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

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. FOR THOSE interested in the processes by which various cell types are differentiated and organized into a tissue system, the vascular system of plants is particularly valuable because it has been studied so extensively (see Esau, 1965). Studying the differentiation of each type of cell in the system is necessary to understand the mechanisms which control and determine the pattern of differentiation of various types of cells in an organized tissue. For this work I have chosen primary phloem fiber cells in the vascular strands. These fibers are found in the periphery of the primary sieve elements. Because they are located inside the innermost cortical layer and on the periphery of the central cylinder, they were first termed pericyclic fibers. However, ontogenetic studies showed that they are developed from the procambrium and they were, therefore, renamed primary phloem fibers (Esau, 1938, 1943; Blyth, 1958). Primary phloem fibers are common in monocotyledons and dicotyledons (Esau, 1969). They are very long and narrow cells that have 1 Received for publication 3 July 1975. I wish to thank the Lady Davis Fellowship Trust whose support made this work possible, Professor William P. Jacobs for advice and critical reading of the manuscript, and the Biology Department at Princeton University for use of their equipment and for financial aid. 2 Present address: Department of Biology, TechnionIsrael Institute of Technology, Haifa 32000, Israel. thick cell walls which are usually heavily lignified. The primary phloem fibers arise from short initials which greatly elongate in the course of their maturation, mainly by intrusive growth at their apical and basal ends. Aldaba (1927) found that the initials in Boehmeria nivea were 20 /m long and grew to be 550,000 /m (55 cm). These fibers are present through several internodes in the mature stem. Development of fibers in this species was gradual and may take months. In Boehmeria and Linum the secondary cell wall of the primary phloem fibers developed when the ends began to elongate by intrusive growth. Then a series of telescoping, open secondary wall lamellae were gradually added centripetally along the non-elongating middle parts of the fiber cell. These secondary wall lamellae developed from the base to the apex (Aldaba, 1927; Anderson, 1927; Kundu and Sen, 1960). In Boehmeria, the upper end of the fiber cell grows for a longer period than the basal end (Kundu and Sen, 1960). In Cannabis and Corchorus, however, the deposition of the secondary layers in the primary and secondary phloem fibers seems to occur simultaneously over thewhole fiber cell (Kundu, 1942). Primary phloem fibers are longer than secondary phloem fibers which develop from the fusiform initials of the cambium. For example, In Cannabis the primary phloem fibers averaged 13 mm and the secondary fibers about 2 mm (Kundu, 1942). In