INFLUENCE OF CLIMATE ON YEGETATION. 661 



at the foot of mountains are well supplied with moisture if the altitude 

 is sufficient, as at the south slope of the Himalayas and the east slope of 

 the Andes. ./Etna, which does not reach the line of perpetual snow in 

 Europe, has arid and barren slopes in the upper regions. 



Habit in relation to Moisture. Plants are divided into Xerophiles, or 

 those capable of existing in very dry climates ; Hygrophiles, or those 

 which can only exist in the presence of abundant moisture ; an inter- 

 mediate group is called Noterophile. Xerophiles are frequently marked 

 by structural characters, such as the succulent tissues encased within a 

 thick leathery rind which admits of little transpiration, Szdum, Cactus, &c. 



The presence of bulbs or of thick stocks and large rigid or fleshy 

 leaves is another characteristic of drought-resisting plants. Another 

 characteristic type of vegetation in such districts is that represented by 

 dry, much-branched spiny trees or shrubs with scanty foliage and small 

 leaves. Resinous exudations, dotted leaves, dense covering of hairs, are 

 other structural peculiarities characteristic of plants of dry regions. Such 

 variations in " habit " are common to most natural orders, and are indica- 

 tions, not of genetic relationship, but of adaptation to circumstances : thus 

 Cacti and Euphorbias have often the same "habit," because living under 

 similar conditions, but, genetically, they are very wide apart. A complete 

 account of this branch of the subject would require a volume for itself. 



Of all the influences to which plants are exposed, climate is the 

 most important ; it sets absolute limits to species, which is not the 

 case with any other of the causes affecting distribution. 



Secondary Natural Influences. 



There is no doubt that the distribution of plants is greatly 

 affected by the conveyance of seeds, fruits, &c. from place to place 

 within the same climatal region, by marine currents, rivers, winds, 

 &c., and by animals, especially birds. 



Currents. An example of diffusion by marine currents is furnished by 

 the occurence of the Cocoa-nut Palm fringing the islands of the Pacific ; 

 and it is probable that the mixed vegetation of many islands is to be 

 accounted for in part by such causes. Marine currents are of most im- 

 portance when they pass along coasts, or across tracts of ocean in similar 

 latitudes such as that part of the Gulf-stream running in the Mexican 

 Gulf, the currents running from Madeira to the Canaries, from the last to 

 Senegal, the east-to-west current of the Society Islands, &c. The seeds 

 of Guilandina Bonduc, a West-Indian tree, have been found in a ger- 

 minating condition on the Cornish coast, where, however, the climate 

 is not sufficiently favourable for it to establish itself. Rivers are most 

 influential when they flow from the east or west. Currents of any kind 

 passing northwards or southwards are less likely to convey seeds &c. into 

 a suitable climate for their naturalization. 



Winds. The winds do not appear to be very influential in transporting 

 Phanerogamous plants ; but the cosmopolitan distribution of the Crypto- 

 gams is at least partially attributable to the facility with which "their 

 microscopic spores are carried away by currents of air. 



