204r HABITATION OF PLANTS. 



plants are of this description. Atmospheric air is necessary to 

 tlie liealth and vigor of plants ; if a plant is placed nnder a glass 

 into which no air can entei* it withers and dies. Most plants 

 contain a certain jTortion of salts absorbed from the soil by 

 which it is nonrlshed. No plants can grow without some de- 

 gree of Juat^ though some require a greater portion of it than 

 others. I'lants nuiy be made to grow without llght^ but they 

 will not exhibit the' verdure, or any of the properties of health. 

 The atmosjplicre lohich is contaminated by the respiration of 

 animals is restored to piivity hy the "vcyetation of plants ; thus 

 animals and ])lants depend on each other for existence. Se- 

 cluded from light, vegetables are no longer capable of convert- 

 ing a portion of the fixed air to their use, or of supplying the at- 

 mosphere with the oxygen on which its importance in sup- 

 porting animal life chieily depends. By the action of lights the 

 carbon of the fixed air (carbonic acid gas) is intei'vwven loith tlie 

 texture of2:)lants. The aromatic plants, as the clove, cinnamon, 

 and Peruvian bark, all ow^e their chief excellences to the in- 

 tense light of the equatorial regions. Gases of different kinds 

 affect vegetation very differently. Carbonic acid gas, though 

 prejudicial to the germination of the seed, has been found 

 when properly applied to hasten the process of vegetation in 

 the plant. Undiluted, or pure carbonic acid gas, destroys vege- 

 table life ; thus, a growing plant placed over w^ort in a state of 

 fermentation dies in a few hours. Dr. Priestley, 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 atmosphere, at least within a certain 

 degree, assists vegetation. In the shade an excess of carbonic 

 acid gas is found to be hurtful to plants. Oxygen gas is essen- 

 tial to the germination of the seed, and to the growth of the 

 plant. Flower-buds confined in an atmosphere deprived of 

 oxygen fade without expanding. Neither Nitrogen nor Hydro- 

 gen^ when unmixed with other substances, afibrd an atmosphere 

 favorable to vegetation. 



313. 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 according to certain fixed 

 principles ; we find not only a wonderful adaptation of plants 

 to the physical necessities of animals in general, 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 w^ould have been inaccessi- 

 ble to the foot of man until the vegetation was fi^rst destroyed' 



Atmospheric air— Sails— Heat— Light— Gases.— 313. Habitation of Plants— Herbs. 



