150 GROWTH OF PLANTS 



we shall see later, illuminating gas contains cyanides which are soluble; in 

 heavier flow of gas these accumulate in the soil in sufficient concentration 

 to kill the plants, so the ethylene response can not occur. Wallace ^^ found 

 that intumescences in the apple stem in response to stimulation by ethylene 

 gas arise through three fundamental changes in the tissues affected, namely, 

 solution of cell walls already showing marked secondary thickening, en- 

 largement of cells, and proliferation of cells. Ethylene induced intumes- 

 cence in apple tudgs in the extreme dilution of 1 to 100 million of air. This 

 approaches the sensitiveness of the potato and African marigold leaves 

 which require over 1 to 300 million and 1 to 1 billion of ethylene in the air, 

 respectively, to induce epinasty. We shall see later that many of the plant 

 hormone types of chemicals induce proliferation of tissues. 



Abscission of leaves, flowers, and fruits. One of the more general effects 

 of ethylene and other effective gases is to cause leaf, petal, flower, and 

 fruit fall by inducing growth in cells of the abscission layer. This is in part 

 because the flat cefls of the abscission layer enlarge and become spherical 

 (hypertrophy) , although proliferation of cefls may occur in some cases. Xo 

 doubt abscission is also furthered by the tendency of ethylene to induce 

 the solution of msoluble pectins of the middle lamella of the cell walls. We 

 shall discuss this effect of ethylene later in this chapter. Fig. 57 shows 

 leaf fall in Crassula arhorescens and petal fall in Salvia induced by ethylene 

 in cigarette smoke in the first, and by ethylene in illuminating gas in the 

 second. 



Ethylene has been used in Oregon to defoliate roses at time of digging. 

 For this purpose a moderately tight chamber having high humidity and a 

 temperature of 70° to 75° F (21° to 24° C), not exceeding 85° F (30° C), 

 is used. Tank ethylene may be used at the optimum rate of 1 cu. ft. to 

 100,000 cu. ft. of space. Apples stored in the chamber give the optimum 

 ethylene concentration when there is a bushel to every 400 to 500 cu. ft. 

 of space. Kerosene stoves and rose hips proved a less desirable source of 

 ethylene because the ethylene concentration was not sufficiently high. 

 The defoliation required four days under optimum conditions. Ethylene 

 has been used to loosen the shucks of English walnuts ^^ and pecans,-- -^ 

 ^\^th improvement in the color of the product in the former. In both cases 

 the nuts could be harvested earlier, as soon as the meats were ripe, mthout 

 waiting for the shucks to loosen on the tree. This prevented later injury 

 in the dry, hot air of California and Arizona. The optimum conditions in 

 the shucking chambers were 1 to 1000 of ethylene and a temperature some- 

 what above 70° F (21° C) with a period of two to four days. Emanations 

 from pecan shucks induced epinasty in potato plants; the pecan tissue pro- 

 duces ethylene, but evidently not enough to induce quick shuckmg. 

 Ethylene probably induces earlier gro^vth in the abscission layer between the 

 shuck and shell. The cytological changes, however, have not been studied. 



Anesthesia and inhibition of growth. Doubt" found that the proper 

 concentration of ethylene produced complete rigor in Coleus, from which the 



