Holland, I first suggested the inquiry respecting the proportions of the primary divisions of plants as connected with climate; and I then ventured to state that "from the equator to 30° lat. in the northern hemisphere at least, the species of Dicotyledonous plants are to the Monocotyledonous as about 5 to 1, in some cases considerably exceeding, and in a very few falling somewhat short of this proportion, and that in the higher latitudes a gradual diminution of Dicotyledones takes place until in about 60° N., and 55° S. lat. they scarcely equal half their intratropical proportion."[1]
Since the publication of the Essay from which this quotation is taken, the illustrious traveller Baron Humboldt, to whom every part of botany, and especially botanical geography, is so greatly indebted, has prosecuted this subject further, by extending the inquiry to the natural orders of plants; and in the valuable dissertation prefixed to his great botanical work,[2] has adopted the same equinoctial proportion of Monocotyledones to Dicotyledones as that 423] given in the Paper above quoted; a ratio which seems to be confirmed by his own extensive herbarium.
I had remarked, however, in the Essay referred to, that the relative number of these two primary divisions in the equinoctial parts of New Holland appeared to differ considerably from those which I had regarded as general within the tropics; Dicotyledones being to Monocotyledones only as 4 to 1. But this proportion of New Holland very nearly agrees with that of the Congo and Sierra Leone collections. And from an examination of the materials composing Dr. Roxburgh's unpublished Flora Indica, which I had formerly judged of merely by the index of genera and species, I am inclined to think that nearly the same proportion exists on the shores of India.
Though this may be the general proportion of the coasts, and in tracts of but little varied surface within the tropics,
- ↑ Flinders' Voyage to Terra Australis, 2, p. 538. (Antè, p. 8.)
- ↑ Nova Genera et Species Plantarum, quas in perigrinatione orbis novi collegerunt, &c. Amat. Bonpland et Alex. de Humboldt, ex. sched. autogr. in ord. dig. C. S. Kunth, 1815, Parisiis.
