PHYSIOLOGICALLY ACTIVE GASES 147 



air. A medium-sized thimble holds about 2.5 cubic centimeters. One such 

 thimbleful of ethylene placed in a room 80 feet long, 32 feet wide, and 

 32 feet high gives 1 part of ethylene to 1 billion parts of air, or just enough, 

 after it is equally diffused throughout the room, to induce epinastic response 

 in the leaves of the African marigold. An- is a rather rare medium compared 

 ^^^th the density of water or plant tissue, also ethylene has low solubility m 

 water and probably in plant tissues, especially at 25° C (77° F), a good 

 grounng temperature. The percentage by weight of ethylene in the plant 

 tissue in the atmosphere described above must be almost fancifully small. 

 The epinastic response of the marigold to ethylene must rank high in the 

 extreme sensitiveness of an organism to a chemical. Vitamins and hormones 

 are noted for producing physiological effects in extreme dilutions and, as 

 we shall see later, there are good reasons for considering ethylene a plant 

 hormone. 



Destructive or dry distillation or mcomplete combustion of carbon com- 

 pounds produces ethylene, carbon monoxide, and other carbon gases. As 

 a result, there are several sources of ethylene and carbon monoxide in the 

 air: artificial illuminating gas; automobile exhaust and exhaust from other 

 internal combustion engines; furnaces when the oxygen supply is made- 

 quate; improperly trimmed or adjusted oil or gas stoves or torches; "^ pipe, 

 cigarette, or cigar smoke; and burning brush or rubbish piles. Finally, 

 burning a sheet of paper ^"^ in the air produces some ethylene and carbon 

 monoxide because the heat is not great enough to burn all distillation gases. 

 There are three kno\\-n natural sources of ethylene: it is given off by coal 

 in the mine or in storage ; one natural gas ^ is kno^vn to contain a trace of 

 ethylene; and respiring living plant tissues produce ethylene. In traces of 

 the gas mixtures mentioned above, it is the ethylene that induces the re- 

 sponses and not the other three. This is because ethylene is so much more 

 effective than the others; also propylene and acetylene when present are 

 generally in lower concentrations than ethylene in these mixtures. In 

 some, CO is more concentrated than ethylene; hi water gas there are about 

 24 per cent CO and about 3 per cent of ethylene. But in the lowest concen- 

 tration of this gas in air that will mduce epinasty, because of the ethylene 

 present, CO exists in about 3^25 sufficient concentration to induce the 

 response. Epinasty does, however, serve as an indirect test for CO for it 

 generally accompanies ethylene in the gases mentioned above. 



Epinastic response of leaves has proved a very useful and delicate test 

 for traces of unsaturated C-gases both in practice and in research.i^. i5 

 This test has been accepted m courts as evidence of illummatmg gas m 

 greenhouses.'* Fifteen or twenty years ago there were frequent and some 

 very extensive injuries to plants in commercial greenhouses by artificial 

 illuminating gas that escaped from leaking pipes, seeped along under the 

 frozen crust, and came up into greenhouses. Numerous researches on the 

 effect of illummating gas and its constituents, mainly ethylene, upon green- 

 house plants have reduced this loss tremendously. The epinastic response 



