278 



POPULAR HISTORY OF LICHENS. 



polyhedral cellules intimately united ; in others there is no 

 distinct epidermic layer, the plant consisting wholly of the 

 filaments, tubes, and green granules above mentioned. In 

 the latter case, the structure of the thallus so closely resem- 

 bles that of the common Nostoc } that the Collemas and Nos- 

 toc have frequently been classed together, either among the 

 Algse or Lichens, according to different views on their mor- 

 phology."* But Nostoc consists wholly of moniliform fila- 

 ments embedded in a mucilaginous basis, never possessing 

 the continuous tubes described as occurring in the Collemas. 

 In rudimentary Collemas the individual cells or articulations 

 of the moniliform filaments develope tubes which ramify and 

 become interlaced, as in the hypothallus of other Lichens. 

 The thallus varies considerably in ordinary physical charac- 

 ters. In some species it is very thin, and when moistened 

 becomes pellucid or somewhat diaphanous, having a uniform 

 and beautiful dark-green colour; in this case the thallus 



* For observations on the relation of Collema to Nostoc and on the deve- 

 lopment and minute anatomy of various species of Collema, vide ' Botanische 

 Zeitung/ July 24, 1854 ; Julius Sachs, January 5, 1855 : also Bernhardi, 6 11- 

 lustratio Lichenum Gelatinosorum/ in Schrader's ' Journal fur die Botanik,' 

 vol. i. 1799 : Cassini and Duchesne, 'Bulletin de la Soc. Philomath,' 1817, 

 or 'Journal de Physique/ 1817: Fee, 'Mem. de la Soc. du Mus. d'Hist. 

 Nat. de Strasbourg/ vol. ii. 1835. 



