SOME ULTRASTRUCTURAL ASPECTS OF METAPHLOEM SIEVE ELEMENTS IN THE AERIAL STEM OF THE HOLOPARASITIC ANGIOSPERM EPIFAGUS VIRGINIANA (OROBANCHACEAE)

Abstract

A light and electron microscope investigation was conducted on phloem in the aerial stem of Epifagus virginiana (L.) Bart. Tissue was processed at field collection sites in an effort to overcome problems resulting from manipulation. At variance with earlier accounts, Epifagus phloem consists of sieve elements, companion cells, phloem parenchyma cells, and primary phloem fibers. The sieve elements possess simple sieve plates and the phloem is arranged in a collateral type of vascular bundle. In addition, this constitutes the first study on phloem ultrastructure in the aerial stems of a holoparasitic dicotyledon, an entire plant which could be viewed as an “ideal sink.” Epifagus phloem possesses unoccluded sieve plate pores in mature sieve elements and a total lack of P‐protein in sieve elements at all stages of development. Mature sieve elements lack nuclei. Plastids were rarely observed in mature sieve elements. Vacuoles with intact tonoplasts were encountered in some mature sieve elements. Otherwise, the ultrastructural features of sieve elements appear to differ little from those described by investigators of non‐parasitic species.