities. But 

 in those endogenous plants where lateral buds are freely 

 produced, and leaves and branches are scattered along 

 the stem, it tapers upward as in the exogenous forms. 

 The Asparagus is the most familiar instance of this ar- 

 rangement, while the Indian Corn furnishes a fine exam- 

 ple of the ordinary structure of Endogenous stems. 



224. ExoGENS AND Endogens. — Exogens have two 

 seed-leaves ; they grow by depositing new cells in Con- 

 centric Circles, on the outside of the wood, or between 

 it and the bark ; they form lateral buds ; and their 

 stems are distinguishable into Pith, Bark, and Wood. 

 Endogens have but one seed-leaf; they do not deposit 

 their wood in Layers, nor on the outside, but in the 

 centre; they have no definite distinction of Pith and 

 Bark ; and they seldom form Lateral Buds. 



STEM FORMS. AERIAL MODIFICATIONS. 



225. In respect to their external form, stems may be 



gens? Why do not Palms gi-ow thick and stout, like Oaks — or Oaks tall, liko 

 Palms ? What hindera lateral growth in the latter ? In what case do they 

 continue to expand horizontally ? What remarkable instance of lateral expan- 

 sion? Distinctions between Exogens and Endogens. Why are Palms not usu- 

 ally branched ? In what cases are they ? What other instances of branched Endo- 

 gens? What happens if the terminal bud is destroyed? What effect has branch- 

 ing on the shape of the stem ? What fine subject for study of the Endogon ? 



