164 GROWTH OF PLANTS 



tomato and in Fig. 63 for the African marigold. In other plants the roots 

 grew only in the region of the stem that was elongating at the time it was 

 subjected to the gas, as in tobacco (Fig. 62) and in a number of other plants. 

 In still other plants the roots developed only at the nodes. The gases also 

 induced development of roots on leaves and roots. The greatest growth of 

 roots was obtained in mtact plants if they were subjected to the gas for the 

 induction period and then placed in gas-free moist air for further growth. 

 While the gases induce rooting they also inhibit later elongation of the 

 roots. These gases induce root-hair formation also, as is shown in Fig. 63- 



While in one respect Zimmerman and Hitchcock had found the thing 

 they had long been seeking, namely, chemicals that induce rooting, these 

 chemicals could not be used in propagation. Since they are gases they move 

 readily throughout the plant, ^^^ " probably traveling through the extensive 

 intercellular system of plants. Because of this they induce roots at places 

 where roots are not desired, all along the stem in some cases and at espe- 

 cially susceptible zones in others. Consequently Zimmerman and Hitchcock 

 sought non-volatile or slightly volatile chemicals that would induce root- 

 ing only at the point of application. This led to the study of other plant 

 hormones discussed in a later chapter. 



Other physiological effects. Several investigators have found that the 

 unsaturated C-gases induce the earlier formation of flowers in pmeapple *''• ^^ 

 and flower and leaf buds in mango.^^ In the Hawaiian Islands acetylene has 

 been used to induce earlier flowering of pineapples and to spread the 

 harvest over a greater period of the year. Use of these gases on tobacco ^' ^s 

 during fermentation improved the color and smoking quality and increased 

 the rate of fermentation of the leaf. Ethylene, 100 ppm,^^ is said to improve 

 the germination and baking performance of freshly harvested and high- 

 moisture wheat in storage. 



Respiring Plant Tissue Produces Ethylene 



Elmer ^^•'^^ in 1932 observed that apples gave off a volatile substance 

 that inhibited the growth of potato sprouts. A year later Oortwjn 

 Botjes ^^ reported that emanations from apples caused epinasty m tomato 

 plants and horizontal nutation of the etiolated pea seedling. She found 

 that ethylene absorbents would remove the effective gas from apple emana- 

 tions. Gane -^ found that aerobically gro^ving yeast (but not anaerobically 

 growing yeast) produced a substance that prevented the growth of a pea 

 seedUng. The substance was absorbed by bromme water, an ethylene 

 absorber. He showed ^^ that ripe apples in ah- produce a substance that 

 modifies the growth of several kinds of plants. In an atmosphere of nitro- 

 gen the apple did not produce emanations that modified the gro\vth of the 

 pea seedling, but seemed to hasten ripening of bananas. Apples killed by 

 freezing no longer produced effective gases. The effective gases from apple 

 were absorbed in bromine water ^^ and ethylene dibromide was identified 



