204 
HABITATION OF PLANTS. 
plants are of this description. Atmosplierio air is necessary tj 
the health :ind vigor of plants ; if a plant is placed under a glass 
into which no air can enter it withers and dies. Most plants 
contain a certain portion of salts absorbed from the soil by 
which it is nourished. JN'o plants can grow without some de- 
gree of heat, though some require a greater portion of it than 
others. Plants may be made to grow without Ught^ but they 
will not exhibit the verdure, or any of the properties of health. 
The atmosjpliere which is contaminated hy the respiration of 
animals is restored to purity hy the vegetation of plants / thus 
animals and plants 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 chiefly depends. JBy tJie action of lights tJie 
carbon of the fixed air (carbonic acid gas) is interwoven loith the 
texture of plants. The aromatic plants, as the clove, cinnamon, 
and Peruvian bark, all owe their chief excellences to the in- 
tense light of the equatorial regions. Gases uf different kinds 
affect vegetation very differently. Caroonic 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 wort 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. ISTeither Nitrogen nor Hydro- 
gen^ when unmixed with other substances, afford an atmosphere 
favorable to vegetation. 
313. Habitation of Plants. — Yegetation 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 
wordy, the greater part of the earth would have been inaccessi- 
ble to the foot of man until the vegetation was first destroyed 
Atmospheric air— Salts— Heat— Light— Gases.— 313. Habitation of Plants— Heobt. 
