Development and Structure of the Watermelon Seedling

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Abstract
The primary root of the watermelon seedling is tetrarch exarch. In transition each xylem strand is divided into 2. These 2 divisions swing outward through arcs of 180[degree], and converge by pairs in front of the 4 phloem groups. When transition is completed there are 4 bundles composed of the 4 original phloem groups intact and of xylem which comes in equal amounts from 2 adjoining groups of xylem of the original alignment. The transition is rapid and is completed at the level of the peg. There is 1 common histogen in the primary root; all primary tissues are derived from a common meristematic region. The endodermis and inner cortical layers, as well as the peri-cycle, of the primary root contribute to the building up of the tissues of the secondary roots. There are usually 6 bundles in the hypocotyl: the 4 transition bundles and 2 others which are anastomosed branches from the transition bundles. All these bundles diverge into the cotyledons. For a considerable period of time the cotyledons serve as photosynthetic organs. The epicotyl develops slowly at first, because its vascular connections with the axis are, in the main, differentiated after germination. The bicollateral bundles extend from the level at which transition is completed upward through the hypocotyl into the cotyledons and into the epicotyl. The internal phloem is derived from the parenchyma which lies adjacent to the inner faces of the bundles. It does not extend into the roots, but is connected laterally with the external phloem in the transition zone.

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