6io FACTORS AFFECTING GROWTH 



in Chap. XXV and this topic will not be considered further. Similarly the 

 part played by nitrogen in the growth of plants has already been discussed 

 in Chap. XXVI. 



The Influence of Atmospheric Gases on Growth. — The biologically 

 important gases which are invariably present in the atmosphere are oxygen, 

 carbon dioxide, and water-vapor. The first two of these are so nearly con- 

 stant in their atmospheric concentrations that they seldom need to be con- 

 sidered as variables in interpreting the developmental behavior of plants under 

 natural conditions. The only exception to this statement is the occasional 

 enrichment of the lower strata of the atmosphere with carbon dioxide as a 

 result of the respiration of soil micro-organisms (Chap. XXI). The water- 

 vapor content of the air, on the other hand, varies considerably. The pro- 

 nounced influences of this factor upon transpiration, the internal water rela- 

 tions of plants, and indirectly upon growth have already been considered. 



Sometimes gases other than those normally present in the atmosphere be- 

 come a part of the environment of plants. Smelters, for example, release 

 considerable quantities of sulfur dioxide into the atmosphere. This gas, in 

 any appreciable concentration, is highly toxic to plants, hence the countryside 

 in the vicinity of smelters is often virtually denuded of vegetation. Most 

 species of plants are injured by an exposure of only one hour to an atmosphere 

 containing as little as one part of this gas in a million (Zimmerman and 



Crocker, i934)- 



Escaping illuminating gas often produces injurious or lethal effects upon 

 plants. Leaky gas mains sometimes cause the death or injury of shade trees 

 along city streets. Similarly injury or death of greenhouse plants has some- 

 times been found to result from leakage of gas from underground mains or 



other sources. 



Manufactured illuminating gas contains both carbon monoxide and ethy- 

 lene, the former being present in much larger proportions than the second. 

 Ethylene, however, is the chief toxic constituent of artificial illuminating gas. 

 Prolonged exposure to even very small concentrations of this gas in the at- 

 mosphere results in death or profound physiological disturbances in most 



species of plants. 



Some species, of which the tomato is an example, are exceedingly sensi- 

 tive to ethylene. The leaves of tomato plants soon show epinasty (Chap. 

 XXXVII) at concentrations as low as o.i part of ethylene to a million of 

 air. A few species are even more sensitive to ethylene (Crocker, et al., 1932). 



Since such minute concentrations of ethylene are far less than can be 

 detected by odor, or even by chemical tests, the simplest method of detecting 

 this gas when present only in traces is to stand several young potted tomato 



