196 GROWTH OF PLANTS 



Hence it is probable that the free chlorine in city water supplies sometimes 

 reaches a concentration that \vill injure or kill fish and even water plants, 

 but it is questionable whether it ever contains enough free chlorine to be 

 dangerous for watering or syringing land plants. 



Comparative Effect of Five Toxic Gases on Plants and Animals 

 Several members of the Institute staff 2. is. la. s^, ss ugg(^ o^. continu- 

 ous-flow fumigation apparatus to study the relative toxicity of the following 

 five gases upon various animals, plants, and plant organs: ammonia (NH3), 

 chlorine (CI 2), hydrogen cyanide (HCN), hydrogen sulphide (H2S), and 

 sulphur dioxide (SO2). Each gas was studied at concentrations of 1, 4, 16, 

 63, 250, and 1000 ppm of the air and the periods of exposure were 1,4, 15, 

 60, 240, and 960 minutes. The full set of experiments produced a great 

 number of detailed facts, many of which are of interest. Space demands, 

 however, that we must present only the general findings with a few of the 

 more interesting details in connection with some of the organisms. 



Fig. 74 shows the number of minutes of exposure required for the highest 

 concentrations of the gases used, 1000 ppm, to kill one-half of the various 

 organisms (LD50) insofar as 960 minutes was sufficient to accomplish this 

 end. One must realize in this figure that, due to the limited page size, the 

 time is recorded as a geometric series on the ordinate rather than as an 

 arithmetical series. If the figure were made on the arithmetical basis and 

 each minute given the same space as the first minute, the figure would be 

 about 45 inches high and the points in the upper part would be very much 

 farther apart, increasingly so as the top of the figure is approached. With 

 this in mind, let us see what the figure shows. 



Beginning with the eight actively growing plant pathogens, it is evident 

 that they vary greatly from each other in their resistance to the several 

 gases. They are arranged from left to right in their increasing order of 

 resistance. These pathogens are in general most readily killed by SO2. 

 The one exception is Rhizoctonia tuliparum, which proved a little more 

 sensitive to CI2 than to SO2. Chlorine is the next most toxic. Then follow 

 NH3, H2S, and HCN in succession, with no significant difference between 

 the last two. Each of the last two failed to kill 50 per cent of four of the 

 eight plant pathogens even in 960 minutes. In the case of the two animal 

 pathogens tested, CI2 proved by far the most toxic, with SO2 second. The 

 other three gases showed low toxicity. Sclerotia of fungi and seeds proved 

 resistant. These reproductive organs are well protected by non-living 

 coats. Soaked seeds, of course, were more sensitive than dry ones. 



Green leaves were, on the whole, the most sensitive to the five gases of 

 any organisms tested, with CI2 the most toxic, SO2 next, followed by NH3 

 and HCN, and with H2S least toxic by a large margin. 



As has already been mentioned, CI2 and SO2 both caused interveinal 

 spotting of the blades of the leaves. Ammonia in much higher dosages 

 brought about spotting of the leaves, but the spots were often black instead 



