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In this article we will discuss about the four main types of root stem transition in angiosperms.
Type A (Fig. 16.1.1A-1E):
This type is represented by Dipsacus, Mirabilis and Fumaria. In the root there are four strands of phloem and four strands of xylem with exarch protoxylem. They are arranged radially.
The stem shows four conjoint collateral vascular bundles with endarch protoxylem. The vascular bundles of stem are located in direct line above the phloem of root. Phloem during its upward course from root to stem has not changed its position.
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Each xylem strand of root, as it passes upward, forks radially into two halves. Among the two halves of xylem one swings to the right and the other to the left as they pass upward. Before joining with the phloem of stem inside the two halves of forked xylem rotate through 180°.
Thus, the exarch xylem of root turns to endarch xylem of stem. So each vascular bundle of stem consists of an entire phloem, half xylem of one xylem strand and half xylem of another xylem strand of root.
Type B (Fig. 16.1.2A-2E):
This type is exhibited by Acer, Cucurbita, Phaseolus and Tropaeolum. In the root there are two strands of phloem and two strands of xylem with protoxylem exarch. They are arranged radially. The stem shows four vascular bundles with conjoint collateral arrangement of vascular tissues with protoxylem endarch.
The two strands of phloem of root, as they pass upward, divide and thus form four phloem strands. The entire phloem strand of root during its upward course elongates. Constriction appears in the middle and thus the phloem strand becomes dumb-bell shaped. The constriction becomes complete and as a result the number of phloem strands is doubled. As the phloem strands pass upward, each strand occupies the characteristic position in stem.
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The xylem strand of root also divides. During division, the xylem forks radially into two halves. Among the two halves of xylem one swings to the right and the other swings to the left as they pass upward.
Before joining with the phloem of stem inside the two halves of forked xylem rotate through 180°. Thus the exarch xylem of root turns to endarch xylem in stem. So each vascular bundle of stem consists of half xylem strand and half phloem strand of root.
Type C (Fig. 16.1.3A-3E):
Medicago, Lathyrus and Phoenix exhibit this type of vascular transition. In the root there are two strands of phloem and two strands of xylem with protoxylem exarch. They are oriented radially. The stem shows two conjoint collateral vascular bundles with endarch protoxylem. The collateral vascular bundles are located in direct line above the xylem strands of root.
Xylem strands of root as they pass upward rotate through 180° and thus they become endarch in stem. Xylem strands of root do not divide and continue their course into stem directly. They rotate only to become endarch. Phloem strands of root as they pass upward elongate. Constriction appears in the middle and thus the phloem strands become dumb-bell shaped.
The constrictions become complete and thus four strands of phloem are formed. Two strands of phloem, each derived from separate strands, unite above the xylem strand of stem. Each vascular bundle of stem is made up of one xylem strand and two half phloem strands of root. The number of vascular bundles of stem is equal to the number of phloem strands of root.
Type D (Fig. 16.1.4A-4E):
Anemarrhena, a monocotyledon, exhibits this type of vascular transition. In the root there are four strands of phloem and four strands of xylem with exarch protoxylem. They are arranged radially. The stem shows two vascular bundles with conjoint collateral orientation of vascular tissues with protoxylem endarch.
During vascular transition one diagonal pair of xylem strands (black-filled in figure) of root does not divide and continues its upward course into the stem directly. These two xylem strands rotate through 180° during their upward course to stem and thus they become endarch. Two vascular bundles of stem are located in direct line above the xylem strands (that do not split) of root.
Each of the two other xylem strands of root, as passes upward, forks radially into two halves. Among the two halves of xylem one swings to the right and other to left as they pass upward. The two halves of xylem rotate through 180° and join with the entire xylem strand on both sides.
Two of four phloem strands of root, as they pass upward, become close to each other and join above the entire and forked xylem strands in stem. Five vascular strands of root make a single vascular bundle of stem. Two phloem strands, one entire xylem strand and two forked xylem strands of root unite to form a single vascular bundle of stem. As a result the number of vascular bundle in stem is reduced to half the number of phloem strands present in the root.
