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In this article we will discuss about the root stem transition in plants.
The change in arrangement of vascular tissues of roots having separate strands of phloem and xylem with exarch protoxylem to collaterally placed phloem and xylem with endarch protoxylem of stem is commonly referred to as root-stem transition.
In higher plants root and shoot form a continuous axial structure. The simple form of axial structure is exhibited in Psilophytales. In higher plants the differentiated plant body into root, stem and leaf is regarded as an evolutionary specialization of simple axial structure.
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The differentiation of root and stem is initiated in the embryo axis. An embryo is made up of one or two cotyledon(s). Towards the upper end just above the insertion of cotyledon(s) there occurs the primordium of shoot-plumule. The portion between the insertion of cotyledon and plumule is the epicotyl (epi = above).
Towards the lower end just below the insertion of cotyledon there occurs the primordium of root- the radicle. The portion between the insertion of cotyledon and radicle is the hypocotyl (hypo = below). Radicle and plumule form a continuous axial structure with the intervening tissues of hypocotyl and epicotyl. Radicle grows below to form root. Plumule grows above to form shoot.
The plant that develops from the embryo axis shows externally a continuity of root and stem. Internally also the tissues of axis, i.e. epidermis, cortex, endodermis, pericycle and secondary vascular tissues are directly continuous from root to stem. Continuity is also present in primary vascular tissues, but they do not merge directly because the orientation of primary vascular tissues is markedly different in root and stem.
In root the primary vascular tissues have independent strands of phloem and xylem that show alternate arrangement, the xylem being exarch. In contrast the stem shows collateral arrangement of primary vascular tissues with endarch protoxylem. So the primary vascular tissues of root and stem though apparently continuous merge with each other by changing one type of arrangement and structure to the other type.
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This change of arrangement of primary vascular tissues is usually referred to as vascular transition. The change occurs at successive levels of radicle-plumule axis. The place, where the change of orientation of vascular tissues occurs, is designated as transition region.
The exact region of vascular transition varies. The transition region may be at the top, centre or base of hypocotyl. Vascular transition may also occur at the top of radicle. The length of transition region is also variable. It may be short and the length ranges from one to three millimeter where the vascular transition occurs abruptly.
In long transition region it may be several centimeters in length and the vascular transition occurs gradually. The transition region may be visible externally as a zone of depression or change in diameter. However in many plants the external morphology and transition region do not correspond exactly.
