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Special Morphology and Classification. 201. 
so simple as in these cases ; for although the bundles of most 
Dicotyledons run in a vertical direction, they nevertheless 
occasionally unite, and form combinations which are often 
rendered yet more complicated by new separations. In the 
Labiate, Umbelliferee, Balsamineze, and many other her- 
baceous plants with jointed stems, the vascular bundles are 
arranged in a circle, and all run in a parallel direction 
through the internode. In the node, however, they branch, 
and are here united with the vascular bundles of the leaves, 
as we have seen to be the case with Grasses (see p. 341). 
The vascular bundles always become closer as they 
descend in the stem, the separate bundles approaching 
nearer and nearer to one another through increase in their 
thickness, often coalescing into a ring which is interrupted 
only by narrow medullary rays (Fig. 95, 11. p. 68). The 
development of the vascular-cells or vessels then always 
begins on the inside of the cambium-bundles, and advances 
thence outwardly with centrifugal growth, so as to render 
possible a smaller or larger development of pith. 
It will be seen from this description that there is no dif- 
ference between the mode of growth of annual and perennial 
Dicotyledons, when young plants of the latter are examined, 
or shoots still in their first year of growth. After this, growth 
ceases in annual plants; while in those that are perennial 
further changes, which will be afterwards referred to, take 
place in the spring, when vegetation wakes from its winter-rest. 
With reference to the composition of the vascular 
bundles, a distinction must be drawn between herbaceous 
and woody plants. In herbaceous plants they are separated 
by layers of an intermediate tissue of more or less con- - 
siderable thickness, so. that they either lie isolated in the 
tissue of the stem, or form a ring which is only inter- 
rupted by narrower intervals of intermediate tissue (Fig. 
472). This intermediate tissue, a part of the fundamental 
tissue, 1s often composed of cells which are narrower and 
otherwise of different form from those of the pith and cortex, _ 
