Monographs Details: Hypheothrix acutissima N.L.Gardner
Authority: Gardner, Nathaniel L. 1927. New Myxophyceae from Porto Rico. Mem. New York Bot. Gard. 7: 1-144. pl. 1-23.
Description:Species Description - Filaments not branched, more or less flexuous; trichomes sparse, 1-5, and closely associated in the sheath, 7-7.5 µ diam., short, rigid, straight for the most part; apices sharply attenuated for 7-10 cells, moderately constricted at the dissepiments; cells quadrate or slightly longer or shorter than the diameter, homogeneous, pale aeruginous, cross-walls thin but conspicuous; sheath thin, membranaceous, hyaline and homogeneous, close fitting.

Distribution and Ecology - Growing on moss along the way north of Maricao, no. 1271 a, type.

Discussion:The material of this collection is sparse, but quite distinct. The specimens upon which this species of Hypheothrix is founded are not typical of the genus in every particular. The sheath is thin and close-fitting, and when only a single trichome appears within a sheath, a condition of common occurrence, there is no way to distinguish it from the genus Phormidium. When several are closely entwined within a sheath it resembles some species of the genus Microcoleus, although typically that genus has a more ample sheath and more trichomes are associated within it. The size of the trichomes and their paucity within a sheath relate the species to those of the genus Hydrocoleum as founded by Kuetzing, 7 and as emended by Gomont. The trichomes of Hydrocoleum are attenuated and capitate, as diagnosed by Gomont. In his original diagnosis of the genus Hypheothrix, Kuetzing (loc. cit., 229) does not mention the character of the apices of the trichomes. The presence of two or more trichomes within a sheath according to Kirchner led Gomont to reduce the genus to a subgenus of Schizothrix. But neither in his original description (Essai, 352. 1890) nor in his second publication of the subgenus10 did he mention the character of the apices of the trichomes. Forti, in De Toni (Syll. Alg. 5: 315. 1907), restores the subgenus to generic rank and emends it so as to include only those species with trichomes having straight, attenuated, and capitate .apices. Accepting this limitation (purely arbitrary) of the genus would exclude the material under discussion from Hypheothrix, on account of its very acute ajDices. The only real distinction between the two genera, Hypheotlirix and Hydrocoleum, as diagnosed by Forti (loc. cit.), seems to be that in the former the trichomes are not capitate and in the latter they are. The Porto Bican material conforms to the former in this, as well as in other characters, and I am placing it there as a very distinct species of the genus. The only species of the forty-three which Forti recognizes which approximates to it is Hypheotlirix pallida (Naeg.) Kuetz. It differs from that species in the absence of an ample lamellose sheath, as well as in other minor details.