BOOK XX. Continuation of the Theory of Geography. Of the Earth, considered as the residence of organic beings. SECTION I. BOOK XX. OF THE GEOGRAPHICAL DISTRIBUTION OF VEGETABLES. We have decomposed the terrestrial globe into its solid, liquid, and aeriform parts. Let us now proceed to the consideration of those innumerable beings which exhibit the spectacle of life upon every point of the globe; which embellish its surface, which feed upon its inexhaustible stores of nutricious juices, and which, by one common destiny, find in it a thousand different graves. These productions, and these inhabitants of the earth, are not scattered over it by the hand of blind chance; general laws have assigned to each class of these organic beings its cradle and its grave; and these laws it becomes us to study before we commence the description of the different parts of the world. Vegetables, from the abundance in which they are produced, and from their intimate connection with the soil, claim the first rank. It is for the botanist to examine in detail the treasures of the vegetable kingdom; the business of the physical geographer is only to mark its general arrangements; and here he finds abundant reason to admire that wisdom which presides over the constitution of the globe.* * Humboldt, Essai sur la Géographie des Plantes, Stromayer, Specimen Geographiæ botanicæ. Gottinge, 1804. Bergmann, Geograph. Phys. Sect. vi. upon vege The temperature of the air appears to form the only phy- BOOK sical limits to the extension of vegetable nature. The scale XX. of atmospherical heat serves accordingly as the ordinary Infiuence scale for the progress of vegetation. Hence, under the of tempeburning climate of the torrid zone, we have only to ascend rature the mountains to enjoy the fruits and flowers of the tempe- tables. rate regions. Tournefort found, at the base of Mount Ararat, the common vegetables of Armenia; half way up, those of Italy and France; and, upon the summit, those of Scandinavia. Forster saw several alpine plants upon the mountains of Terra del Fuego.* If the valleys of the Andes are adorned with bananas and palm trees, the moro elevated regions of that chain support oaks, firs, barberries, and a number of kinds common to the north of Europe.† Man, availing him of this circumstance, has transported and disseminated almost over the whole surface of the globe, those herbaceous plants which supply him with his principal nourishment. Some useful plants have been rendered common to every climate by nature herself. Antiscorbutic vegetables, so salutary for the mariner, when languishing from the long use of salt provisions, are met with wherever there is a vestige of life. Cresses, succory, and wild sorrel, are found upon the ever-frozen banks of Hudson's Bay, and in Siberia, as well as in those blissful islands which are scattered in the midst of the Pacific ocean. The shrubs which produce berries, and small fruits agreeable to the taste, thrive in the more inhabited countries. Even in Greenland, the currant bushes bear very good fruit. Lapland possesses a considerable resource in its shrubs, such as the barberry, the dwarf mulberry tree, the wild wood vine, (vitis idæca) and others. Nei- To what ther external cold, nor the absence of the light, entirely point plants sup extinguishes vegetable life. Caverns and mines give birth port cold. to a certain number of plants, particularly to those of the *Bemerkungen, p. 154. (in German.) + Humboldt, Essai sur la Géog. des Plantes, p. 34. XX. BOOK cryptogameous class. Several of the saxifrage and ranunculus, the dwarf willows, as well as all the lichens, like the cold. The snow, far from impeding the vital functions of these vegetables, secures them against the effect of frosts, and furnishes them in abundance with the oxygen which it contains, and which, by increasing the vigour, accelerates the germination of the seeds.† Plants in hot waters. Ramond has proved, that plants covered by the snow for several years, have yet continued to live beneath it. The organization of alpine or polar plants, admits of a growth and development so rapid, that a few warm days are sufficient to fructify them. Perhaps even perpetual snow may be the abode of a species of vegetation; at least Saussure has discovered in it a kind of reddish dust, very probably of a vegetable nature. Patrin, and Sokolof, saw in Daouria, ground covered with vegetables, though entirely surrounded by perpetual snow.|| Extreme heat checks still less the productive energies of which grow nature in the vegetable kingdom, provided that it be attended with humidity. We see plants grow not only upon the borders of hot springs, but even in the bosom of those waters which seem likely to destroy them. Examples of this kind are to be found from Iceland to the Cape of Good Hope, and from Kamtschatka to the island of Amboyna.¶ The sulphurous exhalations, and the foul air of volcanic caverns, seem to exert upon vegetation only a slow and limited influence, while to animals they prove instantly fatal.