Etymology: Hypo-; refers to Hypoderma, the genus from which H. scirpinum has been segregated. -helion = swamp or marsh dwelling; refers to the habitat of the species included in this genus.

Johnston (1988a, 1988b, 1989, 1990) discussed the unsatisfactory nature of currently accepted generic limits in the Rhytismataceae, and showed that, at least for nonconiferous species, an alternative is possible. Rather than using single characters chosen a priori as important, genera may be based on sets of several characters, which in this case are mostly associated with ascomatal structure and development.

The characters used to define Hypohelion and to distinguish it from other genera in the Rhytismataceae are:
1. Subcuticular ascomata.
2. The upper wall of unopened ascomata is homogeneous in structure, comprising dark brown, thick-walled, angular cells.
3. In opened ascomata the upper wall is more or less uniform in thickness.
4. The edge of the ascomatal opening is not lined with differentiated cells.
5. The ascomata lack a darkened lower wall.
6. The paraphyses are swollen at the apex.

Hypoderma, in the sense accepted by Johnston (1990), possesses only the first of the characters listed above, and the upper wall of the ascomata has thickened labia and a characteristic internal differentiation. Powell (1974) considered that vestiges of thickened labia, in the form of a medial keel, could be seen in the upper wall of the ascomata in many collections of H. scirpinum. My observations showed that although a keel-like ridge is often seen on unopened ascomata, this is due to a folding, rather than a thickening, of the ascomatal wall. It cannot be compared with the thickened labia characteristic of Hypoderma rubi and other species of Hypoderma.

Hypohelion scirpinum and H. parvum are morphologically and anatomically similar, and are also similar with respect to host preference. Both are found on Cyperaceae growing in swampy habitats. The same kind of relationship between ascomatal structure and host substrate was demonstrated for a group of Lophodermium, Lophomerum, and Meloderma spp. found on the closely related families Epacridaceae, Ericaceae, and Pyrolaceae (Johnston 1988b). This correlation between morphological and biological characters suggests that the character set used in this paper, and by Johnston (1990) for Hypoderma, may define rhytismataceous genera in a way which reflects natural relationships amongst these fungi.

Ascomata subcuticular. In vertical section upper wall of unopened ascomata up to 20 µm thick, comprising brown to dark brown, thick-walled, angular cells. In opened ascomata upper wall 15-20 µm thick, becoming gradually narrower toward outer edge, comprising dark brown, thick-walled, angular cells. No differentiated layer associated with edge of the ascomatal opening. Darkened lower wall absent.

Paraphyses 1.5-2 µm diam., swelling up to 5 µm diam. at the clavate apex, extending 5-10 µm beyond asci. Asci 65-90 x 9-10 µm, clavate, tapering gradually to rounded apex, wall undifferentiated at apex, 8 spored, spores extending almost to base, maturation sequential. Ascospores 23-32 x 3-3.5 µm apex rounded, tapering to more or less acute base, 1 septate, surrounded by narrow gelatinous sheath.

In surface view ascomata 0.8-3(-7) x 0.5-0.8 mm, elliptic to broad-elliptic in outline, with rounded ends. In unopened ascomata the wall is uniformly black, usually developing a fold-like ridge along the long axis of the ascomata. Ascomata opening by an elongate split which usually forms more or less along the fold-like ridge. No differentiated zone visible along the edge of the opening split. Parts of the upper wall adjacent to the opening split often break off and fall away, leaving the hymenium partially exposed.

Ascomata develop between the host cuticle and epidermis, although when ascomata develop over stomata in the host leaf, hyphae invade and break down a few host cells immediately below the stomata. In vertical section, at an early stage of development, the ascomatal initial comprises 2-3 layers of hyaline plectenchyma, covered with a single layer of brown, thick-walled, angular cells, which comprises the first stage in the development of the upper wall (Fig. 2A). As the ascomata develop the upper wall thickens to 20-30 µm, comprising thick-walled, brown, angular cells, and a cavity develops within the hyaline plectenchyma, with paraphyses growing upward from the base of this cavity (Fig. 2B). From this stage the upper wall does not become thicker, but it darkens and becomes distorted and folded by the pressure exerted from the developing hymenium. The upper wall shows no internal differentiation, comprising uniformly thickened and darkened, angular cells, but shows a distinct upward fold near the centre of the ascomata (Fig. 2C). The upper wall splits open in the region of this fold (Fig. 2D). No layer of differentiated cells is present along the edge of the opening split. A darkened lower wall does not develop, the hyaline tissue below the hymenium resting directly on the more or less intact epidermal cells of the host.

Paraphyses 1.5 µm diam., swelling more or less abruptly to 4-6 µm diam. at the clavate to knob-like apex. Asci 130-160 x 15-18 µm, clavate, tapering gradually to rounded apex, wall at apex sometimes slightly thickened with indistinct central pore, 8 spored, spores extending two-thirds down the ascus, maturation sequential. Ascospores 40-75 x 4.5-6.5 µm, tapering slightly to both ends, 0-1 septate, surrounded by narrow gelatinous sheath.

Conidiomata 0.2-0.3 mm diam., round in outline, dark brown to black, pustulate, subcuticular. In vertical section lenticular in shape, upper wall 3-5 µm thick, comprising dark brown material without obvious cellular structure. Darkened lower wall absent, the conidiogenous layer developing on 2-3 rows of thin-walled, hyahne, angular cells. Conidiogenous cells 9-15 x 2-3 µm, cylindric, solitary, forming a palisade-like layer, with sympodial proliferation, often with more than one developing conidium held at the apex. Conidia 3-4.5 x 1 µm, oblong-elliptic with rounded ends, or short-clavate with a broadly rounded apex and tapering to a truncate base, hyahne, 0 septate.