Pharmaceutical botany - Lighthouse Survival Blog

ITBEOVASCULAR BUNDLES 13
Fibrovascular Bundles are groups of fibres, vessels and cells cours-
ing through the various organs of a plant and serving for conduction
and support. According to the relative
structural arrangement of their xylem
and phloem masses they may be classed
as follows:
I. Closed Collateral, consisting
of a mass of xylem lying alongside of a
mass of phloem, the xylem facing toward
the centre, the phloem facing toward
the exterior. Stems of most
Monocotyledons and Horsetails.
II. Open Collateral, consisting of
a mass of xylem facing toward the pith
and a mass of phloem facing toward the
exterior and separated from each other
by a cambium. Stems and leaves of
Dicotyledons and roots of Dicotyls and
Gymnosperms of secondary growth.
III. BicoLLATERAL, characterized
by a xylem mass being between an in-
ner and an outer phloem mass. There
are two layers of cambium cells, one be-
tween the xylem and inner phloem mass,
the other between the xylem and outer
phloem mass. Seen chiefly in stems of
the Cucurbitacecs.
IV. Concentric, characterized by
a central xylem mass surrounded by a
phloem mass or vice versa. No cambium
present.
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Fig. II.- -Stages in the development
of sieve tubes, companion
cells, and phloem parenchyma.
A, a and 6, Two rows of procambial
ceUs; in and d, a has divided
longitudinally and c is to become
companion cells; d, a sieve tube,
and b, phloem parenchyma. B, c,
Companion cells, and d, a beginning
sieve tube from c and d, respectively
in A. The cross-walls in d are
pitted; b, phloem parenchyma
grown larger than m A. C, The
same as B with the pits in the crosswalls
of the sieve tubes become perforations,
and the nuclei gone from
the cells composing the tube.
(From Stevens.)
later stage with cells longer and more pointed and walls becoming thickened and
pitted; 4, complete wood fibers with walls more thickened than in the previous stage
and lignified, as shown by the stippling. The protoplasts in this last stage have
disappeared and the fibers are dead. D, steps in the formation of wood parenchyma
from cambial or procambial cells, i, Group of cambrial or procambial cells; 2,
the same enlarged in all dimensions; 3, the same with walls thickened and pitted;
4 and 5 show the same stages as 2 and 3, but here the cells have enlarged radially
or tangentially more than they have vertically. The walls of these cells are apt to
become lignified, but the cells are longer lived than the wood fibers. {From
Stevens.)