184 PHYTOHORMONES 



observation that in Commelina the yellow varieties do not 

 root while green ones do, but the difference is not due to 

 nutrition because addition of sugar or peptone does not 

 affect it. 



In the second place the effects of various empirical treat- 

 ments have been studied; Curtis (1918) found that perman- 

 ganate promotes rooting of cuttings, while inorganic nutrient 

 solutions have no effect, and a number of authors have 

 shown that oxygen is necessary {e.g. Zimmerman, 1930, 

 Graham, 1934). Treatment with carbon monoxide, ethylene, 

 and other unsaturated gases stimulates root formation (c/. 

 XI C). Graham and Stewart (1931) and Graham (1934, 

 1936) have studied the optimum experimental conditions 

 for making cuttings of a large number of different plants. 

 The most important factors are (1) time of year at which 

 the cuttings are taken, which differs for different plants, 

 (2) temperature relations of the cutting, and (3) ample 

 watering without interference with aeration. They found 

 that if these factors are considered, practically any plant 

 can be induced to give 90 per cent rooting from stem or leaf 

 cuttings. 



Loeb, in 1917, suggested that root formation in Bryo- 

 phyllum is controlled by a special root forming substance or 

 hormone. About this hormone he made some statements 

 which in the light of our present knowledge seem quite 

 remarkable. Thus, ''In Bryophyllum the hypothetical geo- 

 tropic hormone is associated (or identical) with the root- 

 forming hormone" (1917). Further, "these (bud-) inhibi- 

 tory substances may be identical with or may accompany 

 the root-forming hormones" (1917a). The inhibition of 

 buds is, as will be shown in Chapter XII, also brought about 

 by auxin. Loeb's subsequent change of view from special 

 substances to mass action relations is discussed in II E. In 

 a later study of Bryophyllum, F. A. F. C. Went (1930a) re- 

 turned to the concept of root-forming substances, these 

 substances being considered to be formed mainly in the 

 older leaves. 



