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Contributions to the knowledge of the Development of the 

 GoNiDiA q/LicHENS^ 111 7'elation to the Unicellular ALGiE^ 

 &c. By J. Braxton Htcks, M.D. Lond., F.L.S., &c. 



Fasciculus III. 



COLLEMA AND NoSTOC^ &C. &C. 



Before entering upon the subject I have proposed for con- 

 sideration in the present contribution, it will be needful to 

 remark that, in 1854', H.Itzigsohn,inthe 'Botanische Zeitung' 

 (" Wie verhalt sich Collema zu Nostoc und zu Nostochineen) " 

 page 521, and J. Sachs, in the same journal, in 1855, 5th 

 January ('^Zur Entwicklungsgeschichte des Collema, &c.'^), 

 insisted upon the origin of Nostoc from Collema ; and they 

 have pointed out one method by which Nostoc springs from 

 that lichen; namely, from a small ball of the jelly-like mucus, 

 enclosing a fev/ of the beaded cells, which becomes extruded 

 from the parent thallus. When one such ball becomes 

 free, it may take one of two modes of development : 1st, 

 it may produce the continuous colourless threads, and thus 

 pass into Collema (fig. 7) ; or 2nd, without any tendency to 

 the formation of these fibres, the ball will increase in size 

 and transparency, become less dense, while the green beaded 

 filaments increase by subdivision, and the heterocysts common 

 to the Nostochaceee arc found at intervals. This latter is 

 Nostoc, and this is one method by which it may arise. 

 In the same condition the development may continue for an 

 indefinite period. The first stages are shoMTi at PI. V, figs. 5 

 and 6. 



But there are other modes by which Nostoc may spring 

 from Collema ; I am not aware of their having been noticed 

 before, and they form the subject of these remarks. H. 

 Itzigsohn has gone further than any botanist in considering it 

 higldy probable that all the Nostochacese are derived from 

 lichen-gonidia. Be this as it may, there are many points 

 in his observations which are worthy of more careful con- 

 sideration and following out than they have liitherto received 

 in this country ; the more so, as some of his instances had 

 already been agreed upon by other excellent observers. How- 

 ever, I shall revert to this subject presently. 



The Collemas, like the other lichens, expel certain gonidia 

 from their surface, which can be recognised even -s^ithin the 

 thallus as larger and lighter green than the others. "When 

 they arrive on the exterior, they appear to undergo segmenta- 



