OTHER ACTIVITIES OF AUXINS 227 



(La Rue, 1936). Probably many other pathological out- 

 growths are explicable in terms of auxin — a development 

 which will open important fields in plant pathology. 



D. Miscellaneous Effects 



A number of other effects of auxin have been recorded. 

 Laibach (1933) has described how tendrils of Cucurbita 

 coil up when auxin paste is applied on one side, the reaction 

 not being restricted to the place of application. This, how- 

 ever, is not generally true, for tendrils of Passiflora, similarly 

 treated, show only local curvatures (u). 



The reactions of leaves are of some interest. Laibach 



(1934) found that when Phaseolus leaves are smeared over 

 with auxin in lanoline, they bulge and curl, while Avery 



(1935) found that the midrib of the Nicotiana leaf curves 

 readily away from auxin paste applied on one side. The 

 authors' experiments {u) indicate that auxin causes elonga- 

 tion of the cells of the midrib and lateral veins, but not 

 increase of surface of the mesophyll. This results in dif- 

 ferential growth with bulging of the leaf-blade (particularly 

 marked in Aristolochia) . In varieties like dejorrnis of tobacco 

 (Honing, 1923) or wiry of tomato (Leslie, 1928), mesophyll 

 growth is practically absent in the mature plant, so that 

 the leaves are restricted to a midrib, with the lateral veins, 

 which are still present, contracted against it. Thus the 

 deformis and wiry genes affect only mesophyll growth. On 

 the other hand, Aphids (see Maresquelle, 1935) and ''curly- 

 top" \drus cause curling of the leaf, due to deficient vein 

 growth, while the blade develops normally. Grieve (1936) 

 has indicated that in ''spotted wilt" virus this curling may 

 be due to lack of auxin. It is significant that it is usually 

 associated with reduction in total growth. All these cases 

 fit in with the view that auxin causes elongation of leaf 

 veins, while the growth of mesophyll depends on other 

 factors, such as that postulated by Gregory (1928). 



Certain other effects are not readily explicable on the 

 basis of growth promotion. Thus if the leaf-blade of Coleus 



