70 ACEOGENOUS OE ACOTTLEDONOUS STEM. 



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 in part below ground in the form 

 of bulbs, as in Lilies, Tulips, and Hyacinths ; or as corms, in Ool- 

 chicum, Crocus, Gladiolus, and Arum. 



In some instances the aerial stem has the usual endogenous struc- 

 ture, while in the underground stem the vascular bundles are in the 

 form of -wedges, with cellular tissue in the centre, thus resembling 

 Exogens. This structure has been remarked in the Smilax or Sarsar 

 parilla family. Lindley calls these plants Dictyogms (dlxruov, a net), 

 from their netted leaves, by which they differ from most Endogens. 

 Henfrey holds that the ring of woody fibres in these plants, as seen 

 in Tamus and Smilax, is an alteration of the parenchymatous cells 

 of the periphery, and is not produced 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 



Acrogenous or Acoiyledonous Stem. 



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

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

 diameter, and produces leaves (fronds) 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, 

 and it often attains the height of 120 feet (fig. 135). A transverse 

 section of the stem (fig. 136) exhibits an irregular circle of vascular 

 bundles, composed of masses, z I, of various forms and sizes, situated 

 near the circumference ; the centre, m, being formed of cellular tissue, 

 and often becoming hoUow. 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, and marked with the scars of the 

 fronds. 



The vascular bundles are formed simultaneously, and not pro- 

 gressively, as in the stems already noticed; and additions are 

 made in an upward direction. The stem is formed by additions 

 to the summit, and by the elongation of vessels already formed; 

 hence the name Acrogenous (oJzgos, summit). The plants are also 

 called Acrohrya {axgog, summit, and /3gii£/i/ to germinate). The 

 vascular system is of greater density than the rest of the tissue, and 

 is usually distinguished by the dark colour of the pleurenchyma or 

 prosenchyma (fig. 136 /), which surrounds the paler vessels in the 

 centre (fig. 136 v v). There is a continuous woody cylinder in the 

 Fern stem. The vascular bundles, however, do not follow a straight 

 course, but unite and separate, leaving spaces between them, similar 



