338 PLANT PHYSIOLOGY 



discuss respiration and then growth, the plant does not respire and 

 then grow; it does both simultaneously and interdependently. 



Chemicals. — The essential elements are necessary for growth 

 for reasons already given (Chaps. X and XI). There are also 

 many compounds which hinder growth. Some of these like 

 formalin, mercury bichloride, and copper sulphate have lent them- 

 selves to use as disinfectants, sprays, etc., because of their growth- 

 prohibiting properties. 



Many gases, especially illuminating gas and its chief constit- 

 uents, ethylene and acetylene, have an injurious effect upon plants. 

 These gases not only check growth but they also cause marked 

 epinastic movements of the leaves and floral parts (Chap. XXIX) 

 and an early dropping of the lower leaves. This is associated with 

 the fact that the gases hinder the development of supporting tis- 

 sues as well as of cutin and cork. Etiolated shoots are especially 

 sensitive and may be used to test the presence of ethylene, which 

 causes marked epinasty in tomatoes when present in only 1 part 

 in 200,000 (Crocker, 1929). One can smell illuminating gas when 

 1 part is present in 400 of air, but many plants are damaged when 

 ethylene is present in proportions of only one part to a million! 

 In fact, Zimmerman reported that these gases were more dangerous 

 to roses when present in these very low concentrations than in 

 higher ones. 



In smelter smoke, the destructive effects of which are sometimes 

 observed for many miles from the source (50 miles according to 

 unpublished work of Hedgecock), the most dangerous component 

 is generally sulphur dioxide. In the many lawsuits that have re- 

 sulted between growers and smelter owners, the latter have tried 

 to prove that the damage was too far from the smelter to be caused 

 by gases therefrom, but analyses of the air and leaves have gener- 

 ally shown the growers to be right. Plants should have as pure 

 air as possible with a sufficient amount of carbon dioxide for 

 maximum growth. 



On the other hand, some gases (Chap. XXVI) have a stimulating 

 effect upon growth and are used for the forcing of plants. Among 

 these are the vapors of the common anaesthetics, chloroform and 

 diethyl ether, as well as many others, including ethylene chlorhy- 

 drin, ethyl bromide, carbon bisulphide, etc. 



Wounds. — Darwin (1880) noticed that plant roots when 

 wounded slightly bend in such a way that the tip moves away 



