ACROGENO1JS OR ACOTYLEDOXOUS STEM. 55 



same way as the removal of the outer part of the wood Avould destroy 

 an exogenous stem. The branches in such plants are formed on the 

 same principle as the stems ; but their fibres, when reaching the stem, 

 dp not proceed to the centre, but extend outside the previous layers, 

 between them and the outer false bark (fig. 113 I e), and thus it 

 is that they give rise to lateral increase. In Grasses, the stem 

 or culm is usually hollow or fistular (fig. Ill), in consequence of 

 the outer part, by its rapid increase, causing the rupture and ulti- 

 mate disappearance of the internal cellular portion. The fibres in 

 some Grasses cross from one side to the other, forming partitions, as 

 in Bamboo. 



100. In many Endogenous or Monocotyledonous plants, the stem 

 remains below ground, developing shoots which are simple, as in 

 Banana and Plantain, or branched, as in Asparagus. In the former, 

 the stem above ground is an herbaceous shoot, composed of the 

 sheaths of the leaves. It dies after fruiting, and is succeeded by 

 other shoots from the subterranean stem. The shoots or buds from 

 such stems occasionally remain below ground in the form of bulbs, 

 as in Lilies. 



101. In some instances, the aerial stem has the usual endogenous 

 structure, while the under-ground stem has the vascular bundles 

 developed in the form of wedges, with cellular tissue in the centre, 

 thus resembling some Exogens. The structure has been remarked in 

 the Smilax or Sarsaparilla family. Lindley calls these plants Dictyogens 

 (bixrvov, a net), from their netted leaves, a character by which they 

 differ from other Endogens. Henfrey holds that the ring of woody 

 fibres in Tamus and Smilax, is merely an alteration of the parenchy- 

 matous cells of the periphery, and is not produced, as some have sup- 

 posed, in the same way as the zones of Dicotyledons. He considers 

 this ring as probably analogous to the liber, and not to the indefinite 

 vascular bundles of Exogenous stems. 



Acrogenous or Acotyledonous Stem. 



102. This stem, in its general external aspect, resembles that of 

 Endogens. It is unbranched, usually of small, nearly uniform diameter, 

 and produces leaves at its summit. It is easily distinguished by its 

 internal structure. Tree Ferns furnish the best example of this kind of 

 stem. In them it is denominated a Stipe, or a Rachis, and often attains 

 the height of 120 feet (fig. 116). A transverse section of the stem 

 (fig. 117) exhibits a circle of vascular tissue composed of masses, z, I, 

 of various forms and sizes, situated near the circumference; the centre, 

 m, being either hollow or formed of cellular tissue. On the outside 

 of the vascular circle, cells exist, p, covered by an epidermal layer or 

 cellular integument, e, often of hard and dense consistence, formed 



