THE LEAF 37 
system, as seen in cross section, usually has the form of a ring 
(Fig. 24), a crescent-shaped ring, a crescent (Fig. 26), or scat- 
tered patches, but may have other shapes. 
If it is in the form of a ring, parenchyma 
cells are usually found within the ring 
(Fig. 24). The inner part of the ring is 
composed of zylem, which is conspicuous 
on account of its having thick-walled 
cells. Xylem conducts water and, on 
account of its thick-walled cells, gives 
strength. The conducting elements of 
Fie. 29. Arrangement the xylem are long tubes composed of 
of netted veins and the = quad cells. 
pee the,veinlets The xylem ring is surrounded by a 
in a small portion of a : = ay oe : 
ets Mime (Citrus ring of phloém, which is composed of thin- 
lima). (x 6) walled cells (Fig. 24) and serves for the 
transportation of food materials that 
come from the chlorenchyma of the leaf. When the xylem, 
instead of being in the form of a ring or a crescent-shaped ring, 
has the form of a crescent, the phloém may occur only below 
the xylem (Hig. 24) or 
may be both above and 
below it. 
The conducting sys- 
tem is better developed 
in the stem than in the 
leaf and will be consid- 
ered in greater detail 
in the chapter dealing 
with the stem. 
Sclerenchyma. Thick- 
walled, dead cells are 
frequently found scat- 
tered in the parenchyma just outside of the phloém (Fig. 24), 
These are strengthening cells and are known as sclerenchyma, 
They are greatly elongated in the longitudinal direction of the 


Fia. 80. Section of a vertical leaf of Huca- 
lyptus, showing a palisade layer on each side. 
(x 275) 
