The control of the differentiation of vascular networks | Control of the differentiation of vascular networks [Peas, kidney beans]
1975
Sachs, T.
A polar control of vascular differentiation by stimuli moving from the shoot to the root has been demonstrated many times and accounts for some traits of the pattern of vascular strands. The problem dealt with here is whether this polar mechanism controls the formation of the vascular networks which are common in leaves. Networks generally, though not invariably, include strands which do not connect the shoot to the root, and therefore cannot be considered as polar. A small transverse vascular strand in Phaseolus stems includes neighbouring vessels which have opposite shoot to root directions. A developmental study indicated that vessels with opposite polarities do not mature at the same time, and suggested that there are repeated changes in the direction of the flow of the polar stimuli which control vessel formation. Experiments with Pisum show that auxin can induce the formation of a non-polar strand of xylem when the location of the source of auxin, and the resulting direction of auxin flow, are changed repeatedly. It is concluded that in the first stages of vascular differentiation there is a determination of the axis, but not of the direction, of the movement of differentiation-inducing stimuli. When the rate of development and of the production of stimuli is not synchronous throughout the organ, this early determination of an axis of movement leads to the differentiation of vascular networks.
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