ORIGIN AND GROWTH OF FIBRO-VASCULAR BUNDLES. 95 
Internal Structure.—When we make a transverse section of 
a Palm stem, it presents, as we have seen (page 75), no such 
separation of parts into pith, wood, medullary rays, and bark, 
as we have described as existing in an exogenous stem ; but the 
fibro-vascular system is seen to consist of bundles (figs. 177, f, 
and 192, A, b, c, d), which have no tendency to collect together 
so as to form rings of wood as in exogenous stems, but are 
arranged separately from one another in the mass of parenchy- 
matous cells (figs. 177, m, and 192, A, a, a), of which the ground 
substance or fundamental tissue is composed. The whole is 
covered externally by a fibrous and parenchymatous layer, which 
is called the false bark or rind (fig. 177, b); because this is not 
a distinct and parallel formation to the wood, as is the case with 
the bark of exogenous stems, but is formed essentially by the 
ends of the fibro-vascu- 
lar bundles, as will be Fic. 193. Fie. 194. 
presently noticed, and 
cannot. therefore be’ { ¢ 
separated from the mass 
beneath (see page 96). 
In annual or herba- 
Fia. 195. 
ceous monocotyledonous * : 
stems the parenchyma 
between the fibro-vas- 
cular bundles is soft and : 4 
delicate ; but in trees 
which grow to any 
height, as Palms, the 
cell-walls become thick- 
ened and hardened, and ° ja 
thus form the tissue 
termed  sclerenchyma, 
which ultimately binds 
the original separate 
pone les a into a_ solid Figs. 193 and 194, Diagrams showing the course 
aradenead mass TreseM- of the fibro-vascular bundles in a monocotyle- 
bling wood. donous stem. 4a, 0, & d, Fibro-vascular bundles, 
ints ‘ Fig. 193. Exhibits the course of the bundles as 
Origin and Growth formerly supposed. fig. 194. According to 
of the Fibro-vascular Mohl’s view, as now proved to be correct. 
Bundles. — The  struc- — Fig. 195. Vertical section of the stem of 
; s a Palm, showing(/v) the fibro-vascular bundles 
ture of the fibro-vascu- intersecting each other as they pass down- 
lar bundles thus distri- — wards. 
buted in the parenchy- 
matous system has been already referred to under the name 
of definite or closed (page 77); but we have still to describe their 
origin and direction through the stem. It was formerly supposed 
that these bundles, as they were successively developed, were 
at first directed towards the centre of the stem, and continued 
their course in the same direction down to its base as seen 
