What is vascular bundle?
The plant tissues transport water and nutrients that are pumped through the plant's whole structure, often against gravity. Plant vascular tissue is arranged in vascular bundles. The vascular bundle is the unit of vascular tissue system consisting of xylem, phloem and sometimes cambium.
Types of vascular bundle
The vascular bundles are of four types depending on its structural types, those are
Collateral and Conjoint vascular bundles
The vascular bundle in which xylein and phloem are together present and remain side by side is called conjoint, collateral.
Structure of Collateral vascular bundle
i) In this vascular bundle, the phloem lies on the outer side facing the cortex (external phloem) and the xylem remains towards the pith (internal xylem).
ii) The vascular bundle is conjoint because both xylem and phloem are present.
iii) On the basis of the presence of cambium the vascular bundles can be of the two following types :
a) Conjoint collateral open vascular bundle
b) Conjoint collateral closed vascular bundle
Conjoint collateral open: - In this, the cambium is present between the xylem and phloem, which initiates secondary growth (e.g. dicot stem).
Conjoint collateral closed: -: In this, the cambium is absent, so secondary growth is not observed (e.g. monocot stem, leaves).
Bi-collateral vascular bundle
Bi-collateral vascular bundle are special type of collateral vascular bundles, where the phloem patches are present on both sides of the xylem, seperated from the xylem by the strip of cambium.
Structure of Bi-collateral vascular bundle
i) The vascular bundle contains xylem, phloem and cambium and hence it is also called conjoint, bicollateral, open type and is observed in Cucurbita stem.
ii) The patch of phloem facing the cortex is external phloem, while that facing the pith is internal or included phloem.
iii) Like that, the cambium is facing the external and internal phloem is called outer and inner cambium respectively.
Radial vascular bundle
The vascular bundle in which xylem and phloem strands occur separately on alternate radii in a ring-like fashion separated by non-vascular tissues within the vascular cylinder, is called radial vascular bundle.
Structure of Radial vascular bundle
i) Radial vascular bundle are present in dicot and monocot roots.
ii) The xylem is metaxylem towards the centre while protoxylem towards the margin.
iii) The number of vascular strands may be 4-6 in dicot root but in monocot root there are 8-10 strands.
Concentric vascular bundle
The vascular bundle in which xylem and phloem are remain together in such a way that one encloses the other.
Structure of Concentric vascular bundle
i) Concentric vascular bundle are also called conjoint vascular bundles.
ii) There is no cambium present in between xylem and phloem, so these vascular bundles are always closed.
Special type of vascular bundle
Special type of vascular bundles are not included in the main classification of vascular bundles and they are of the following types :
i) Cortical bundles: These bundles are scattered in the cortical region of the stem found in the stems of families like Cactaceae and Oleaceae.
ii) Cortical bundles : These bundles are scattered in the cortical region of the stem found in the stems of familes like Cactaceae and Oleaceae.