Monographs Details: Scytonema evanescens N.L.Gardner
Authority: Gardner, Nathaniel L. 1927. New Myxophyceae from Porto Rico. Mem. New York Bot. Gard. 7: 1-144. pl. 1-23.
Family:Scytonemataceae
Description:Species Description - Filaments 450-650 µ long, adhering by their gelatinous sheaths into a dense stratum, mostly erect, considerably contorted and more or less parallel, 25-32 µ (up to 40 µ in the apical region) diam.; trichomes clavate, the widest part at the apex of filament, gradually diminishing toward* the base where it very frequently vanishes while the apical portion is yet actively growing, 10-14 µ diam. at the apex, more'or less deeply constricted at the dissepiments, at times moniliform; cells one-half to one-third the diameter long at the apex of the trichome, increasing in length to 2-3 times as long at the base, homogeneous to finely granular, bright olive-green; heterocysts sparse, subspherical to cylindrical; sheath thick, faintly lamellose and ocreate, hyaline at the apex and uniformly yellowish in the lower parts of the filament, ragged along the margins; branching sparse, mostly single under the heterocysts and in the upper parts of the filament.

Distribution and Ecology - Growing on limestone between Arecibo and Utuado, no. 1482, type, and 1481; on limestone between Hatillo and Arecibo, no. 1397 g; at Hato Arriba, Arecibo, no. 1430

Discussion:

PLATE 15, FIGURE 26

Scytonema evanescens seems from its structure to be a close relative of S. crassum Naegl. Comparison with the material distributed in Rabenhorst's Exsiccatae, no. 1843, the nearest to authentic material I was able to examine, shows some important differences. The sheath in S. evanescens is much more uneven and ragged and lacks the clearly defined lamellose and ocreate sheath characteristic of S. crassum. The color of the sheath in S. evanescens is uniformly distributed, while that of S. crassum appears first in the parts nearest the trichome, and is less intense in successive layers toward the surface, where it is hyaline. The branching is mostly single and under the heterocyst, but occasionally it is geminate in S. evanescens.