Tissues i?i Certain Monocotyledons. 
49 
The secondary tissues form two distinct regions of con- 
spicuously different structure. The outer zone is characterized 
by scattered, sharply defined secondary bundles imbedded 
in comparatively thin- walled, radially arranged parenchyma. 
The inner secondary zone has the bundles densely crowded, 
so as not to be readily distinguishable, with but little paren- 
chyma between them (see Fig. io). Here there is no obvious 
radial arrangement. We will first describe in detail the 
structure of the outer zone. 
On its exterior side it is surrounded by a regular cambial 
layer which is manifestly the seat of formation of the 
secondary tissues (see Fig. 14). The details of development 
will be considered later. On its outside the cambium pro- 
duces secondary cortex, which eventually grows to a great 
thickness (see longitudinal section, Fig. 15). 
The secondary vascular bundles, like all other bundles 
in the stele, are concentric. The ring of xylem consists 
chiefly of long tracheides, with a very tortuous course. Their 
walls have corresponding bordered pits with slit-like openings ; 
among the tracheides a few parenchymatous elements are 
scattered, some of which border on the phloem. The latter 
presents no peculiarities; as the constituent elements of the 
sieve-tubes are short, their sieve-plates, which are horizontal, 
are often met with in transverse sections. It is very common to 
find two groups of phloem in the same bundle ; they may be 
placed either tangentially or radially. This is due to the fact 
that the secondary bundles often anastomose in both planes, as 
is easily seen in the corresponding longitudinal sections. The 
system of secondary bundles thus forms a continuous network. 
The tracheides form much the greater part of the bundle. 
We found the average numbers to be for each bundle, as seen 
in transverse section, forty tracheides, nine cells of the xylem- 
parenchyma, and eight phloem-elements. The rectangular 
pitted cells of the secondary ground-tissue have a very 
regular radial arrangement, which is only disturbed where 
the vascular bundles occur. The latter are arranged generally 
in concentric series. 
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