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Special Morphology and Classification. 369 
the stem only increases for a short time, and then only by 
the incréase in size of the separate cells, and occupies there- 
fore but a small space in comparison with the wood and the 
fundamental tissue of the medullary rays. It consists of a 
parenchymatous tissue, in which other elements, as latici- 
ferous vessels, are sometimes, though rarely, enclosed. 
. Deviations are not unfrequent from the normal structure and course 
of the vascular bundles here described. In many water-plants, such as 
Hiippuris, Myriophyllum, Ceratophyllum, and Trapa, a sheathis formed 
which limits the increase in diameter of these plants. (Fig. 94, p. 66.) 
The roots of Dicotyledons always possess, when young, 
a thin-walled epidermis, which soon becomes replaced, like 
that of the stem, by cork-tissue. The cortex of the root 
is also very similar to that of the stem, consisting of the 
same elements; and its cells contain the same substances 
within them. It is very usual for a vascular bundle-sheath © 
to separate the axial bundle from the cortex. While, as has _ 
already been mentioned (p. 362), the vascular bundles of 
the stem have a centrifugal growth, the development of 
those of the root is in many plants centripetal ; z.¢., the 
first vessels are formed on the outside of the formative 
tissue, and their further development and legnification ad- 
vance from the centre towards the axis (Fig. 477, 1.). The 
development of the pith is by this means so much checked 
that it is always less developed in the root than in the stem ; 
and in many roots it has entirely disappeared when their 
vascular bundles have attained their full development. As 
soon as the vascular bundles have met in the axis of the 
root, their development progresses outwardly, the new 
vessels, the bast-bundles, pressing themselves towards the 
periphery (Fig. 477, 11.). A growth of this nature is found 
especially in herbaceous plants, ¢.g. Czcuwfa, and in those in 
which the stem has isolated vascular bundles. The root of 
other, especially of woody Dicotyledons, shows a similar 
structure to that of the stem. The cell-cavities, both 
of the xylem- and bast-cells, are however considerably 
BB 
