140 LAKE SUPERIOR. 



To satisfy ourselves of the powerful influence of electricity upon 

 vegetation, we need only remember the increased rapidity with which 

 plants come forth, during spring, after thunder storms. 



Many other causes still more intimately connected with the aspect 

 of our globe have also a great influence upon the distribution of the 

 animals and plants which live on its surface. The form of continents, 

 the bearing of their shores, the direction and height of mountains, 

 the mean level of great plains, the amount of water circumscribed 

 by land and forming inland lakes or seas, each shows a marked influ- 

 ence upon the general features of vegetation. Small low islands, 

 scattered in clusters, are covered with a vegetation entirely different 

 from that of extensive plains, under the same latitudes. .The bearing 

 of the shores again, modifying the currents of the sea, will also react 

 upon vegetation. Mountain chains will be influential not only from 

 the height of their slopes and summits, but also from their action 



togames. The attempts made to group the former according to the nature of the soil 

 upon which they grow, have afforded no satisfactory results. It is otherwise when we 

 consider the hydrodynamic capacity of the soil, that is to say, the property which it 

 has to retain the water for a longer or shorter time. Tracing our investigations in 

 this direction we arrive, on the contrary, at very important conclusions. A sandy 

 desert and a peat-bog for instance, as the two extremes, have quite peculiar floriE, 

 which stand completely isolated from the vegetation of soils whose essential component 

 material is humus. This fact is in perfect accordance with recent discoveries in vege- 

 table physiology, which seem to prove that plants extract nothing from the soil except 

 water, or nourishment in a liquid state, and that their other components, the carbon 

 in particular, are furnished them from the atmosphere. 



As we descend the scale, and arrive at the cryptogames, the chemical influence 

 of the soil is gradually more and more felt in the distribution of the genera, and 

 even of the species. The mosses even may be readily grouped according to the locali- 

 ties where they live. The Orthotrichoe occur almost exclusively upon the bark of trees, 

 and upon granite and limestone ; the Phascaceaj inhabit clayey soils, with the Gym- 

 nostomeae, Pottieae, Funarieae and some Weissiee. The Sphagneae occur only in peat- 

 bogs, or in waters charged with ulmic acid ; the Splachneae generally upon animal sub- 

 stances in decomposition; the Grimmiere upon granitic rocks ; whilst the greatest num- 

 ber of the Hypnums and Dicranums cover large surfaces of rotten vegetables. And 

 if we take into consideration the modifications which temperature introduces in the 

 habitation of some mosses, we are enabled to account even for the cosmopolitism of 

 some species which, like the Bryums, would seem to be less influenced than others by 

 the nature of the soil upon which they grow. 



The examination of the lichens which attach themselves commonly to the surface of 

 ■woods and rocks leads to conclusions still more striking. Some species live exclusively 

 upon limestone ; others upon mica schist ; others upon various kinds of granite ; and 

 others finally upon certain species of trees or other vegetables. The analysis of the 

 substances upon which lichens live, has, if not completely explained, at least led to 

 the understanding of the causes of the remarkable distribution of these plants. 



