The vascular pattern of Italian ryegrass (Lolium multiflorum Lam.) 2. development of the seedling axis to maturity
1976
Bell, A.D.
The vascular system present in a grass seedling axis persists in a functional state at the base of a maturing plant, but undergoes a number of modifications. Two strands of phloem, accompanied by some internal xylem, differentiate in association with the bicollateral mesocotyl trace at right angles to the existing phloem, resulting in a tetrarch bundle. Lateral seminal roots are themselves tetrarch and the vascular insertion of a seminal root on to the mesocotyl is a distinctive three-dimensional feature. At the base of the mesocotyl the tetrarch bundle merges with the tetrarch bundle of the primary seminal root via a transition zone. The four phloem poles unite and then diverge again; the central xylem strand splits into three and then reunites, the two tissues being intimately interlocked by this rearrangement. The additional vascular tissue of the mesocotyl extends up into the coleoptilar node and becomes involved in the vascular attachment of nodal roots at this point. Additional vascular tissue continues to differentiate in the periphery of the maturing stem and is here termed the ‘peripheral plexus’. In the seedling, the xylem of the ‘bridge’ linking the mesocotyl trace with the scutellar trace is associated with xylem transfer cells and also contains tracheids with distinctive, thin-barred scalariform thickening. These transfer cells disappear as the plant matures but numerous tracheids with thin-barred scalariform thickening are then to be found. The possible significance of transfer cells in the coleoptilar node is discussed.
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