HABITATION OF PLANTS. 211 



No plants can grow without some degree of heat, though some re- 

 quire a greater portion of it than others. 



Plants maybe made to grow without //g-A)', but they will not exhibit 

 the verdure, or any of the properties of health. The atmosphere, 

 which is contaminated by the respiration of animals, is restored to 

 purity by the vegetation of plants; but secluded from light, vegeta- 

 bles are no longer capable of converting a portion ofthe'fixed air to 

 their use, or of supplying the atmosphere with the oxygen, on which 

 its importance in supporting animal life chiefly depends. By the ac- 

 tion of light, the carbon of tlie fixed air* is interwoven with the tex- 

 ture of the plants. The aromatic plants, the clove, cinnamon, and 

 the Peruvian bark, all owe their chief excellences to the intensse light 

 of the equatorial regions. 



Gases of different kinds affect vegetation very diflferently. Car- 

 bonic acid gas, though prejudicial to the germination of the seed, has 

 been found, wlien properly applied, to hasten the process of vegeta- 

 tion in the plant. Pure carbonic acid gas destroys vegetable life; 

 thus, a growing plant placed over wort in a state of fermentation, 

 dies in a few hours. Dr. Priestly, a celebrated chemist, proved that 

 this gas is of great utility to the growth of plants vegetating in the 

 sun, and that whatever promotes'the increase of it in their atmos- 

 phere, at least within a certain degree, assists vegetation. In the 

 shade, an excess of carbonic acid gas is found to be hurtful to 

 plants. 



Most kinds of manure afford large portions of carbonic acid gas. 



Oxygen gas is essential to the germination of the seed and to the 

 growth of the plant. Flower-buds confined in an atmosphere de- 

 prived of oxygen, fade without expanding. 



Neither Nitrogen nor Hydrogen, when unmixed with other sub 

 stances, afford an atmosphere favourable to vegetation. 



Habitation of Plants. 



Vegetation is not scattered by chance over the surface of the globe 

 but we perceive that the Creator has regulated its distribution ac 

 cording to certain fixed principles ; we find not only a wonderfu. 

 adaptation of plants to the physical necessities of animals in gener 

 al, but that they are also varied to correspond to the peculiar wants 

 of animals in different climates. 



First, we would notice the herbs which cover the surface of the 

 earth ; had their stems been hard and woody, the greater part of the 

 earth would have been inaccessible to the foot of man, until the vege- 

 tation was first destroyed by fire, or by some other means. Can we 

 imagine that the grass and herbs which now afford a soft carpet for 

 our feet, came by chance to grow thus, rather than hard and woody, 

 like the trees? Can we imagine, that by chance the prevailing col- 

 our of vegetation is green, that colour upon which, above all, the eye 

 rests with the most agreeable sensations? Suppose tlTfe grass and 

 herbs to have been red or yellow, and with our present organs of 

 sight, how painful would be the sensations excited by these bright 

 colours ! Instead of beholding the face of nature with delight, we 

 should turn from it, and vainly seek some object on which the eye 

 might repose. 



Woody shrubs occasionally alternate with herbs, but they are so 

 placed as not to offer obstructions to the foot of man ; they often 

 grow out of the clefts of rocks, affording a means of climbing almost 



* Carbonic acid gas. _ 



Heat— Light— Gases— Habitation of plants— Hei bs— Woody shrubs 



