Xylem Development in Young Internodes of the Kentucky Wonder Bean

Abstract

1. The rate of stem elongation in Phaseolus vulgaris L. var. Kentucky Wonder is related to (a) internodal length (the inverse relation between stem length and daily period of illumination results from greater elongation of individual internodes in shorter photoperiods rather than from the production of a greater number of internodes); (b) pith cell number in a longitudinal direction (etiolation results from vertical proliferation of pith cells rather than from greatly increased length of individual cells); (c) the lignification pattern of the earliest differentiated xylem (the ratio of spiral to annual elements adjacent to the pith in the fifth and sixth internodes below the stem apex increases with the daily length of light exposure); and (d) the ratio of total spiral to total annular elements (the incidence of spiral markings increases with the length of the photoperiod). 2. The rate of stem elongation has no appreciable effect on (a) the total length of individual pith cells or (b) the spacing of spiral or annular markings in the first differentiated, longest internode. The distance of separation is determined by the final length of individual cells, which appears to vary little in corresponding positions in etiolated plants, in those given a 13-hour photoperiod, or in those grown in continuous illumination. 3. In the first xylem elements differentiated at the stem tip lignin is deposited as close spirals. In the fourth, fifth, and sixth internodes below the stem apex there are occasional exceptions to a strictly centrifugal pattern of xylem maturation.