400 THIMANN 



to the production of auxin by buds and young leaves and its polar transport 

 down the stem. It is not excluded that the cambium itself may produce 

 auxin within the stem also. Another example is the cell proliferation that 

 occurs in galls, tumors, and nodules, which can sometimes be ascribed to the 

 overproduction of auxin by the parasite, though the elaborate structural pat- 

 terns of many insect galls suggest that probably numerous factors are con- 

 cerned. Galls and nodules certainly contain an excess of auxin, and in some 

 cases galls can be produced by applying auxin, or better, by the combination 

 of auxin and kinetin. Furthermore, the parasites causing galls have in several 

 instances been shown to produce auxin in culture, but because this property 

 is shared by other non-parasitic microorganisms, it seems that it is not the 

 auxin-forming (or kinetin-forming) property alone, but the combination of 

 this with the ability to enter and survive in plant tissues, that is the critical 

 factor in parasitism. In the case of crown gall, the galls continue to grow 

 and will give rise to new galls when pieces are transplanted, even though the 

 bacteria which started the gall have all disappeared. This is interpreted to 

 mean that the role of the bacteria was primarily to stimulate the plant cells 

 to produce, and go on producing, the auxin and/or kinetin. Such an induced 

 change in the host cells has been ascribed to a special "tumor-inducing 

 principle," but this matter is still under investigation. 



The practical result just mentioned is the development of the whole new 

 field of plant tissue cultures. Gautheret and Nobecourt, both in France, 

 discovered almost simultaneously in 1937 that isolated tissue fragments 

 would grow indefinitely if traces of an auxin were added to the medium. 

 Recently a number of additional growth-promoting or -controlling factors 

 have been isolated from various sources, including especially coconut milk. 

 These findings have led to a marked revival of experimental morphology 

 and anatomy, which is beginning to make comprehensible many of the details 

 of tissue formation and differentiation. The discovery that when tissues have 

 been cultured for a time with auxin they sometimes develop the ability to 

 grow without it, and now prove to contain some auxin, may throw light on 

 problems of tumor development and perhaps be of interest to workers with 

 animal diseases. 



Promotion of cell division undoubtedly plays a part in another dramatic 

 effect of auxin — the initiation of roots on stems. It was long known that 

 buds or young leaves on the stem promote the formation of roots at the base 

 of cuttings, and this action was at first ascribed to a root-forming hormone; 

 later on there was some surprise when the postulated hormone turned out 

 to be identical with auxin. The discovery was at once applied by nurserymen 

 for the large-scale rooting of cuttings, and indeed this was, in order of time 

 (1935), the first of the horticultural uses of auxins. The first sign of root 

 initiation is normally given by divisions in the pericyclic layer or close to it, 

 or in multiseriate ray tissue, though on various occasions with cuttings of 



