Root Stem Transitions in Plants with PPT

Root Stem Transition: 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.

The current post describes the various types of root-stem transitions found in Angiosperms.

Ø  The epidermis, cortex, endodermis, pericycle and the secondary vascular tissue are continuous from root to stem.

Ø  Only the primary vascular tissue in the axis undergoes a change in their position and orientation.

Ø  The exarch arrangement of xylem in root changes to endarch position in the stem.

Ø  In the transition zone, the vascular tissue undergoes many changes such as Forking, Multiplication, Rotation and Fusion.

Ø  The vascular tissue also increases their diameter.

Ø  The exact position of the transition zone is different in different plants.

Ø  Usually, the transition zone occurs at the tip of the radicle or at the basal or middle or at the top portion of hypocotyl.

Ø  The length of transition zone also varies from few millimeters to several centimeters.

Ø  The transition of vascular tissue may be gradual or abrupt in different plants.

Ø  The phloem tissue, in most of the cases, remains more or less same position.

Root Stem Transition

Different types of Root Stem Transitions in plants

Ø  Eames and MacDaneal (1947) described FOUR types of root stem transition in plants.

Ø  They are:

(1).  Fumaria Type (A – Type)

(2).  Cucurbita Type (B – Type)

(3).  Lathyrus Type (C – Type)

(4).  Anemarrhena Type (D -Type)

Fumaria Type Root-Stem Transition (Type A)

Ø  Occurs in Fumaria, Mirabilis and Dipsacus.

Ø  Each xylem strands in the root divides radially into two branches.

Ø  As the branches go upward they swing laterally.

Ø  Among these two branches, one swing to the right and one swing to left and join to the phloem strand inside.

Ø  Phloem strand do not change its position or orientation and they pass directly from the root to the stem.

Ø  The number of vascular bundles formed in the stem will be equal to the number of phloem strands occurs in the root.

Cucurbita Type Root-Stem Transition (Type B)

Ø  Found in Cucurbita, Acer, Phaseolus and Tropaeolum.

Ø  In Cucurbita type both the xylem and phloem strand divide into two strands.

Ø  Each xylem swing as in Fumaria type, and join with a strand of phloem.

Ø  Phloem does not change its orientation, but the xylem becomes inverted.

Ø  Number of vascular bundles formed in the stem will be twice the number of phloem strands in the root.

Lathyrus Type Root-Stem Transition (Type C)

Ø  Found in Lathyrus and Medicago.

Ø  Here the xylem strand does not divide but pass continuously into the stem.

Ø  However the xylem strands twist 180 degrees.

Ø  Meantime the phloem strands divide.

Ø  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

Ø  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.

Anemarrhena Type Root Stem Transition (Type D)

Ø  It is a very rare type of root-stem transition.

Ø  Found in some monocot plants such as Anemarrhena.

Ø  Here half of the xylem strand divide radially, the rest remain undivided.

Ø  The branches of the divided xylem swing laterally and join with the undivided xylem strand which has now inverted its position.

Ø  The phloem strands do not divide but fuse in pair with the xylem strand.

Ø  Thus a single vascular bundle in this type of stem composed of two phloem strands and three xylem strands.

Ø  The number of vascular bundles formed in the stem will be half the number of vascular strands in the root.

Review Questions

1.      With suitable diagrams, write an essay on root-stem transitions in Angiosperms.
2.      List the four types of root-stem transitions you studied.
3.      What are the peculiarities of root-stem transition zone?
4.      Explain the Fumaria type root-stem transition with diagram.
5.      Explain the Cucurbita type root-stem transition with diagram.
6.      Explain the Lathyrus type root-stem transition with diagram.
7.      Explain the Anemarrhena type root-stem transition with diagram.

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