Xylem Development in Young Internodes of the Kentucky Wonder Bean

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Abstract
Seedlings of Phaseolus vulgaris var. Kentucky Wonder were grown for 150 hrs. in continuous light, a 13-hr. photoperiod, or total darkness. The rate of stem elongation was related to: (a) internodal length, greater elongation of individual internodes occurring in shorter photoperiods; (b) pith cell number in a longitudinal direction, etiolation in the pith resulting from vertical proliferation of cellls; (c) the lignification pattern of the earliest differentiated xylem, the ratio of spiral to annular elements adjacent to the pith in the 5th and 6th internodes below the stem apex increasing with daily length of light exposure; and (d) the ratio of total spiral to total annular elements, the incidence of spiral markings increasing with length of photoperiod. The rate of stem elongation had no appreciable effect on total length of individual pith cells or spacing of spiral or annular markings in the first differentiated, longest internode. In the 1st xylem elements differentiated at the stem tip lignin was deposited as close spirals. In the 4th, 5th, and 6th internodes below the stem apex there were occasional exceptions to a strictly centrifugal pattern of xylem maturation.

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