The structure and development of the haustorium in parasitic Scrophulariaceae*

Abstract

The structure and development of roots and haustoria in 37 species of parasitic Scrophulariaceae was studied using light microscopy. The mature haustorium consists of two regions: the swollen “body” and the parent root, which resembles non‐haustorial roots in structure. The body arises from the parent root and is composed of an epidermis, cortex, central region of xylem (the vascular core), a region of parenchyma (the central parenchymatous core), and the portion of the haustorium contained in the host tissue (the endophyte). The xylem of the vascular core is composed predominately of vessel elements. The central parenchymatous core is composed of parenchyma and col‐lenchyma. Vessels extend from the vascular core through the central parenchymatous core to the endophyte. The endophyte is composed of parenchyma cells and vessel elements. No phloem is present in the body of the haustorium. Early stages in the development of the haustorium are exogenous. Initial periclinal divisions in the epidermis or outer cortex are followed by hypertrophy of cortical parenchyma. These events are followed by development of the vascular core from the pericycle, attachment of haustorium to the host by a specialized layer of cementing cells or root hairs, and penetration of the host by dissolution of host cells.

Anatomical confirmation of root parasitism in Brazilian Agalinis Raf. species (Scrophulariaceae)

“…In some species they were produced close together in a bead-like fashion, such as A. genistifolia (figure 3) and A. nana. The general morphology of these nodules was very similar to the haustoria morphology found in the North-American species of Agalinis studied by Musselmann & Dickison (1975). The authors referred to the mature haustorium as a bell-shaped to globose structure, although this shape might be altered when sandwiched between host roots and other obstacles.…”

“…Once the nodules were really haustoria, we adopted a specific nomenclature to describe their morphology and anatomy. The terminology used here was based mainly on Musselmann & Dickison (1975). We could distinguished two general regions of the Brazilian Agalinis haustorial structure: the swollen portion which kept fixed to the host root and a segment of root that gave rise to this swollen portion (figure 2).…”

“…Cells with the same features also appear in Agalinis linifolia (Nutt.) Britton haustoria (Musselmann & Dickison, 1975). Recently they were also mentioned and discussed in studies about Struthanthus vulgaris Mart.…”

“…Approximately at the centre of the haustorium there existed a vascular region provided with tracheary elements, interspersed or not with parenchyma cells (figures 6, 8 and 9). This region is called vascular core by Musselmann & Dickison (1975). Surrounding the vascular core there are cells with a meristematic aspect resembling a vascular cambium (figure 8 and 9), which could become continuous with the parent root cambium.…”

“…Most anatomical studies on parasitic Scrophulariaceae during the first part of the twentieth century were tangential to taxonomic or ecological studies. Between 1930 and 1950 few papers on parasitic Scrophulariaceae appeared and since 1960 there has been a resurgence of interest in these plants (Musselmann & Dickison, 1975). Malcolm (1966) reported that the majority of chlorophyllous parasites belonged to the Scrophulariaceae and Santalaceae.…”