Essay on Stem. After reading this essay you will learn about: 1. Origin of the Stem 2. Root-Stem Transition.
Essay # Origin of the Stem:
The first stem meristem is organized during the development of the embryo. The fully developed embryo commonly consists of an axis, the hypocotyl-root axis. The axis bears at its upper end, one or more cotyledons and the shoot primordium, whereas at its lower end it bears the root primordium covered with a root-cap.
The radicle (embryonic root) is found at the lower end of the hypocotyl and the embryonic shoot is found above the insertion of the cotyledons. The embryonic shoot is composed of an axis bearing un-extended internodes and one or more leaf primordia. This shoot (first bud) is commonly known as plumule and its stem part is termed epicotyl.
The origin of shoot organization is found in the hypocotyl- cotyledon system where the hypocotyl is the first stem unit of the plant and the cotyledons are the first leaves. The hypocotyl is located below the cotyledonary node, but not in between nodes.
During the germination of the seed, the root meristem forms the first root, whereas the shoot meristem develops the first shoot by adding new leaves, nodes and internodes to the shoot system formed in the embryo.
The lateral stems normally arise by the development of new apical meristems laterally in the terminal meristem of the mother axis. The adventitious branches develop on both stem and roots, by the formation of meristems secondarily in the pericyclic, phloic or cambial regions.
Essay # Root-Stem Transition:
The root and stem make a continuous structure called the axis of the plant. The vascular bundles are continuous from the root to the stem. The epidermis, cortex, endodermis, pericycle and secondary vascular tissues are directly continuous in the two organs, root and stem, but the arrangement of vascular bundles is quite different in the two organs.
The stems possess collateral bundles with endarch xylem, whereas the roots possess radial bundles with exarch xylem. Of course, a region exists where these changes occur and the two different types of vascular tissues maintain their continuity.
The change of position involving inversion and twisting of xylem strands from exarch to endarch type is referred to as vascular transition, and the part of the axis where these changes occur is called transition region.
Commonly this region is quite small and rarely of several centimetres. These changes may be found gradually or abruptly in the top of the radicle or at the base of the hypocotyl, near its middle region, or in the upper part. The phloem bundles remain practically in the same position. The transition may be of four main types which vary from species to species.
Type A:
In Mirabilis, Fumaria and Dipsacus, and other plants each xylem strand of the root divides by radial division in the branches. As these branches pass upward, they swing in their lateral direction; one bends towards right and the other goes to the left. Simultaneously these branches join the phloem strands on the inside.
The phloem strands, however, do not change their position and also remain unchanged in their orientation. They remain in the form of straight strands continuously from the root into the stem. In this type as many primary bundles are formed in the stem as many phloem strands are formed in the root.
Type B:
In Cucurbita, Phaseolus, Acer and Trapaeolum and several other plants the xylem and phloem strands fork, the branches. It is to make a point here that the strands of xylem and phloem both divide. The branches of the strands of both swing in lateral direction and pass upward to join in pairs.
After joining in the pairs they remain in the alternate position of the strands in the root. The xylem strands become inverted in their position and the phloem strands do not change their orientation. This way, in the stem, the number of bundles becomes double of the phloem strands found in the root. This type of transition is more commonly found.
Type C:
In Medicago, Lathyrus and Phoenix, the xylem strands do not fork and continue their direct course into the stem. These strands, however, twist through 180 degrees. The phloem strands divide soon and the resulting halves swing in the lateral direction to the xylem positions. The phloem strands join the xylem strands on the outside. In this type as many bundles are formed as there are phloem strands in the root.
Type D:
This type of root-stem transition is rarely found and is known in only a few monocotyledons (e.g., Anemarrhena). In this type half of the xylem strands fork and the branches swing in their lateral direction to join the other undivided strands of xylem. Soon after the xylem strands become inverted.
However, the phloem strands do not divide, but on the other hand they become united in pairs. Simultaneously these united phloem strands unite with the triple strands of the xylem. This way, a single bundle of the stem consists of five united strands, and thus half as many bundles are formed in the stem as there are phloem strands in the root.
In the stems where internal phloem is present, the forked branches of the phloem strands of the root depart at the level at which the roots begin to change into the stems. These branches of phloem strands pass inward and lie inside the new xylem strands giving rise to bicollateral bundles. In some of the monocotyledonous plants the transitory region is very short and from that too several lateral roots are given out.
The seed plants are generally divided into two groups—the angiosperms and the gymnosperms. The angiosperms are further sub-divided into dicotyledons and monocotyledons. In a cross section of the stems of dicotyledons and gymnosperms the vascular bundles are found to be arranged in a ring, whereas in most of the monocotyledonous stems the vascular bundles are numerous and scattered.