474 UNTOWARD INFLUENCES. 



especially those which contain sulphur compounds as impurities.^ 

 Formerly, in the vicinity of large chemical factories, the escaping 

 gases were productive of wide-spread injury to vegetation ; but 

 improved methods of manufacture have diminished this evil to 

 a considerable extent. 



1240. Sulphurous acid, formed by combustion of sulphur in 

 the open air, produces, even when existing in the air in the pro- 

 portion of only one part in 9,000, the following effects upon 

 leaves : their blades shrivel from the tips, become grayish yel- 

 low, and soon drj' so that they fall off at a slight touch. The 

 phenomena observed are somewhat lil^e those occurring at the 

 time of the fall of the leaf in autumn. Yet in the experiments by 

 Turner and Christison mentioned in the note,^ the amount of 

 sulphurous gas present in the air was so small as to escape 

 detection by smell. 



H^-drochloric acid gas, nitric acid in vapor, and chlorine are 

 also very destructive to plants, even when in such minute 

 amounts as to be unnoticed on account of their odor. 



Injurious effects are often produced upon shade trees by the 

 leakage of illuminating gas from street mains. 



1241. Wardian Cases. In 1829 Ward accidentally discovered 

 that plants could thrive in tightly closed cases, in which there 

 could not be any interchange of the air with the outside atmos- 

 phere.' This discovery led him to institute experiments rela- 



1 E. Angus Smith : Air and liain, 1872, pp. 465, 553. 



2 For accounts of experiments in this interesting field, the student may 

 consult the following works : Turner and Christison, Edinburgh Medical and 

 Surgical Journal, xxviii. p. 356; and Gladstone iu Report of British Association 

 for Advancement of Science, 1850. 



3 N. B. Ward : On the Growth of Plants in Closely Glazed Cases, 1852. 

 The table on the following page, based on researches by T. W. Harris, shows 



the agents, the effects of which were tried upon chlorophyll, and the results in 

 each case as to the extrusion of chlorophyll pigment (see 772). The figures 

 in the third column indicate results as follows ; — 



1. Chlorophyll grains large and well defined. Sponge-like stracture evi- 

 dent. One or two globules of large size on almo.st eveiy grain ; sometimes 

 almost as large as the grain itself, which is colorless or nearly so. 



2. Globules still plentiful but smaller ; frequently several on each grain. 

 Structure of the grains evident. The protoplasm in this and the two following 

 grades (3 and 4) is often contracted by the chemicals used, rendering the result 

 more or less obscure. 



3. Globules small, and fewer than in 2. Grains still retain some coloring- 

 matter in their substance, and are not so well defined either in form or 

 structure. 



4. Globules few; only seen on a few grains. Structure of the grain not 

 defined, but under a high power it frequently has a granular and sometimes a 



