876 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



of Collema are hardly distinguishable from Nostoc (Fig. 79, a), 

 though there does not seem any reason to believe, with some 

 authors, that the two are merely different stages of one and 

 the same plant, or that one passes or is transformed into the 

 other. If the views of Fries are to be adopted, Nostoc liche- 

 noides, and foliaceum, are merely Collema limosum and 

 fiaccidum, growing in a place so saturated with moisture 

 that no shields can be produced, and that these species, if 

 removed to a drier situation, perfect their fruit. He informs 

 us that after he had ascertained this rule, he was able to dis- 

 cover the fruit of many species, as Parmelia conoplea, lanu- 

 ginosa, gelida, &c., which he had never found in fructification 

 before, and the abnormal Leprarice of dark fissures in rocks, 

 &c., were referred to their proper origin.* 



412. There is no doubt that Lichens are normally aerial 

 plants ; but a few sjoecies, as Veri'ucaria submersa, are always 

 immersed, and some species of Endocavpon flourish when ex- 

 posed to the spray of torrents, by which they are occasionally 

 submerged. Were there no distinct mode of fructification in 

 Nostoc, we might readily allow its identity with Collema, or 

 at least there would be nothing to excite much reluctance in 

 admitting it ; but the mode of reproduction in Nostoc is well 

 ascertained (p. 139), and the species are so closely connected 

 with Anahaina and other genera, as to make the matter at 

 least highly improbable. Meanwhile it is quite certain that 

 many Lichens, in their origin, appear under the form of minute 

 Algge, and that several so-called species of Hcematococcus, &c., 

 are merely early stages of the development of Lichens, which 

 may in their infancy present either a filamentous or furfura- 

 ceous mass. Facts like these, when well ascertained, illustrate 

 the true nature of organisms and their hidden affinities ; but 

 it is only when they are superficially examined, that they lend 

 any support to the doctrines of equivocal generation. Meyer 



* The rule may hold good in many cases as to the effect of variation 

 of locality, on siich nearly allied organisms ; but it does not follow, that 

 because the early stage of a Collema may resemble a Nostoc, every 

 Nostoc is an early stage of a Collema. 



