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Show 294 PHYSIOGNOMY OF PLANTS. of the earth's surface. (Compare my memoir, entitled" De distributione geographica Plantarum secundem cooli temperiem et altitudinem montium, 1817, pp. 24-44; and the farther development of the subject of these numerical relations given by me in the Dictionnaire des Sciences naturelles, t. xviii. 1820, pp. 422-436; and in the Annales de Chimie et de Physique, t. xvi. 1821, pp. 267-292.) The numerical relations of the forms of plants, and the laws observed in their geographical distribution, may be considered in two very different ways. If plants are studied in their arrangement according to natural families, without regard to their geographical distribution, it is asked, What are the fundamental forms or types of organization to which the greater number of species correspond? Are there on the entire surface of the earth more Glumacere than Compositre? Do these two orders make up between them one-fourth part of the whole number of phrenogamous plants? What is the proportion of Monocotyledons to Dicotyledons? These are questions of General Phytology, or of the science which investigates the organization of plants and their mutual connection, or the present state of the entire vegetable world. If, on the other hand, the species of plants which have been grouped according to the analogy of their structure are considered, not abstractedly, but according to their climatic relations, or according to their distribution over the surface of the earth, we have questions offering quite another and distinct interest. We then examine what are the families which prevail more in proportion to other Phanerogamre in the torrid zone than towards the polar circle ? Are Compositre more numerous, either in the same geographical-latitudes or on the same isothermal lines, in the New than in the Old Continent? Do the forms which gradually lose their predominance in advancing from the Equator towards the Poles follow a similar law of decrease in ascending mountains situated in the equatorial regions? Do the proportions of particular families to the whole mass of Phanerogamre differ in the temperate zones, and on equal isothermal lines, north and south of the Equator? These questions belong properly to the Geography of Plants, and connect themselves with the most important problems of meteorology and terrestrial physics. The character of a landscape or country is also in a high degree |