The Control of the Differentiation of Vascular Networks

A polar control of vascular differentiation by stimuli movingfrom the shoot to the root has been demonstrated many timesand accounts for some traits of the pattern of vascular strands.The problem dealt with here is whether this polar mechanismcontrols the formation of the vascular networks which are commonin leaves. Networks generally, though not invariably, includestrands which do not connect the shoot to the root, and thereforecannot be considered as polar. A small transverse vascular strandin Phaseolus stems includes neighbouring vessels which haveopposite shoot to root directions. A developmental study indicatedthat vessels with opposite polarities do not mature at the sametime, and suggested that there are repeated changes in the directionof the flow of the polar stimuli which control vessel formation.Experiments with Pisum show that auxin can induce the formationof a non-polar strand of xylem when the location of the sourceof auxin, and the resulting direction of auxin flow, are changedrepeatedly. It is concluded that in the first stages of vasculardifferentiation there is a determination of the axis, but notof the direction, of the movement of differentiation-inducingstimuli. When the rate of development and of the productionof stimuli is not synchronous throughout the organ, this earlydetermination of an axis of movement leads to the differentiationof vascular networks.