Internal Phloem Development in Pharbitis nil Chois. (Convolvulaceae)

Abstract

Internal and external phloem in Pharbitis nil differentiate in procambium of the aerial axis and foliar organs Internal phloem differentiates centrifugally as discrete strands following the differentiation of external phloem and xylem. Internal phloem is usually interior to protoxylem elements. External phloem, as viewed in cross section, differentiates centripetally and becomes a continuous band in stems and hypocotyls following the production of secondary phloem by the vascular cambium. Strands of internal and external phloem nearly surround the crescent-shaped xylem mass of petiole midveins. The widest phloem strands and the longest sieve-tube elements differentiate in internal phloem of mid and terminal stem segments. These segments also contain the widest sieve-tube elements; however, elements differentiate in external, rather than internal, phloem. Anomalous development of cambium and adventitious roots are associated with internal phloem development in older portions of aerial axes. Arcs of interna...