Shrub or subshrub 0.3-1(1.5) m tall, erect to decumbent, branching at the base or usually 1-stemmed, with branches erect or divaricate-ascending, pseudo-dichotomous or lateral, numerous and often fastigiate, soon naked below. Stems dull red-brown, 4-lined and ancipitous when young, eventually terete, cortex exfoliating in strips; internodes 1-25 mm long, usually shorter than leaves. Leaves sessile, appressed to ascending; lamina 4-13(-18) x 0.8-3(-4) mm, narrowly elliptic-oblong to linear, plane, concolorous or almost so, not glaucous, subcoriaceous; apex acute to obtuse or sometimes rounded, base cuneate, not decurrent, free; basal or near- basal veins 5(3), midrib with 1-2 obscure pairs of branches, tertiary reticulum obscure or absent; laminar glands dense, visible above. Inflorescence l(2-7)-flowered, dichasial/ monochasial with lateral branches from up to 18 nodes below, the whole cylindric or fusiform; primary pedicels l.5-3(-6) mm long; bracts and bracteoles linear-subulate. Flowers 10-16 mm in diam., stellate; buds ovoid-ellipsoid to narrowly ovoid, acute. Sepals 3.5-5 x 0.8-1.5 mm, subequal, not or scarcely imbricate, linear-lanceolate to narrowly lanceolate, acute; veins 5(3), only midrib sometimes prominent; glands wholly linear or punctiform near apex and margin. Petals yellow, 5-12 x 2-4.5 mm, 1.7-2.5 x sepals, oblanceolate; apiculus acute; glands mostly linear, distally punctiform. Stamens 30-50, obscurely (3-?) 5-fascicled, longest 3-6 mm long, 0.5-0.6 x petals. Ovary 1.5-2.5 x 1-1.5 mm, narrowly ellipsoid; styles 3-5, 1.5-3 mm long, 1-1.2 x ovary, outcurving; stigmas broadly capitate. Capsule 5-6.5 x 2.5-3(-4) mm, narrowly ovoid-pyramidal to ovoid, exceeding or rarely equalling sepals. Seeds 0.6-0.7 mm long; testa ribbed- scalariform.
Wet meadows, marshes, and bogs (thickets in Rio Grande do Sul); 720-1200 m (lowland in Rio Grande do Sul?).
Brazil (Paraná to northern Rio Grande do Sul), Argentina (Misiones).
H. denudatum resembles 21. H. brasiliense in having a relatively narrow capsule that nearly always exceeds the sepals. Its branching, however, is sometimes pseudo- dichotomous, whereas that of H. brasiliense is always lateral, the usually shorter leaves are concolorous and sub-coriaceous, with only 3-5 basal veins that branch obscurely, if at all. The inflorescence is l(2-5)-flowered (not 8-30-flowered), the stamens are fewer, the capsule usually narrowly ovoid-pyramidal (not cylindric), and the seeds somewhat longer.
H. denudatum has been misunderstood since Reichardt treated it as a prostrate variety of H. rigidum, which (in turn) he had wrongly associated with H. cordiforme (= H. cordatum). He also confused it with H. linoides. In fact it is quite distinct from these species and is easily recognizable by its short narrow leaves with 3-5 nerves and its small flowers, usually solitary, with narrow sepals and narrow capsules. The most primitive form (erect with relatively few bran- ches, larger leaves and flowers, and 5 styles) occurs in Paraná and adjacent Santa Catarina. From this there are two reduction trends: (i) northward, where the branching becomes more diffuse, the habit decumbent (or prostrate), the leaves and flowers much smaller, and the styles gradually reduced to 3; (ii) southward, where the stem remains erect but the more numerous fastigiate branches give the plant a broom-like appearance. Here, too, the leaves and flowers are reduced in size and the styles are occasionally reduced to 4 or 3. Although the extreme forms of these morphoclines look very different, the occurrence of a series of intermediates linking both to the primitive form precludes the recognition of infraspecific taxa.