DISTRIBUTION OE PLANTS IN ZONES. 669 



Climatal Zones. In the preceding Chapter we have seen that 

 climate has a most important influence upon vegetation ; and pro- 

 ceeding on this ground, it is possible to divide the surface into 

 climatal zones, within which a certain average character of vegeta- 

 tion exists. But mere temperature is but one of the influences ; 

 and it is evident that many diverse conditions must exist within 

 such climatal zones, dependent upon the other influences above 

 referred to. Hence, although the general views afforded by mark- 

 ing out climatal regions are useful to the beginner, it is necessary 

 to bear in mind that they are essentially superficial. In Meyen's 

 subdivision of the globe, the zones were denned by parallels of 

 latitude ; but the distribution of temperature, the chief agent here 

 regarded, is so irrelative to the parallels, especially in the northern 

 hemisphere, that we have modified them by isothermal lines ob- 

 tained from Dove's maps. The isotherms selected are mostly 

 annual temperatures ; but in defining the Arctic regions it has 

 appeared more natural to take the line indicating an equal tem- 

 perature in the months of September and July. 



In the following summary the names of Meyen's zones are 

 retained ; the peculiar limitation by isothermal lines is indicated 

 for each zone. 



1. The Equatorial Zone. This zone, as limited by us, comprehends but 

 a comparatively small range in the New World, and is most developed in 

 the Old, especially in Africa. On consulting an isothermal map, it will / 

 be observed also that the larger portion of it lies on the north side of the 

 equator, since the preponderance of land in the northern hemisphere 

 deflects the isothermal lines in this direction. The boundaries are the 

 annual isotherms of 79'3 F. (26 C.) on each side of the equator j but it 

 may he noticed that in Africa, as well as in Hindostan and in the Indian 

 archipelago, there exist between these lines circumscribed regions in 

 which the annual isotherms rise to 81'5 F. (27 C.). 



The characteristics of this zone are marked by the extreme luxuriance 

 of vegetation, from the great heat, together with the abundant moisture. 

 The trunks of the trees attain enormous diameter ; the flowers have most 

 brilliant colours ; and not only is the earth clothed most profusely with 

 numberless forms of plants, but the trees are overgrown by Orchids, 

 Aroids, Bromeliacese, and Ferns, and matted together by Lianes, or 

 gigantic rope-like woody climbers; so that the primceval forests present 

 such a dense mass of vegetation as to be almost impenetrable, even to 

 the explorer who advances axe in hand. The Palms, the Banana tribe 

 (Musacese), arborescent Grasses, Pandanus, Scitamineae, and Orchideae 

 are very striking features j the Fig-trees of most varied kinds, the Silk- 

 Cotton-trees (Bombaceas) also abound both in the Old and New Worlds ; 

 the Csesalpinise, Malpighiaceae, Anacardiacese, Swietenise, Anoneae, Ber- 

 tholletise, and Lecythidese especially mark the forests of America ; the 

 Sapindacese, Artocarpi, Sterculiaceae, Ebenaceae, Meliaceae, Laurineae, 

 &c. those of the Old World. In this zone also, in the Indian archipe- 

 lago, occurs the most remarkable of the Khizanths, the gigantic parasite 



