Evolution of the Collemacei. 371 



scarcely ever produce apothecia. They appear to be multiplied 

 by the Nostoc-like gemmules excreted from the upper surface 

 of the thallus, and not by any sexual propagation. And pro- 

 bably fertile thalli are only evolved from spores. 



We may therefore conclude that the genus Nostoc of modern 

 algology, in part at least, if not entirely, may be regarded as 

 the initial or metamorphic states of the Collemata. 



I have already added to the family of the Lichenes various 

 Scytonemata and Sirosiphones j and, indeed, certain Gloeocapsce 

 (such as Synalissa jpicina^ 8. meladermtaj Collema evilescenSj 

 and Bryophagus Gloeocapsa^ Nitschk., op time forte dicendus 

 Gloeocapsa hryopJiaga) may be added. 



The lichenose nature of these vegetations can only be mani- 

 fested by the fructification. I have formerly stated that the 

 various sterile states of them which are so abundantly met 

 with are analogous to the Leprarim of the family of the Li- 

 chenacei, and that LeprariiB never pass into typical Lichens 

 producing apothecia, but always remain leprose and sterile. 

 In like manner the Nostoc-like and analogous states of the 

 Scytonemata^ Sirosipliones^ and Gloaocapsce remain in their 

 peculiar inferior grade, atypical and sterile, without any evo- 

 lution into a more perfect and fertile condition. It may be 

 also suspected that not a few other vegetable forms at present 

 received into algology ought to be transferred to lichenography ; 

 but a fuller experience is needful to determine how far this is 

 true. 



On the sandy roads about Paris and elsewhere in France I 

 have observed that in a rainy season a small Nostoc often 

 quickly springs up, and in the selfsame localities Collema 

 pulposum in a sterile condition afterwards occurs ; and we 

 may hence probably infer that this derives its origin from the 

 Nostoc. But the evolution of this adult Lichen, as of all 

 others, is slow. 



It may also be noted that the age of Lichens may be best 

 determined in cemeteries, where they abundantly occur on 

 sepulchral stones bearing the dates of their erection, and upon 

 which, no doubt, the Lichens, everywhere abundant, speedily 

 disseminated themselves. Thus the specimens would coincide 

 in size with the varied ages of such stones. For example, 

 in the cemetery outside the city of Helsingfors, I have noticed 

 that it required a period of twenty years for a Lichen (a com- 

 mon Physcia) to attain to a moderate or adult size : consequently 

 their growth must be excessively slow. 



26* 



