SS thy \ ‘ ry aoe Z _- ~ 
/ . 
342 Structural and Physiological Botan y- 
rangement of these separate elements of the vascular bundles — 
of the stem is in general as follows (see Figs. 93, 94, pp. 65,° 
66).. At their inner side next the pith is a bundle of prosen- 
_chymatous cells, and in close contact with this are the 
vessels and vascular cells, which are partly surrounded by — 
the parenchymatous cells of the xylem, partly intermingled — 
with them. ‘Then follows an easily distinguished thin-walled 
part consisting of sieve-tubes, conducting sieve-cells, and 
parenchymatous cells belonging to the bast-portion ; and 
finally, towards the cortex, a bundle of bast-fibres. Some- 
times the bundles of fibres of the xylem and the bast coalesce 
laterally with one another by means of narrower or broader 
groups of prosenchymatous cells, so as to form a closed.ring 
. surrounding the other elements of the vascular bundle. 
' 
The root of Monocotyledons has an epidermis pro- 
vided with numerous root-hairs. The cortex is parenchy- 
matous, and is often differentiated into an outer and an 
inner cortex. It is frequently separated from the inner por- 
tions by a sheath. ‘The vascular bundle forms a closed 
hollow cylinder which encloses the central pith. The bast- 
cells, which are usually elliptical, lie between the radially 
placed vessels of the xylem. The xylem- and the bast-portions 
of the vascular bundie are separated by prosenchymatous 
cells. The bast-portion consists of sieve-tubes lying within 
and of bast-parenchyma pressed outwards. ~ 
The leaves are penetrated by fibrovascular bundles. 
When several enter a leaf, they may first unite into a leaf- 
‘stalk, or may be separated in their entire course, as occurs 
in many sessile leaves, like those of Grasses. The xylem- 
portion of the bundle faces the upper, the bast-portion the 
under side of the leaves. ‘The skeleton of the leaf, formed 
of the vascular bundles, is surrounded by the parenchyma 
of the leaf. This parenchymatous portion or mesophyll 
separates—independently of the epidermis, which is always 
present and is provided with stomata and often with hairs— 
into two layers, the upper layer consisting of cylindrical, the er 
