GEOGRAPHICAL DISTRIBUTION OF PLANTS. 43 



ashes of Mtna,, and in the sand of Calabria. The plants of Aira, a calca- 

 reous mountain, grow equally well on the argillaceous rocks of the Vosges, 

 or the granitic Alps. But though the kinds of earths in which plants 

 grow naturally, seem of no great importance, yet the presence of metallic 

 oxides and salts, such as sulphate of iron or copper, or sulphur alone, or 

 alum, or other similar substances hi a state to be soluble hi water, are found 

 to be injurious to all plants, of which the maremmes of Tuscany, and some 

 parts of Derbyshire, are examples. As a general result of the facts which 

 have been collected relative to the influence of soil on the distribution of 

 plants, it may be stated that the chemistry and the geology of soils have 

 much less influence on plants than their temperature, moisture, and texture; 

 and that it is often a very bad method of culture to imitate exactly the soil 

 in which a plant is found growing. 



147. The influence of the atmosphere, considered with reference to its 

 chemical composition, and the gaseous matters which may be suspended hi 

 it, or its motion as wind on the distribution of plants, is not supposed to be 

 great ; or at all events, that influence is not yet so far understood as to be 

 reduced to any general law. Its difference of density at different elevations, 

 produces, as we have seen, a corresponding increase in the intensity of light, 

 and it is also found that humidity decreases as we ascend. This last result 

 must be attended with some effects on plants ; but as the ratio of the de- 

 crease of humidity has not been determined, its effects, separated from those 

 of temperature and light, are not sufficiently understood. De Candolle 

 remarks, that the rarification of the atmosphere by elevation may diminish 

 the quantity of oxygen for absorption by the leaves, and may at the same 

 time facilitate evaporation; but the precise result of these conditions is 

 unknown. 



148. The following are the principal stations of plants which require to 

 be known by the cultivator, and all of which he can imitate by art. 



1. Marine plants, which grow in or on the surface of the sea, and which, 

 though practicable, it has rarely been attempted to cultivate by art. 



2. Maritime districts, as the sea-shore, where the soil is more or less 

 impregnated with salt, which must be absorbed by the roots of plants, while 

 those parts which are above gronnd must be affected by the spray and sea- 

 breezes. Some are absolute sea-shore plants, such as salicornia, but others 

 grow equally well on the sea-shore and in inland situations, as the Eryngium 

 campestre and the common Thrift. 



3. Saline steppes, where the soil is impregnated with salt, but where the 

 foliage is not influenced by a saline atmosphere. 



4. Aquatic plants, or such as grow in fresh-water rivers and lakes, either 

 immersed and rooted in the soil forming the bottom on which the water 

 rests, or floating on the surface and sending down roots so as to touch the 

 soil ; in some eases scarcely doing so, as in Lemna. This kind of habitation 

 is imitated by artificial ponds or currents, or by basins in which the surface 

 of the water is kept in motion by jets or fountains. 



5. Marshes, bogs, and fens, easily imitated by suitable soil kept constantly 

 saturated with water. 



6. Meadows and pastures, the plants inhabiting which may generally be 

 cultivated in common soils and situations. 



7. Cultivated lands, of which the same may be said. 



8. Rocks, which are chiefly the habitations of cryptogamic plants, and 



