218 PHYTOHORMONES 



considerably higher than is Ukely to occur physiologically. 

 The problem is further complicated by the fact, observed 

 by Wilhelm (1930) and by Orth (1934), that the pith cells 

 of Vicia Faba show growth and division on treatment with 

 a variety of extracts and solutions, including sugar solution. 

 Wehnelt (1927) and Jost (1935a) obtained cell divisions in 

 Phaseolus pod parenchyma with 2 per cent levulose, 0.1 per 

 cent NaCl, 0.01 per cent acetic acid, and 0.01 per cent citric 

 acid. The apparent non-specificity of the reaction leads one 

 to suspect that some of these materials may act indirectly 

 by merely setting free the active substance in some way 

 from the tissue, while others, such as the bean juice, really 

 contain the active growth-promoting substance. To which 

 type the action of auxins belongs cannot yet be definitely 

 said. Thus both in the above work, and in that of Laibach 

 and others on swellings (see XIII C), auxins bring about 

 cell divisions in many kinds of tissue only when applied in 

 high concentrations. 



The first stages in the initiation of new roots on stems 

 have frequently been shown to be divisions in the pericycle, 

 and it was shown in XI C that such root formation is 

 brought about, and controlled, by auxin. This fact cannot, 

 however, be taken as proof that auxin controls cell division 

 in general, and since the phenomena of root formation are 

 complex, it will be better to examine first the clearest case 

 of the action of auxin on cell division. 



B. Cambial Growth 



The one type of cell division which appears to be really 

 controlled by auxins under physiological conditions is the 

 formation of, and division in, the cambium. This applies 

 to the cambium of the stem, and to a large extent to both 

 cambium and pericycle of the root. It was first shown by 

 Jost (1891, 1893) that the activity of the stem cambium 

 of dicotyledons is greatly stimulated immediately below 

 the growing leaves. The effect is the same even if they are 

 etiolated, and it is transmitted only in the morphologically 



