278 FRESH-WATER ALG^E OF THE UNITED STATES. 



Genus 93, NOSTOC, Vauch. 



Filaments nioniliforni, enclosed in a thin gelatinous envelope 

 more or less distinct, often invisible, flexuonsly curved, irregu- 

 larly interwoven ; cells globose or elliptic, more or less closely 

 connected. Heterocysts intercalated or more rarely terminal. 

 Cells about the size of the heterocysts or a little larger, which 

 occur in the older forms, have been considered spores. Clusters 

 of such filaments form distinct bodies, thalli, which are usually 

 surrounded by a firm membrane, variously colored, most fre- 

 quently olivaceous, but sometimes dark bluish green, dark 

 brown, or light yellow ; sometimes colorless. In form either 

 globose or subglobose, irregularly divided or often indefinitely 

 expanded. 



The value of Nostocs as algae has frequently been questioned ; 

 they have been associated with the lichens ; however true, that 

 in the life-history of some lichens a Nostoc-foTm may occur, there 

 can be no doubt that the usual forms of Nostocs belong to the 

 algae, but there may be a question, whether they are true, nor- 

 mally developed, plants. Many observations and studies lead 

 us to believe that they are merely intermediate life conditions, 

 arrested growths of algae life. It has already been shown ( Plate 

 CLXXXVII, figs. 1-15 ), that a Nostoc is the matrix of Scytonema; 

 also, how Nostocs are developed into ticytonema; compare Plate 

 CLXXXII, figs. 9-13, also fig. 8 ; Plate CLXXXVII, figs. 27-33, 

 and Plate CLXXXIV, figs. 34-40. 



Dr. Itzigsohn in his studies of the life-history of Hapalosiphon 

 found that Nostocs constitute a stage of development, Plate 

 CXCVI, figs. 16, 17. Where Scytottema prevails to any extent, 

 there Nostocs are sure to occur also. Sometimes one may greatly 

 predominate over the other, but they are usually consociated. 



Nostocs form an intermediate life-stage, an abortive existence 

 in which they grow and multiply, producing their like for a 

 number of generations, as do many of the unicellular forms of 

 algae, without developing the true plant. There appears to be a 

 struggle for existence in which various forms are produced ; 

 there may be thousands of them and only here and there one 

 which is destined through its fertility, to propagate the species ; 

 the others are abortive and soon fade away. Many of the 

 varieties of Nostoc- forms have given rise to a host of specific 

 names which have no value as fully developed plants. 



Nostocs constitute a condition of plant-life which occurs fre- 

 quently ; they are objects which attract the attention of ama- 

 teurs and older collectors, and must needs have a name to be 