** * Scopoli, Diss. ad Scient. Natur. part i. p. 84-120. Humboldt, Flore Friberg. subterr. &c. + Hassenfratz, Journal de l'Ecole Polytechnique, 4e cahier, an. IV. p. 570576. amond, Observat. p. 51. Martens, Voyage to Spitzbergen, p. 54, (in German.) Linnæi, Flor. Lapp. præf. pl. xx. || Patrin, Voyage, p. 19. Olafsen et Povelsen, Travels in Iceland, (in German) ii. p. 31, and 181. Krascheninikow, Kamtschatka, (in German) p. 91. Sparman, Voyage, (in German,) p. 142. Labillardiere, tom. i. p. 324. **Soulavie, Works of Hamilton, p. 246. Smith's Tour, &c. ii. p. 103. XX. settle on such a Moisture necessary to vegeta bles. Not a seed can to ve The most formidable obstacles to vegetation is the ab- BOOK sence of humidity. Look to those sandy deserts under the equator, as well as towards the Pole, condemned to perpetual sterility. Not a drop of rain can loose soil, always moving with the winds: strike root into it. It is unquestionably to milar that we must ascribe the nakedness of several mountains; precipitous sides, or flat summits, afford no shelter to the vegetable colonies which the winds transport to them, whilst on other mountains under a colder temperature several plants are still supported. For example, the shivering mountain in Derbyshire produces no herbs, because its sides are daily decomposed in their schistous plates, which are constantly gliding down to the bottom.* The pressure of the atmosphere exercises à striking in- Pressure of fluence upon the configuration and life of plants. In vegetables the functions essential to life are performed chiefly sphere. at the surface; hence their great dependence on the medium by which they are surrounded. Animals are affected rather by internal excitement, and acquire of themselves the temperature which suits them. Respiration by the epidermis is the most important vital function of plants; and this function, in so far as it is subservient to the evaporation and secretion of fluids, depends on the pressure of the atmosphere; on this account the Alpine plants are so aromatic, so downy, and so plentifully furnished with secretory vessels. On the contrary, these plants grow with difficulty in the plains, where their respiration by the epidermis is impeded by the increased pressure of the air.t the atmo The chemical nature of the soil manifests its influence Chemical upon vegetables, rather by modifying their juices, their nature of fruits, and the stateliness of their appearance, than by set- the soil. ting limits to their cultivation. Common salt, however, dissolved and scattered over the soil in considerable quan * Kuttner, Beytræge, &c. that is, Mém. sur l'Angleterre, cah. vii. p. 20. + Humboldt, Tableaux de la Nature, ii. p. 115. n. 14. VOL. I. 62 XX. BOOK tities, almost entirely prevents the growth of vegetables,* The fusion which lava undergoes, is probably the only cause which, for some centuries has retarded the progress of vegetation on its surface, whilst the volcanic cinders raise most abundant crops.† In general, the soil serves only as a support and shelter to plants; they derive their nourishment from the water and the oily fluids which are collected together in the earth, and imbibed by their roots. A very small quantity of earth, dissolved in these fluids, is absorbed by the plant. Other causes also contribute to the support of vegetable life. Plants respire through their airvessels the different fluids of the atmosphere; the presence of light especially is indispensable to the chemical operation, by which the pabulum or food of the plant is assimilated into its substance. The elementary earths obtained from plants by chemical analysis, appear rather to be the product and remains of the digestion by which the vegetable assimilates its food, than particles immediately derived from the surrounding soil. The experiments of M. Schrader have shown, that the plants which vegetate in sublimed sulphur, give, when analyzed, the same earth as those which grow in the ordinary way. These observations, furnished abound in by vegetable physiology, may explain why the siliceous earth forms so large a portion of the substance of plants, although the calcareous soils are generally covered with a more vigorous and more abundant vegetation than the granitic soils. The calcareous earth attracts humidity, diffuses warmth, and supplies the plants with fixed air; but it is the silex which predominates in the best vegetable earth. The siliceous matter abounds also in grasses, and in several varieties of rushes. There have been found in the ashes of rye straw no less than 70 parts of silex in 100 of Lands which plants. Forskal, Flor. Egypt. p. 45. † Hamilton's Works, p. 33. Brydone's Tour, vol. i. p. 116. Senebier, Encyl. Method. Veget. Physiol. vol. i. Schrader, deux Mémoires couronnés sur la nature des parties terreuses deg plantes, Berlin, 1800. (in Germ.) Expériences de Fourcroy de Giobert, &c. Encycl. Nat. Physiol. Veget, vol. 3. p. 276. |