The tissue responsible for the conduction of food and water is called Vascular Tissue. The xylem and phloem are the two major components of vascular tissues. They together form the vascular bundles in dicot stem, the xylem and phloem remain alienated by a patch of meristematic tissue called cambium, so the vascular bundle is open and it shows secondary growth. In monocot stem, the cambium is absent, so the vascular bundle is closed. The vascular bundles along with other tissues, remaining within the endodermis is called stele. In both dicot and monocot roots, cambium is absent and the xylem and phloem remain in alternate patches in the form of radial vascular bundles. In leaves, the vascular bundles are closed without any secondary growth.
The vascular tissues are distributed all over the place in a plant, i.e., in the root, stem and leaves. They are absent in epidermal trichomes and prickles.
The vascular tissues originate from the pro-cambium or plerome of the apical meristem, the xylem originating from xylic pro-cambium, while phloem originating from phloic pro-cambium respectively.
The primary vascular bundles consist of primary xylem; primary phloem and cambium, while in the secondary wood, the secondary xylem and phloem are cut off from the cambium.
The primary xylem of vascular tissue differentiates from the xylic pro-cambium in the early stage of the development of the plant. The major elements of xylem include tracheid, trachea or vessel, xylem parenchyma and xylem fibre. On the basis of the maturity of xylem elements, xylem can be classified as protoxylem and metaxylem. The first formed xylem elements with comparatively smaller diameter are called protoxylem. The later-formed xylem elements with comparatively broader lumen are known as metaxylem.
It develops from the phloic procambium of the primary meristem in a young plant body. The various elements present in the primary phloem are sieve cell, sieve tube, phloem parenchyma, phloem fibre and companion cells. On the basis of maturity, phloem can be of 2 types viz., protophloem and metaphloem. The first fanned phloem elements with a comparatively less lumen is protophloem. The later-formed phloem elements with a broader lumen is known as metaphloem. Phloem usually remains as external phloem on the outer side of the xylem elements in the vascular bundles of stem. In root, it remains alternately with xylem elements forming alternate vascular bundles. In the leaves, the phloem elements are located below the xylem elements. In certain stems as gourd and potato, phloem also remains in the inner side of the primary xylem elements and are called internal or included or intra-xylary phloem.
The cambium or lateral meristem is a strip of procambium situated in between xylem and phloem in the vascular bundles of dicots and gymnosperms, which remains meristematic even after primary growth is over. Cambium can be of three types : Fascicular cambium : The cambium present in between the xylem and phloem within the vascular bundles is called fascicular cambium. Inter-fascicular cambium : The cambium developing in between two vascular bundles in the medullary rays is called inter-fascicular cambium, which joins with the fascicular cambium to form the cambial ring that cuts off secondary xylem towards the Centre and secondary phloem towards the periphery. Cork cambium : They are formed from cortical cells during secondary growth in the extrastelar region, it is also called phellogen, which forms the bark or periderm.
This website uses cookies.
