24 ON CALIFORNIAN MOSSES. 



cese only, considered as a variety of Orthotrichum Lycllii, seems to have an exceptional 

 distribution, inhabiting as it does the bark of trees of the temperate zone. This would 

 perhaps prove that this variety, as Hampe will have it, is a true American species. The 

 name of 0. papillosum Hmp. is well adapted to it. 



It would appear that there is in the composition of some stones, peculiar substances 

 which may affect the distribution of the mosses. Two Eastern North American species of 

 Desmatodon live, one exclusively on conglomerate sandstone, the other on old bricks. Our 

 indigenous species of small Fissidens are found also near the water on sandstone, either 

 rolled in the creeks or cut for constructions, except Fissidens sitbbasilaris Hedw., a true 

 arboreous species. 



It is perhaps to this predilection of some mosses for a peculiar kind of stone, that we 

 owe in our temperate climate the presence of Eustichium Norvegicum Bryol. Eur., inhabit- 

 ing the conglomerate sandstone of Southern Ohio and Southern Kentucky, and considered 

 by European bryologists as an exclusively northern species. 



We might perhaps look for a third cause influencing the geographical distribution of 

 the mosses, in the peculiar tendency of nature of associating in the same region similar 

 forms, nearly allied species or may be varieties. Whatever might be the manner of ex- 

 plaining this grouping, it exists for the mosses far more evidently than for the phcenoga- 

 mous plants. Thus we have here, three species of Cryphaxt, two of Leucodon, three of 

 Leptodon, seven of Anomodon, three of Thelia, three of Pylaiscea, six of Cylindrothecium , 

 and a great number of Hypnum, &c., whose specific characters are sometimes blended 

 together in a way truly perplexing for the examiner. Even these specific differences 

 can be sometimes followed on plants living in close proximity, and their cause apparently 

 traced to natural variations of habitat. The Thelia: live generally attached to the smooth 

 bark at the base of both Carpinus Americana Mich, and Oatrya Virginica Willd. Now 

 Thelia liirtella Sul., with simple papillae on the back of the leaves, is found higher on the 

 trunk. When it descends on the uncovered roots and becomes more exposed to the sun, 

 the color is grayish, the papillae are bilobed at the apex, there is a difference in the peri- 

 stome, and we have indeed another species, Thelia asprella Sul. Southward, in Kentucky 

 and Tennessee, this Thelia asprella is seen sometimes attached to small roots plunging in 

 the sand, from the roots, the moss itself passes to the sand, becomes fixed to the ground, 

 is now different in some characters, areolation and peristome, and a third species, indeed, 

 Thelia Lescurii /SuL, apparently due to the influence of another habitat. Such impercepti- 

 ble changes of forms corresponding with progressive variations, might be followed in the 

 same manner, in other genera, especially in the species of the genus Cylindrothecium. 

 But this examination would be now of small value ; for without long and experimental 

 researches, our belief in the influence of external circumstances of habitat on the form, 

 can be but merely conjectural. 



