138 EFFECTS OF GASES ON PLANTS. 



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

 be breathed by man and animals is capable of supporting vegetable 

 life. The experiments of Priestley, Percival, and Saussure, 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 plants do not thrive 

 in pure nitrogen, nor in hydrogen gas. These gases seem to have no 

 directly injurious effects, but to act chiefly by depriving the plants of 

 carbon and oxygen. 



293. There are certain gases 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 disorganization; 

 others as narcotic poisons, inducing a drooping and decay of the entire 

 plant. To the former class belong sulphurous acid gas, hydrochloric 

 or muriatic acid gas, chlorine and nitrous acid gas; while under the 

 latter are classed sulphuretted hydrogen, cyanogen, carbonic, oxide, 

 and ammoniacal gas. 



294. Sulphurous Acid Ga is highly injurious to plants. It pro- 

 duces greyish-yellow dry-looking spots on the leaves, which gradually 

 extend until the leaves are destroyed and fall. The effect resembles 

 much the ordinary decay of the leaves in autumn. The proportion of 

 gas, in some experiments, was only 1 in 9,000 or 10,000 parts of air, 

 and the quantity $ of a cubic inch; and yet the whole unfolded leaves 

 of a mignionette plant were destroyed in forty-eight hours. This pro- 

 portion of the gas is hardly or not at all discoverable by the smell. 



295. muriatic Acid Gas produced effects similar and scarcely inferior 

 to those of the last-mentioned gas. When | of an inch was diluted 

 with 10,000 parts of air, it acted destructively on Laburnum, and 

 Larch, destroying the whole vegetation in less than two days. Even 

 when in quantity not perceptible by the smell, it still acts as an irritant 

 poison. 



296. Sulphuretted Hydrogen acted in a different way from the acid 

 gases. The latter attacked the leaves at the tips first, and gradually 

 extended their operation to the leaf-stalks. When in considerable 

 proportion, their effects began in a few minutes; and, if diluted, the 

 parts not attacked generally survived if the plants were removed into 

 the air. But in the case of sulphuretted hydrogen, the leaves, with- 

 out being injured in texture or colour, became flaccid and drooping, 

 and the plant did not recover when removed into the air. It 

 required a larger quantity of this gas to produce the effects stated. 

 When six inches were added to sixty tunes their volume of air, the 

 drooping began in ten hours. This gas then acts like a narcotic 

 poison, by destroying vegetable life throughout the whole plant at 

 once. 



* See Edinburgh Medical and Surgical Journal, vol. xxviii. p. 356. 



