INFLUENCE OF HEAT ON PLANTS. 73 



when a plant or tree of temperate climates is transported to the tropics. 

 Within a very short period after one crop of leaves has fallen off, a new 

 one makes its appearance. This goes through all its changes of develop- 

 ment and decay more rapidly than it would do in its native clime ; and 

 in its turn falls off, and is speedily succeeded by another. Hence the 

 fruit-trees of this country, transported to the East or West Indies, bear 

 abundant crops of leaves, three, perhaps, in one year, or five in two 

 years, but little or no fruit ; and the period of their existence is much 

 shortened. 



100. As Plants are almost wholly dependent upon the temperature of 

 the surrounding medium for the supply of Heat necessary for their 

 growth, many regions must have been devoid of Vegetable life altogether ; 

 if there were not a remarkable adaptation, in the wants of different 

 species, to the various degrees of temperature of the habitations prepared 

 for them. Thus we see the Cacti and Euphorbia attaching themselves 

 to the surface of the most arid rocks of tropical regions, luxuriating, as 

 it would seem, in the full glare of the vertical sun, and laying up a store 

 of moisture from the periodical rains, of which even a long-continued 

 drought is not sufficient to deprive them. The Orchideous tribe, on the 

 other hand, whose greatest development occurs in the same zone, find 

 their congenial habitation in the depths of the tangled forests, where, 

 with scarcely an inferior amount of heat, they have the advantage of a 

 moister atmosphere, caused by the exhalations of the trees on which they 

 cling. The majestic Tree-Fern, again, reaches its full development in 

 insular situations ; where, with a moist atmosphere, it can secure a 

 greater equability of temperature than is to be met with in the interior 

 of the vast tropical continents. None of these races can develope 

 themselves elsewhere to their full extent at least, unless their natural 

 conditions of growth are imitated as far as possible ; and in proportion 

 as this imitation can be made complete, in that proportion may the plant 

 of the tropics be successfully reared in temperate regions. 



101. There are some examples of the adaptation of particular forms 

 of Vegetable life to extremes of temperature, which are interesting as 

 showing the extent to which this adaptation may be carried. In hot 

 springs near a river of Louisiana, of the temperature of from 122 to 

 145, there have been seen to grow, not merely Confervae and herbaceous 

 plants, but shrubs and trees ; and a hot-spring in the Manilla Islands, 

 which raises the thermometer to 187, has plants flourishing in it, and 

 on its borders. A species of Ohara has been found growing and repro- 

 ducing itself in one of the hot-springs of Iceland, which boiled an egg in 

 four minutes ; various Confervse, &c., have been observed in the boiling- 

 springs of Arabia and the Cape of Good Hope ; and at the island of New 

 Amsterdam, there is a mud-spring, which, though hotter than boiling- 

 water, gives birth to a species of Liverwort. On the other hand, there 

 are some forms of Vegetation, which seem to luxuriate in degrees of cold, 

 that are fatal to most others. Thus the Lichen, which serves as the 

 winter food of the Rein-deer, spreads itself over the ground whilst thickly 

 covered with snow ; and the beautiful little Protococcus nivalis, or Red 

 Snow, reddens extensive tracts in the Arctic regions, where the perpetual 



