EFFECTS OF GASES ON PLANTS. 159 



ranging on either side. The view generally adopted is, that plants 

 give out carbonic acid at certain times, and that the green parts of 

 plants under the influence of light decompose the gas, fix the carbon, 

 and eliminate the oxygen. 



Experiments have been made as to the effect of the different rays 

 of the spectrum in aiding the decomposition of carbonic acid, by the 

 green parts of plants. The light-giving rays, or those nearest the 

 yellow, appear to have the greatest effect in the fixation of carbon, 

 and in the production of wood ; while the heat-giving, and the tithonic 

 or chemical rays, have scarcely any influence. 



The tropics and warm climates, where a sky seldom clouded per- 

 mits the 'glowing sun rays to shine on a luxuriant vegetation, are 

 the constant and inexhaustible source of oxygen, thus contributing 

 to the respiration of the animals, not only of their own latitudes, 

 but also of the temperate and colder zones, where artificial light and 

 warmth must replace the deficient light and heat of the sun, and 

 which thus produce a copious supply of carbonic acid, to be expended 

 on the nutrition of the tropical plants. The life of animals is thus 

 connected intimately with the vegetable productions of the globe, not 

 merely as regards the materials of their food, but also in reference to 

 the air which they breathe. 



While the breathing of man and animals, and the various pro- 

 cesses of combustion, are constantly abstracting oxygen from the 

 atmosphere, and substituting carbonic acid, plants are decomposing 

 this noxious gas, and restoring the oxygen. 



Effects of certain Gases on liviTig Plants. 



It has been already stated that plants can live in an atmosphere 

 containing a .considerable proportion of carbonic acid, provided they 

 are exposed to the light. Thus, an atmosphere which could not be 

 breathed by man and animals is capable of supporting vegetable life. 

 Experiments show, however, that plants will not continue to exercise 

 their functions in pure carbonic acid gas, but that in all cases a certain 

 quantity of free oxygen must be present. It has been found that 

 though plants do not thrive in pure nitrogen, nor in hydrogen gas, yet 

 their vitality is not destroyed by the presence of these gases. Saus- 

 sure observed that a plant of Lythrum Salicaria lived for five weeks 

 in an atmosphere of hydrogen gas. Nitrogen has been proved to be 

 innocuous. These gases seem of themselves to have no directly 

 injurious effects, but to act chiefly by deprivmg the plants of carbon 

 and oxygen. 



There are certain gases, however, which have very prejudicial 

 effects on plants, as proved by the experiments of Turner and 

 Christison. Some of them act as irritant poisons, causing local dis- 



