APPENDIX D. Ivii 



every form of Hemaiococcus {Gleocapsa, Kiitzing), and also 

 almost every form of Coccochloris {Palmoglcee, Kiitzing) as 

 well as that of Sorospora virescens — that is to say, from the 

 gonidia of a single Lichen there may be produced, by slight 

 variation of the ' conditions ' under which they grow, no less 

 than twenty-three forms, which have been hitherto regarded 

 as distinct species of fresh-water Algae. 



But even this is not all ; such algoid forms may have other 

 totally different modes of origin, to some of which we shall 

 afterwards allude (see pp. Ixiii-lxviii, Ixxiii, Ixxv, and Ixxxviii). 



Itzigsohn ^ and J. Sachs ^ had both insisted upon the fact 

 that another group of algoid forms, known as Nostoc, origi- 

 nates from a Lichen called Collema ; and their observations 

 have since been confirmed and extended by Dr. Braxton Hicks. 

 They had observed the development of Nostoc from changes 

 taking place in a small ball of jelly-like mucus enclosing 

 two or three beaded cells, which becomes extended from 

 the C^//(?w^-thallus. Such a jelly-ball may undergo one or 

 other of two developmental phases : first, it may produce 

 continuous colourless threads, and may thus again pass into 

 Collema ; or, secondly, it may develop no colourless fibres, 

 but the ball may grow in size and transparency, whilst the 

 green-beaded filaments within increase by subdivision, and 

 the heterocysts, common to the Nostochacecea, appear at 

 intervals. This second mode of development, according to 

 Itzigsohn and Sachs, may continue for an indefinite period. 



Dr. Hicks describes still another mode by which Nostoc 

 may originate from Collema. Within the thallus of this 

 Lichen certain gonidia appear, which are larger and of a 

 lighter green than the others. These, when liberated, undergo 

 segmentation for a time, so as to produce a small Chloro- 

 coccus-vci2JS>%. Soon, however, the products begin to take 



* ' Botan. Zeitung,' 1854, p. =21. ^ Loc. cit. 



