HABITATION OP PLANTS. 
211 
No plants can grow without some degree of heat, though some re- 
quire a greater portion of it than others. 
Plant.s'may be made to grow without light, 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 of the 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 the fixed air* is interwoven with the tex- 
ture of the plants. The aromatic plants, the clove, cinnamon, and 
the Peruyian bark, all owe their chief excellences to the intense light 
of the equatorial regions. 
Gases of different kinds affect vegetation very differently. Car- 
bonic acid gas, though prejudicial to the germination of the seed, has 
been found, when properly apphed, to hasten the process of vegeta- 
tion in the plant. Pure carbonic acid gas destroys vegetable hfe ; 
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 unmixe^i 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 the 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— Ligiit— -Gases— Habitation of plants— Herbs— Woody shri^s. 
