22 ON CALIFORNIAN MOSSES. 



the North American continent. It is the adaptation of each species of moss to a peculiar 

 habitat. These humble and apparently useless beings have their geological and lithologi- 

 cal preferences far better marked than any other kind of vegetables. They select also to 

 live upon, different species of trees, different substances, the dung of certain animals, the 

 muddy deposits of a peculiar nature, the barren ground or the meadows inhabited by 

 special grasses, old fences, old roofs, old mud walls, &c. This explains, 1st. Why, ac- 

 cording to the tables of distribution, the mosses living on the ground, Gymnostomum, 

 Weisia, Barbula, Desmafodon, Trichostomicm, &c., are generally identical in Europe and on 

 both sides of the North American continent, varying only according to climatic influ- 

 ences ; and also why, when we come to species living on the bark of trees, or to pecu- 

 liar substances, the specific and even the generic differences are manifest. Thus we find 

 in Eastern North America, all the species of Drummondia, Plychomitrium, CrypTuxa, Leu- 

 codon, Leptodon, Clasmatodon, Thelia, Homalothecium, Pylaiscea, and in Western North 

 America, Alsia, exclusively arboreous and exclusively indigenous, most of these genera 

 having not even any specific representative in Europe. In other genera, like Orthotrichum, 

 Anamodon, Leskea, Cylindrothecium, Climacium, Hypnum, we find equally identity or 

 near affinity of forms for the species living on the ground, or on rotten, decomposed sub- 

 stances already nearly transformed into humus, while we have generally specific differences 

 for the truly arboreous species. 



There are indeed exceptional cases, apparently tending to weaken the prevalence of 

 this rule. But I think that they might be easily explained if we were well acquainted 

 with the true nature of the substances supporting mosses, and with the local influences 

 which may act sometimes on their distribution. To exemplify this assertion, we may ex- 

 amine a few of those apparent exceptions. All the species of the genus Sphagnum live in 

 bogs, and their existence is apparently depending on water only. We have, neverthe- 

 less, in Eastern North America, eight species different from those of Europe. But the 

 water of the bogs where these mosses* are generally living is more or less impregnated 

 with ulmic acid, resulting from the decomposition of wood, and there may be accord- 

 ingly some chemical influence acting on the distribution of the species. Moreover, most 

 of the exclusively American species of this genus belong to a warmer climate, or to a pa- 

 rallel of latitude where in Europe the destruction or the scarcity of the forests has prevented 

 the formation of bogs. We have only two indigenous species : Sphagnum cydopliyllum 

 Sul. & Lesq. and Sphagnum sedoides Brid., with a northern range of distribution. They 

 live either on the sand, or on granitic rocks, covered with water part of the year and en- 



* According to Professor Schimper, the genus Sphagnum does not belong to the mosses, but to a separate 

 family, the plants having a thallus like the Lichenes, and a conformation of their own. 



