This PPT intends to present the vascular bundles in plants, its diversity, origin and function in mechanical and physiological roles in higher plants.

1. By
N. Sannigrahi, AssociateProfessor, Department of Botany, Nistarini College,
Purulia, 723101(W.B)India

3.  All land plants need conducting tissues to transport water,
minerals and organic materials from the source to sink inside
the plant body. The tissues having conducting strands
constitute the vascular system of plants. The vascular bundles
originate from primary meristem. According to Hanstein and
Schmidt plerome and corpus respectively give rise to vascular
bundles along with other non-vascular tissues. Haberlandt
(1914) called the meristem of vascular tissues as procambium
that divides longitudinally.
 The peripheral derivatives form phloem while the inner forms
xylem. The first cells of xylem to mature are termed as
protoxylem whose development may be centripetal or
centrifugal or both in relation to the centre of axis of the organ
and respectively termed as exarch, endarch and mesarch.

5.  The former two are the characteristic of root and stem
respectively while mesarch arrangement is found in the stem
bundles of Pteris and ferns. The lately formed xylem is termed
as metaxylem. Protophloem is the first cell of phloem to
mature and their development is acropetal in relation to the
centre of axis of organ. The lately formed phloem is termed as
metaphloem. Vascular tissue transports water, minerals, and
sugars to different parts of the plant. Vascular tissue is made of
two specialized conducting tissues: xylem and phloem. Xylem
tissue transports water and nutrients from the roots to different
parts of the plant, and also plays a role in structural support in
the stem. Phloem tissue transports organic compounds from
the site of photosynthesis to other parts of the plant. The
xylem and phloem always lie adjacent to each other in a
vascular bundle .

6.  Xylem and phloem constitute the vascular bundles. The xylem
of two types-protoxylem and metaxylem. Protoxylem are early
formed xylem in comparisons to metaxylem. Protoxylem
derived from procambium. In exarch condition, metaxylem
remains towards the centre and in endarch, the metaxylem
remains toward the centre. Similarly, protophloem and
metaphloem derived form the same direction . The vascular
cambium composed of two types of cells-uniform initials
having elongated structure and ray initials having more or less
isodiametric.
 The primary growth is brought about by primary vascular
bundles and the secondary growth is achieved by the
secondary cambial due to the addition of secondary xylem and
secondary phloem respectively.

7.  a) Closed collateral bundle:
 In this type cambium is absent in between xylem and phloem.
Therefore stems having this type of bundle do not have normal
secondary growth. Ex. Monocotyledonous stem. Usually these
bundles are enclosed within bundle sheath made up of
sclerenchyma and those that lack the sheath are considered as
anomalous (e.g. Asparagus stem).
 b) Open collateral bundle:
 An open collateral vascular bundle has cambium called
fascicular cambium between xylem and phloem. The bundles
can increase in diameter by normal secondary growth with the
help of fascicular cambium. Ex. Dicotyledonous stem.

8.  Type 2. Bicollateral Bundle:
 A vascular bundle with phloem situated on the peripheral and
inner side of xylem is known as bicollateral bundle. A strip of
cambium termed outer cambium is present between the
peripheral phloem and xylem; another strip of cambium,
termed inner cambium, is also present between inner phloem
and xylem.
 The peripheral or external phloem is termed as outer phloem
whereas the inner or internal phloem is called inner phloem.
The sequence of vascular tissues in the bicollateral bundles
from periphery toward centre is outer phloem, outer cambium,
xylem, inner cambium and inner phloem.

9.  These bundles are open type as strips of cambia are present
but the secondary thickening occurs only by the outer
cambium, i.e. cambium present between the outer phloem and
xylem. Ex. Cucurbita
 Concentric Bundle:
 A vascular bundle in which one type of vascular tissue
surrounds the other is known as concentric bundle.
 In this bundle xylem either encircles or is encircled by phloem
and accordingly the following two types are recognized:
 (a) Amphivasal bundle:
 A vascular bundle in which xylem encircles the central strand
of phloem is known as amphivasal bundle, also called
leptocentric bundle. Ex. Dracaena, Yucca.

10.  b) Amphicribral bundle:
 A vascular bundle in which phloem encircles the central
strand of xylem is called as amphicribral bundle, also
known as hadrocentric bundle. Ex. Selaginella.
 The concentric bundles, either amphivasal or
amphicribral, are closed as there is no cambium in
between xylem and phloem.
 Type 4. Radial Vascular Bundle:
 A vascular bundle, in which the primary xylem and
primary phloem strands are separated from each other by
nonvascular tissues and they are situated on alternate radii
of an axis, is known as radial vascular bundle or radial
bundle.

11.  These bundles are the characteristic of roots. There is no
primary cambium in this bundle and the secondary
thickening occurs by the secondary cambium that
originates at the time of secondary growth in
dicotyledonous root only.
 The dicot roots usually have four to six number of
protoxylem poles in contrast to monocot root where many
poles of xylem (more than six) are present. The number
of protoxylem poles in a root may be 1, 2, 3, 4, 5, 6 or
more. Accordingly they are called monarch, diarch,
triarch, tetrarch and so on.

13.  The vascular system is a continuous system in two parts-stem
and root and the parts are connected with the lateral
expansions-leaves and branches. However, the skeleton differs
in different plant organs which is constant as far as species is
concerned. Every species has its own plant and arrangement of
vasculature. The vascular skeleton shows increasing
complexity from the first vascular land plants, Pteridophytes
to angiosperms. The continuity of the vascular bundles from
the aerial part to the underground parts are achieved by
transition zone with orientation, splitting, swing of the
vascular elements occur.