428 joii.s. mm. I'itkhsen 



soliilion tor o mimites pyrenouls tniild hi- plainly sccn (cf (iciller 

 1922;. Wliile in M. f/lnhosa all cclls show a thickeninj^ of llie wall, this 

 is not tlic case iii M. pi/ ri/orm is . tlio yoiinj^ cclls, which arc splicrical 

 or oval. liavinj^ ralhcr Ihin walls unirorm oii all sides. Il is oiily in the 

 oldcr cells. wliicli I have rcpeatedly ohserved attached hy llieir broad 

 end to the wood Fi}». 34a, thai a thickcninj^ of the wall appears at that 

 end of the ccll which is tiirned away from the substratiini. Meproduc- 

 lion sceins to lake placc in the same way as in M. {^lobnsa, First the 

 cell conlents divide in two, ihen in tour (Fig. 34 i\ elc, i. e. successively 

 in a {^realcr nnmbcr of si)ores: whether thesc arc zoospores or aplano- 

 spores it has been impossibic for mc lo dccide. The basal. Ihin part 

 of the wall of the molher cell biirsts whereby the spores are liijcraled. 

 The chief characters by which M. py ri form is ditFers from M. ylobosa arc, 

 then, brielly Ihe following: 



1. Young cclls show no Ihickening of the cell wall. 



2. Pyrenoids are present. 



3. The cclls are attached. 



4. The cell lumen is pear-shaped, not sphcrical. 



Il f<»rmed a green laycr on a horizontal beam near the ground on 

 a farm in E. Iceland. 



Trebouxia arboricola Puymaly. Puymaly 1924, p. 200. 

 Cystococcus luimicola Boye Petersen 1915. p. 327, Tavle II. fig. 26. 



- — Tréboux 1912, p. 69, 



Chlorococcum sociabile Brand 1925, p. 331. 



N. Icel. 261. 



Il is only in a single sample from woodwork in IsafjonVir thai I 

 lliink I have scen this species which is extremely widespread in Furope 

 on llic trunks and young branches of trees, as well as on thatched 

 roofs and olher subsirata. Lichcns containing this species being very 

 common in Iceland. il mij^ht have been e\|)ccted thai the algæ would 

 be found very fre(iuenlly Iree living. I have. howcvcr, soughl it in vain 

 in localilics corrcsponding lo Ihosc in which I have oflen found il in 

 Denmark, viz. especially tree-trunks and the lenliccls of young branches. 

 Thus I have collecled samples of bark and branches of Beluld from the 

 following localilics: F. Iceland; l-:gilslat^ir, Hallormstatiir — N. Iceland: 

 Hals —W. Iceland: XonMunga. but in none of these placcs have I been 

 able to show wilh certainly the presencc of frec living Trebouxia arbori- 

 rolii. Tréboux found 1. c. thai in Southern Russia the green coatings 

 »tf algæ on Iree-lrunks and woodwork consisl predominantly of this 

 species, while Pleurocorcus rulf/aris Nag. (= P. Xåf/elii Chod. is predomi- 

 nant in Nc.rthern Bussia. Il would seem. then, thai lifc condilions for 

 this pi;int are (|nile diMcrenl in Iceland and .Southern Bussia. and the 

 ellecl menlioned can only be supposed to be i)rodiiccd by difrercnces 

 in climatc, while Denmark seems to occupy an intcrmediale position. 

 In Denmark Trebonxid is not uncommon on tree-trunks, bul il is far 

 In.m predominant in the vegetalion. and according t<i P u y ni a 1 y similar 

 condilions seem lo prevail in Francc. 



