118 INFLUENCE OF THE ELEMENTS. 



from its peculiar productions, a distinct region. 22d. Island* in the 

 South Seas, within the tropics ; it has about 214 genera, but 173 are 

 found in India ; others are in common with America. The bread-fruit 

 tree is a characteristic of these islands. 



It will be understood by the above outlines that genera are referred 

 to as the distinguishing characteristics of these regions which, of 

 course, comprehend innumerable species and varieties. This is a very 

 interesting subject, when pursued farther, but space here admits only 

 of the names of the regions, without particulars as to their peculiar 

 and remarkable vegetable productions which are treated in the 2d part. 



INLUENCE OF THE ELEMENTS ON PLANTS. 



Light) heat, moisture, soil, and atmosphere. 



These, like the effect of natural boundaries, determine with equal 

 certainty the fiat, "thus far shalt thou go and no farther." Some 

 plants, it is seen, are found alone in the tropics, such, for example, 

 as the palms, the tree-ferns, etc. The temperate region contains the 

 cruciferous and umbeliferous plants almost exclusively, while the con- 

 iferous and amentaceous flourish in the regions of the north. These 

 with others are all effected by the great physical agents light, heat f 

 moisture, and soil. 



Solar light on vegetation is no less impoi tant than temperature. It 

 produces, indeed, in vegetable life, the greatest variety of phenomena. 

 Plants, it is well known, imbibe less humidity in darkness than in 

 light and consequently the latter greatly influences their character and 

 growth. It effects materially their decomposition of carbonic acid, 

 so essential to their life and growth, and changes their properties and 

 the composition of their organs. In its absence they absorb oxygen 

 gas, which gives acid qualities to their productions, while in the light 

 they vigorously perform the important function of giving off oxygen 

 to the air. To its absence also is attributable the phenomenon of the 

 sleep of plants. Light is more generally diffused than heat, pervading 

 as it does the cold climates and seasons with little apparent diminu- 

 tion during the day. Its rays are however more oblique, as we pro- 

 ceed to the extreme north, where it is little wanted by vegetation 

 during the winter, as the loss of leaves renders it useless to plants. 

 During the period of vegetation it is, however, almost continuous. 

 Plants, too, which maintain habitually the same position can live in 

 cold climates, where, in summer, light is thus continued ; while those 

 species exist at the south which are in the habit, alternately day and 

 night, of sleeping and waking, or closing and expanding their flowers. 

 From this is seen the difficulty of cultivating tropical plants in north- 

 ern climates. The vine fails to ripen in the foggy air of Normandy, 



