1919 J Setchell-Gardner : Myxophyceae 103 



26. Dichothrix Zanard 



Filaments aggregated into caespitose or cushion-shaped masses, 

 siibdichotomonsly and falsely branched; trichomes 1-6, rarely more, 

 within a sheath, tapering above, often ending in a long, hyaline hair ; 

 heterocyst basal or intercalary, rarely absent, single or seriate ; plants 

 marine or fresh water. 



Zanardini, Plant. Mar. Rubro., 1858, p. 297 (p. 89, Repr.). 



The type of the genus is Dichothrix pemcillata from the Red Sea, 

 but tlie genus now contains about a dozen species distributed through 

 both fresh and salt waters, one of which is devoid of heterocysts. It 

 has been our experience to find two seemingly good and endemic 

 species on our coast. 



Key to the Species. 



1. Heterocysts 2-6 seriate, trichomes 9-11/* thick 1. D. seriata (p 103) 



1. Heterocysts single, trichomes 4-5/u. thick (at base) 2. D. minima (p 104) 



1. Dichothrix seriata S. and G. 



Plate 6, fig. 20 



Filaments forming a caespitose stratum on rocks, 1-1.5 mm. high, 

 25-35/u, diam., erect; repeatedly more or less fasciculately pseudo- 

 branched, ultimate branches mostly strict, acuminate; sheath homo- 

 geneous, ample, hyaline below, yellowish brown above, strict, not 

 ocreate or only slightly so; trichomes aeruginous, 9-11/i, diam., almost 

 cylindrical, with acuminate apices; branches long, included in the 

 common sheath; cells 2.5-3.5/a long; heterocysts basal, 2-6 seriate, 

 diminishing in size downward, subspherical to disk-shaped. 



Growing on rocks interspersed with Rhodochorton Rothii, in 

 sheltered localities along high-tide level. Cape Flattery, Washington. 



Setchell and Gardner, in Gardner, New Pac. Coast Alg, III, 1918a, 

 p. 473, pi. 40, fig. 20. 



Of all the known forms of Dichothrix, D. seriata seems most closely 

 related to D. rupicola Collins (1901, p. 290), from which it differs in 

 having longer filaments, with greater diameter, which are more strict 

 at the apices, in having seriate, intercalary heterocysts as well as basal, 

 and in the sheaths not being ocreate. 



