ENDOGEXOUS OR MONOCOTYLEDONOUS STEM. 



Endogenous or Monocotyledonous Stem. 



91. This kind of stem is composed of cells and vessels which are 

 differently arranged from those of the Exogenous stem. The vascular 

 bundles are scattered through the cellular tissue, and there is no dis- 

 tinction of pith, wood, bark, and medullary rays (fig. 110). In the 

 young state, the centre of the stem is occupied 

 entirely by cells, which may be said to represent 

 pith, and around this the vessels are seen increas- 

 ing in number towards the circumference. The 

 central cellular mass has no medullary sheath. In 

 some cases its cells are ruptured, and disappear dur- 

 ing the progress of growth, leaving a hollow cavity 

 (fig. Ill); but in general it remains permanent, 

 and is gradually encroached upon by the develop- 

 ment of the vascular system. The latter consists 

 of vessels arranged in definite bundles, which do 

 not increase by additions to their outside after 

 being once formed, although they are developed 

 in a progressive manner. These bundles may be 

 considered as representing the vascular wedges, 

 produced during the first year of an exogenous 

 stem's growth (fig. 94). They consist of woody 

 vessels enclosing some cellular tissue between 

 them, spiral, and porous vessels. The outer part 

 of the stem is not formed by a separable bark, but consists of a dense 

 mass of fibrous tissue, mixed with laticiferous vessels and cells. It is 

 intimately connected to the inner part of the stem, without the inter- 

 vention of medullary rays. 



92. On making a transverse section of a young endogenous stem (fig. 

 112), there is observed a mass of cells or utricles, w, of various sizes, 

 often small in the vicinity of the vascular bundles, spiral vessels or 

 tracheae, t, large porous vessels, v p, laticiferous vessels, I, and woody 

 fibres, f, resembling those of liber, thickened by internal deposits. A 

 similar section of a further advanced endogenous stem, as of a Palm 

 (fig. 113), shows numerous bundles of vessels dispersed irregularly in 

 cellular tissue; those near the centre, m, being scattered at a distance 

 from each other, while those towards the outside are densely aggre- 

 gated, so as to form a darkish zone, b, and are succeeded at the circum- 

 ference by a paler circle of less compact vessels, /, with some compressed 



Fig 

 tribut 



Fig. 111. iransverse section 01 stem ot rnragmites commums, or common reed me cellu- 

 lar tissue in the centre has disappeared, leaving a ristular or hollow stem, with a ring of cells 

 and vessels, the latter indicated by dots, n, Node where the fibres cross, so as to form a solid 

 partition. 



