550 



PHYSICAL GEOGRAPHY. 



of the city of Cumana, he found the famous Zamang del Guayre, a species of mimosa, 

 with its trunk nearly ten feet thick, and its hemispherical head more than 600 feet in 

 circumference, the branches bending towards the ground in the form of an immense 

 umbrella. The boababs of Senegal, and of the Cape Verde Islands, exhibit still greater 

 dimensions, several having been noticed with trunks varying from 50 to 100 feet in 

 circumference. 



11. Plants of the thickets or hedges, comprehending the small shrubs which form the 

 hedge or thicket, as the hawthorn and sweet briar ; the herbaceous plants which grow at 

 the foot of these shrubs, as tuberous moschatel, wood sorrel, and violets ; and those which 

 climb among their numerous branches, as bryony, black bryony, and some species of 

 everlasting pea. 



12. Subterranean plants, or those which live in mines and caves, entirely excluded 

 from the light, as byssus> truffles, and some other cryptogamic plants. 



13. Plants of the mountains, which De Candolle proposes to divide into two sections: 



1. Those which grow on alpine mountains, the summits of which are covered with perpe- 

 tual snow, and where, during the heat of summer, there is a continued and abundant 

 flow of moisture, as numerous saxifrages, gentians, primroses, and rhododendrons. 



2. Those inhabiting mountains on which the snow disappears during summer, as several 

 species of snap-dragon, umbelliferous plants chiefly belonging to the genus seseli, or 

 meadow-saxifrage, and labiate plants. 



14. Parasitic plants, which derive their nourishment from other vegetables, and which, 

 consequently, may be found in all the preceding situations, as the misletoe, brown-rape, 

 dodder, and a number of lichens, mushrooms, and mosses. 



15. Pseudo-parasitic plants, which live upon dead vegetables, or upon the bark of 

 living vegetables, but do not derive their nourishment from them, as epidendron, lichens, 

 and mosses. 



16. Plants which vegetate in hot springs, the temperature of which ranges from 80 

 to 150 of Fahrenheit, as vitex agnus casfus, the chaste-tree of the ancients, a species of 

 osier, several cryptogamous plants, and ulva thermalis, the hot-spring laver. 



17. Plants which are developed in artificial infusions or liquors, as the mould found in 

 Madeira wine, a species of conferva. 



This is an arrangement of vegetable tribes, according to the physical nature of the 

 station they occupy, but very different species are found upon similar sites in different 

 parts of the world, a diversity partially referable to a diversity of climate, which we have 

 seen to depend upon elevation and latitude. There are some general laws respecting the 

 distribution of plants, which it is not likely that any subsequent observation will modify. 

 The proportion of the cryptogamic to the phaenogamic species, or those which never bear 

 flowers, as mosses and lichens, to the common flowering plants, increases as we recede 

 from the equator. The cryptogamous plants are to the phaenogamic, in equatorial 

 countries, as 1 to 5 ; in Australia, as 2 to 1 1 ; in France, as 1 to 2, in several countries 

 nearly equal, and over the whole globe as 1 to 7. The proportion of dicotyledonous 

 plants, or those which have two seed-lobes, like most of the European trees, to the mono- 

 cotyledonous, which have only one, as grasses, lilies, and palms, increases as we recede 

 from the poles. The absolute number of species, and also the proportion of woody species 

 to the herbaceous, increases as we approach the equator. The number of species, either 

 annual or biennial, is greatest in temperate regions, and diminishes both towards the 

 equator and the poles. The following table gives the relative proportions which several 

 well-defined orders, or families of plants, bear to the whole mass of vegetation in the 

 zones mentioned, and shows the zone in which they occur in the greatest relative abun- 

 dance. 



