jSTEW MYXOPHYCEAE from PORTO RICO 



69 



and the filaiiient becomes "double-ended," growing in both 

 directions. It seems that no heterocysts are formed intercalary 

 in the main trichome. Branching occurs by tlie death of one or 

 more cells after which one or both ends may push through the 

 sheath at that point, or only the lower end may form a branch, 

 and the upper hormogonium forms a heterocyst at its base. 



Calothrix evanescens sp. nov. 



Plate 13, figure 22 



P^'ilaments erect or prostrate, straight or curved, rigid, 200- 

 300 [1 long, 7-8 m diam. ; trichomes cylindrical from the basal 

 heterocyst throughout the larger portion of the length, then 

 tapering to a very narrow, short hair, constricted at the dissep- 

 iments, 5.5-6.8 |j diam. ; cells cylindrical to dolioform, quadrate 

 to one-third the diameter long, mostly homogeneous, pale aeru- 

 ginous to yellowish or honey-color, mostly showing a clear cen- 

 tral area ; cross-walls relatively thick and conspicuous ; apical 

 cell blunt and rounded, becoming conical in the young stage of 

 growth; the apical portion soon prolonged into a short hair; 

 heterocysts single, basal, spherical to subspherical, same diam- 

 eter as the trichome; sheath thin, firm, smooth, homogeneous, 

 hyaline, vanishing at about half way from the base to the apex. 



Growing in w^ater among other Myxophyceae and debris, 

 Jayuya, no. 1776 a, type; on a wall, Coamo Springs, no. 411. 



In certain stages in the life cycle of this species it resembles 

 the genus MicrocJiaete. The hair dies back, the apical portion 

 becomes dolioform, the terminal cell enlarges and becomes 

 rounded, and a sheath is secreted. The whole trichome is then 

 cylindrical with but a single basal heterocyst. AVhen growth 

 begins at the apex, the diameter begins to diminish, the apical 

 cell becomes conical, and finally a short hyaline hair is produced. 

 It is a very distinct species, related in some of its characters to 

 C. linearis. 



Calothrix Juliana tenuior var. nov. 



Filaments 700-900 m long, 12-14 m diam. at the base, tapering 

 gradually to a hair at the apex ; trichomes 9.5-11.5 n diam. at the 

 base ; cells very short, 1.2-1.8 (.i long. 



Growing on the roots of water plants in Arroyo de los Cor- 

 chos, no. 1696, type; on twigs in a stream, north of Maricao, no. 

 1291. 



