OTHER ACTIVITIES OF AUXINS 221 



it about the same velocity as that found for auxin in the 

 coleoptile (Chapter VI). 



In trees the activation of cambial divisions by auxin 

 has been studied by Soding (1936a), who showed that inser- 

 tion of a crystal of indole-acetic acid into the cambium of 

 woody twigs gives rise to a rapid growth of new secondary 

 wood, which in the willow was up to 1 mm. wide; there was 

 also some new production of cortex. The effect, which is 

 due entirely to cambium activation, was only visible down 

 to about 3 cm. below the point of application (cf. Rogen- 

 hofer, below). An interesting fact which has been described 

 by various investigators, e.g. Coster (1927-1928) and 

 Gouwentak and Hellinga (1935), is that wounding alone, 

 without auxin addition, definitely produces some cambial 

 stimulation; since, so far as we know, wounding will not 

 produce any auxin, but will be more likely to destroy it 

 (cf. lY A), this indicates that some other substance, prob- 

 ably of the type of the lepto-hormone of Haberlandt (see 

 II E) is here involved. This fact, taken together with the 

 relatively short distance of movement in Soding's and 

 Snow's experiments, seems to raise the possibility that the 

 cambial stimulus, which in nature travels very far, may not 

 be entirely explained by the role of auxin. However, Avery, 

 Burkholder, and Creighton (1937) have determined the dis- 

 tribution of auxin in twigs, following bud development, 

 and find a parallelism between the downward spread of auxin 

 and that of cambial activity. 



As was stated above, the action of auxin on roots also 

 gives rise to cell division. The thickening of roots produced 

 by moderate concentrations of auxin (cf. Chapter IX) is 

 largely due to enlargement of the cortical cells, but it is 

 usually accompanied by cell divisions either in the cambium 

 (Jost, 1935a) or in the pericycle (Thimann, 1936) or both. 

 Divisions in these layers give rise to lateral roots, which may 

 be produced directly as a result of auxin application (cf. 

 XI F), and at physiologically low concentrations. The 

 induction of divisions in the stem or root pericycle is evi- 



