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Show A NOTATION AND ADDITIONS. 307 perceive that even in the great families-our knowledge of which has been of late most trikingly enriched by the descriptions of botanists- we are still acquainted with only a small part of existing plants. The Repertorium of Walpers completes Decandolle's Prodromus of 1 25, up to 1846: we find in it, in the family of Leguminosre, 806 species. We may assume the ratio, or relative numerical proportion of this family to all phrenogamous plants, to be 2 1 1-as we find it -/15 within the tropics, -h in the middle temperate, and j 3 in the cold northern zone. The described Leguminosre would thus lead us to assume only 169,400 existing phrenogamous species on the whole surface of the earth; whereas, as we have shown, the Compositre indicate more than 160,000 ah·eady known species. The discordance is instructive, and may be further elucidated and illustrated by the following analogous considerations. The major part of the Compositre, of which Linnreus knew only 785 species, and which has now grown to 12,000, appear to belong to the Old Continent; at least Decandolle described only 3590 American, whilst the European, Asiatic, and African species amounted to 5093. This apparent richness in Compositre is, however, illusive, and considerable only in appearance; the ratio or quotient of the family ( ~~ between the tropiils1 { in the temperate zone, and 1\ in the cold zone), shows that even more species of Compositre than Leguminosre must hitherto have escaped the researches of travellers; for a multiplication by 12 would give us only the improbably low number of 144,000 phrenogamous species. The families of Grasses and Cyperacere give still lower results, because comparatively still fewer of their species have been described and collected. We have only to cast our eyes on the map of South America, remembering the wide extent of territory occupied by grassy plains, not only in Venezuela and on the banks of the Apure and the Meta, but also to the south of the forest-covered regions of the Amazons, in Chaco, Eastern Tucuman, and the Pampas of Buenos Ayres and Patagonia, bearing in mind that of all these extensive regions the greater part have never been explored by botanists, and the remainder only imperfectly and incompletely so. Northern and tJentral Asia offer an almost equal extent of Steppes, but in which, however, dicotyledonous herbaceous plants are more largely mingled with the Graminere. |
