POLARITY OF INDUCTION AND PATTERN OF PRIMARY PHLOEM FIBER DIFFERENTIATION IN COLEUS

Abstract

The differentiation of primary phloem fibers was studied in Coleus blumei on a quantitative basis. The pattern of fiber differentiation in intact, untreated plants was found to be in the acropetal direction (from a mature internode to a young one). The youngest internodes to differentiate primary phloem fibers were those with cambial activity. In plants grown in the winter, fibers started to differentiate in internodes closer to leaf #2 than in spring‐grown plants. A wound changes the pattern of fiber differentiation surrounding it. A wound in which the tissues above and below it were separated with parafilm, prevented fiber differentiation in the tissues directly below the wound, and caused more fiber differentiation in the tissues above and lateral to it. Under wounds with no parafilm separation, few or many fibers differentiated depending on the angle of the wound. The number of fibers under diagonal wounds was five to nine times more than under a horizontal wound. By excision experiments it was found that mature leaves were the source of induction of fiber differentiation. Leaves that produced induction caused fiber differentiation in the internode below them but did not cause fiber differentiation in the internode above. The induction, which can flow through a wound and cause fiber differentiation in at least two internodes below the source, is a polar induction in the basipetal direction (i.e., in the direction from the leaves to the root). Phloem fibers differentiated only in the vascular strands and not from the parenchyma cells between the strands. Therefore, they follow the new regenerative sieve and vessel elements in the pre‐existing vascular strands, but do not follow them in their regeneration between the longitudinal strands.