Root stem transition

The transition of the arrangement of the vascular bundles in roots and stem.
Both stem and roots have different arrangements of the vascular bundles.

1. ROOT STEM TRANSITION BY HETAKUMARI PILUDARIA BSc (HONS) BIOTECHNOLOGY SEM II

2. INTRODUCTION Root and stem form a continuous structure in plants called the axis. The vascular tissue in root is radial (xylem and phloem arranged separately in different radii) whereas in the stem is conjoint (xylem and phloem are arranged together in same radii as vascular bundles). Thus, there should be a region in the axis where the vascular structures of the root and the stem meet and merge together. This peculiar region is called the Root-Stem Transition Zone. HPILUDARIA 2

3. The epidermis, cortex, endodermis, pericycle and the secondary vascular tissue are continuous from root to stem. Only the primary vascular tissue in the axis undergoes a change in their position and orientation. The exarch arrangement of xylem in root changes to endarch position in the stem. In the transition zone, the vascular tissue undergoes many changes such as Forking, Rotation and Fusion. HPILUDARIA 3

4. The vascular tissue also increases their diameter. The exact position of the transition zone is different in different plants. Usually , the transition zone occurs at the tip of the radicle or at the basal or middle or at the top portion of hypocotyl. The length of transition zone also varies from few millimeters to several centimeters. The transition of vascular tissue may be gradual or abrupt in different plants. The phloem tissue, in most of the cases, remains more or less same position. HPILUDARIA 4

5. Different types of Root Stem Transitions are observed in plants Eames and MacDaneal (1947) described FOUR types of root stem transition in plants. They are: (1).   Fumaria Type (A – Type ) (2).   Cucurbita Type (B – Type) (3).   Lathyrus Type (C – Type) (4).   Anemarrhena Type (D -Type) HPILUDARIA 5

6. Fumaria Type Root-Stem Transition (Type A) Occurs in Fumaria , Mirabilis and Dipsacus . Each xylem strands in the root divides radially into two branches. As the branches go upward they swing laterally. Among these two branches, one swing to the right and one swing to left and join to the phloem strand inside. Phloem strand do not change its position or orientation and they pass directly from the root to the stem. The number of vascular bundles formed in the stem will be equal to the number of phloem strands occurs in the root. HPILUDARIA 6

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8. Cucurbita Type Root-Stem Transition (Type B) Found in Cucurbita , Acer, Phaseolus and Tropaeolum . In Cucurbita type both the xylem and phloem strand divide into two strands. Each xylem swing as in Fumaria type, and join with a strand of phloem. Phloem does not change its orientation, but the xylem becomes inverted. Number of vascular bundles formed in the stem will be twice the number of phloem strands in the root. HPILUDARIA 8

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10. Lathyrus Type Root-Stem Transition (Type C) Found in Lathyrus and Medicago . Here the xylem strand does not divide but pass continuously into the stem. However the xylem strands twist 180 degrees. Meantime the phloem strands divide. The phloem strands then move laterally to the position of the xylem strand and then they fuse together and join the xylem strand on the outside In this type the number of vascular bundle formed in the stem will be equal to the number of phloem strands found in the roots. HPILUDARIA 10

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12. Anemarrhena Type Root Stem Transition (Type D ) It is a very rare type of root-stem transition. Found in some monocot plants such as Anemarrhena . Here half of the xylem strand divide radially, the rest remain undivided. The branches of the divided xylem swing laterally and join with the undivided xylem strand which has now inverted its position. The phloem strands do not divide but fuse in pair with the xylem strand. Thus a single vascular bundle in this type of stem composed of two phloem strands and three xylem strands. The number of vascular bundles formed in the stem will be half the number of vascular strands in the root. HPILUDARIA 12

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15. Significance of Root-Stem Transition In the transition zone the internal tissue arrangement is completely different from that of root and stem . Thus, this region does not belong to any of those categories. Several interpretations are available to describe the structure and evo­lutionary significance of the transition zone. HPILUDARIA 15

16. The root and shoot tips of the seedling have their own meristem which forms the root and stem, respectively . During development, vascular connections are esta­blished between radicle and cotyledon via hyopocotyl . The epicotyl traces are joined with the radicle-hypocotyl-cotyledon unit and tissues between the traces are mutually accommodated (Esau, 1965). HPILUDARIA 16

17. The radial arrangement of xylem and phloem is usually considered as primitive where­as the collateral arrangement of them is regarded as advanced . Therefore, the xylem and phloem arrangements at the upstream transition zone represent the different evolutionary stages which culminate into collateral arrangements in the stem. HPILUDARIA 17

18. THANK YOU HPILUDARIA 18