72 



MEMOIRS OF THE NEW YORK BOTAXICAL GAKDEJiT 



filament, gradually diminishing toward^the base v.-hcre it very 

 frequently vanishes while the apical portion is yet actively grow- 

 ing, 1(J-14 M 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, hya- 

 line 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. 



Growing on limestone between Arecilio and Utuado, no. 1482, 

 type, and 1481 ; on limestone between Hatillo and Arecibo, no. 

 1397 g ; at Hato Arriba, Arecibo, no. 1430. 



Scytonema evanescens seems from its structure to be a close 

 relative of -S'. crassum Xaegl. Comparison with the material dis- 

 tributed in Rabenhorst's Exsiccatae, no. 1843, the nearest to 

 authentic material I was able to examine, shows some important 

 ditferences. 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 in- 

 tense 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. 



Scytonema capitatum sp. nov. 



Plate 15, figure 27 



Filaments 22-26 m (up to 30 m) diam., more or less erect and 

 parallel, associated into small, closely crowded fascicles forming 

 a velvety stratmn 2-3 mm. thick, relatively straight and rigid ; 

 trichomes 18-22 \i diam., usually constricted at the dissepiments, 

 especially in the apical portions; cells purplish gray to slightly 

 olive-green, finely granular, one-half to one-fourth as long as 

 broad, ([uadrate in the oldest parts; heterocysts compressed- 

 spherical to cjuadrate; sheath thin, membranaceous, yellow, 

 homogeneous, smooth, decidedly capitate at the apices of the 

 filaments. IIkmi thickening up to 10 m : l)ranching sparse, single or 

 geminate. 



