Vascular bundle is the isolated unit of the longitudinal strands of conducting tissues consisting essentially of xylem and phloem, frequently with a sheath of thick walled cells or other interspersed cells.
Occurrence of Vascular Bundle:
Vascular bundles are present in all plant organs, mainly in stem (including rhizomes or underground stems), leaves and roots (including aerial). In stem they are either scattered or located around the stem axis.
Bundles may also be present in the cortex or pith and respectively they are referred to as cortical bundle (ex. members of Cactaceae, Oleaceae) and medullary bundle (ex. Dahlia, Piper etc.). The location of these bundles is considered as anomalous. In roots they are situated at the centre.
Functions of Vascular Bundle:
Xylem and phloem compose vascular tissues, which transport water and dissolved minerals to the photosynthetic tissues through xylem. The elaborate food and other substances are translocated to all parts of the plant through phloem.
Ground tissues between bundles are the medullary rays that assist in lateral transport of minerals, water and other substances. Vascular bundles also provide mechanical support. The fascicular cambium within bundles forms secondary tissues during secondary growth.
Origin of Vascular Bundle:
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 (Fig. 14.1).
The former two are the characteristic of root and stem respectively while mesarch arrangement is found in the stem bundles of Tmesipteris 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.