PARASITISM IN PHOLISMA (LENNOACEAE): II. ANATOMICAL ASPECTS

Abstract

The anatomy of roots and haustoria of Pholisma depressum is detailed. Lignified xylem consists of vessel elements only, and is associated with some xylem parenchyma. Sieve tube members exist in both the stem and the root. The pilot root is characterized by a restriction of cambial activity to the primary phloem regions, thus resulting in 3–5 bundle-like structures in older roots. From the protoxylem points between these bundles vascular flanges extend radially outward. The steles of lateral roots are extensions of these vascular flanges. A root cap is recognizable but very thin.The endophyte is a lobed organ with sinuous and irregular vascular strands lacking any phloem-like tissues. Strands, occasionally uniseriate, invade adjacent host tissues, usually in phloem-producing regions of the host cambium. These strands establish xylem-to-xylem contact, but are parallel to host xylem elements. They may subsequently become partially encapsuled by the latter. No radial sinkers exist.The nuclei of the endophyte show a more or less permanent type of heteropycnosis, the significance of which is unknown.A comparison of the parasitism of Lennoaceae and Orobanchaceae yields a number of significant differences in the structure and development of roots and haustoria.