8. The plants of sands, or of very barren soil; in the classification of which much difficulty is experienced: thus, plants of the sand of the sea-shore are confounded with saline plants; those of barren soil with the species of cultivated land, and those of coarse sand are not different from those of gravel.

9. Plants of sterile places that are very compact, as stiff clayey soil, or such as have their surface hardened by drought or heat, or those which are trodden hard by man or animals. This is an heterogeneous class, and contains plants of very uncertain characters.

10. Plants which follow man. These are few in number, and more fixed in their station, either in consequence of nitrous salts being necessary to their existence; or because, perhaps, azotized matter is required for their nutriment.

11. Forest plants, among which are to be distinguished, firstly, the trees that form the forest, and the herbs which grow beneath their shade. The latter are to be separated into two kinds, those which can support a considerable degree of shade during all the year, which are found in evergreen woods; or such as require light in the winter, like those which are found among deciduous trees.

12. Bushes and hedge plants. The shrubs which compose this division differ from the plants of the forest in their smaller size, and by the thinness of their leaves; the herbaceous kinds that grow among them are ordinarily climbing plants.

13. Subterranean plants, which live either in dark caverns as the byssus, or within the bosom of the earth, as the truffle. These can dispense altogether with light, and several cannot even endure it. Plants that grow in the hollows of old trees have great analogy with those of caverns.

14. Mountain plants, as subdivisions of which all the other stations may be taken. be taken. We generally class among mountain plants such as, in Europe, are not found lower than 500 yards; but this is quite an arbitrary limit. The most important division is between those which grow on mountains, the summit of which is covered with eternal snow, and those of mountains which lose their crest of snow in the summer. In

the former, the supply of water is not only continual, but more abundant and colder as the heats of summer advance; in the latter, on the contrary, the supply of water ceases when it becomes most requisite. The former are evidently much more robust than the latter.

15. Parasitical plants; that is to say, such as are either destitute of the power of pumping up their nourishment from the soil, or of elaborating it completely; or as cannot exist without absorbing the juices of other vegetables. These are found in all the preceding stations. They may be divided into, first, those which grow on the surface of others, as the Cuscuta and the Misletoe: and, secondly, intestinal parasites, which are developed in the interior of living plants, and pierce the epidermis to make their appearance outwardly, such as the Uredo and Æcidium.

16. Epiphytes, or false parasites, which grow upon either dead or living vegetables, without deriving any nourishment from them. This class, which has often been confounded with the preceding, has two distinctly characterised divisions. The first which approaches true parasites, comprehends cryptogamous plants, the germs of which, probably carried to their stations by the very act of vegetation, develope themselves at the period when the plant, or that part where they lie, begins to die, then feed upon the substance of the plant during its mortal throes, and fatten upon it after its decease; such are Nemasporas and many Sphærias: these are spurious intestinal parasites. The second comprehends those vegetables, whether cryptogamic, such as lichens and Musci, or phanerogamous, as Epidendrums, which live upon living plants, without deriving any nutriment from them, but absorbing moisture from the surrounding atmosphere; these are superficial false parasites: many of them will grow upon rocks, dead trees, or earth.

Thus we see that M. De Candolle has found it necessary to divide vegetation into sixteen stations. I do not attach much importance to several of them, because they are vague and uncertain of application, and frequently common to many plants; but it is, nevertheless, useful to bear in mind, that such distinctions do exist, and to point them out whenever

they take any very decided peculiarity of character. This is, indeed, indispensable, in order to enable us hereafter to form any definite appreciation of the nature of the influence of the combined agency of soil, temperature, and atmosphere.

The next, and by far the most important head under which the geographical distribution of plants is to be considered, is with reference to temperature and light. These depend, firstly, upon latitude; and, secondly, upon elevation above the sea.

As we proceed from the pole towards the equator, we find the temperature gradually increasing: and, as we ascend from the surface of the ocean up into the atmosphere, we find the temperature gradually decreasing, until we reach a point at which perpetual frost holds his throne, and where vegetation

ceases.

In like manner we find, as we recede from the equator to the pole, we quit the country of palms and other arborescent monocotyledonous plants, for the habitations of deciduous dicotyledonous trees, Coniferæ, and cryptogamic plants; and that as we rise into the atmosphere as considerable a change takes place. Thus, in Teneriffe, the foot of the mountain is occupied by Crithmum latifolium, succulent Euphorbias, Plocama pendula, and Prenanthes spinosa: to these succeed vines, corn, Canarina campanula, and Messerschmidia fruticosa: a third class, consisting of laurels, Ilex, Ardisias, heaths, and Viburnums, occupy the succeeding tract. These are surmounted by pines, Cytisus, and Spartium microphyllum; and, finally, the scenery is closed by Spartium nubigenum, Juniperus oxycedrus, Scrophularia, Viola, and Festuca. (See Humboldt's Travels.)

Therefore, in considering the matter of the vegetation of a given climate, it is necessary to take into account the temperature peculiar to the latitude itself, and the reduction caused by elevation.

The decrement of caloric, as we ascend into the air, will be understood by the following table, calculated by Mr. Daniell, from observations made by Mr. Green the aëronaut, in an aërial voyage performed in 1821. These are particularly

instructive; because they were all made within the space of half an hour, under circumstances which varied as little as possible.

The difference between the temperature of the highest elevation and the earth's surface amounting to 36° in the space of twentyseven minutes.

The amount of the decrement of heat, as compared with that of latitude, has been calculated to be, in France, equal to one degree of retrogressive latitude for every 540 feet of vertical elevation; that is to say, the temperature of a district of 3240 feet of elevation, in 45° N. lat., would be equal to the temperature of 51° N. lat. on a level with the sea. But, from Humboldt's computations, it appears that, nearer the equator, this proportion varies. He found, from careful and repeated observations, between 0 and 3000 feet of elevation, that in the middle of the temperate zone, the mean temperature of the year decreased in a degree equivalent to 2° of N. lat. for every 600 feet of elevation; the mean summer heat 1° 30′; the mean autumnal heat 1° 24′; or, on an average, the decrement of temperature was about 1° of latitude for every 396 feet of elevation. Temperature decreasing in this rapid ratio, it is evident that, if vegetation is affected by temperature, it will offer great differences in the ascent of a mountain. And accordingly it is found, as will be seen by the following tables, that the nature of the vegetation towards the upper limits at which plants grow, gradually changes from that of the base of the mountain, until plants entirely disappear at the limits of perpetual snow.