40 F. E. FRITSOH. 



59. NOSTOC LONGSTAFFI,* sp. n. 



(PI. Ill, figs. 132-137.) 



Thallo Isete serugineo, impellucido, primo adnato, demum libere natante, forma 

 disci tenuis ovalis vel circularis cum margine modice irregulari, non confluentis, 

 diametro usque ad 12 mm. ; trichomatibus dense intricatis, valde tortuosis, ssepe cum 

 vaginis distinctis amplis hyalinis margine incrassata et inter cellulas constrictis ; 

 cellulis parvis, plerumque globosis vel interdum ellipsoideis (deplanatis doliformibus 

 post divisionem), plerumque remotis, coutentu homogeneo Isete serugineo ; heterocystis 

 plus minusve globosis, solitariis, diametro trichomatis circiter duplo latioribus, mem- 

 brana incrassata et contentu homogeneo ; sporis globosis, membrana levi, confertis, 

 in catenis longis dispositis ; thallus peridermate tenui, paulo lamellato, et firmo (multo 

 firmiore quam in N. disciformi) circumdatus est ; thalli juvenales elongati sunt. 



Diam. cell. veg. = 3-4 /a ; diam. heterocyst. = 5-6 /a ; diam. spor. = 5 /a ; diam. 

 vaginse = 10-12 /a. 



Hah. — Gap pond, Winter Harbour ; freshwater pond in ice off Black Island, 

 McMurdo Strait, December 81st, 1902 ; dry ponds. Winter Quarters, February, 1902. 



This species has a great superficial resemblance to N. disciforme {pf. fig. 132), but 

 there are important diff"erences. The filaments are much more densely intertwined 

 than in that species (fig. 137), and as a result the whole thallus acquires a coarser, 

 opaque appearance, very different from that of the delicate transparent thalli of 

 N. disciforme {cf. figs. 137 and 131, which show the thalli of the two species, magnified 

 to the same extent). The envelope of the whole colony is also much coarser and firmer 

 than the gelatinous investment of iV". dificiforme. These differences are sufficient to 

 enable one to distinguish between the thalli of the two species with the naked eye. 



The trichomes are very similar in the two species, but in iV. longstaffi, a wide 

 mucilage-sheath is commonly found enveloping the individual trichomes (fig. 136). 

 This sheath is far more distinct than that occasionally found in N. disciforme; it 

 has a thickened outer edge and is often obviously constricted between the cells. The 

 spores are absolutely spherical (fig. 135) and often form long chains, which were 

 abundant on the surface of the Phor7nidiwn-&}ieets in several of the samples, being 

 quite free from the colonies. 



The young colonies appear to be quite different from those of N. disciforme. 

 Those which I have seen were elongated (figs. 133, 134) with a heterocyst at one or 

 both ends and the contained trichome was already closely wound. I have not been 

 able to follow their further development. 



The two species of Nostoc above described obviously belong to Bornet and Flahault's 

 section Cuticularia {see Ann. Sci. nat., Bot., 7 ser., VII. (1888), p. 183). N. disciforme 

 comes nearest to N. maculiforme Bornet et Flahault, but it differs from it in the 



* Named after Mr. Longstaff, whose generous contribution rendered the expedition possible. 



