382 PLANT GROWTH SUBSTANCES 



Similar disorganized growth was observed by de Ropp (13) in frag- 

 ments of sunflower stem tissue cultured on agar containing i mg. per 

 liter of lAA. After a week on this medium the fragments lost their 

 original structure and developed into shapeless, semitranslucent tissue 

 masses. These tissue masses resembled in outward appearance the bac- 

 teria-free crown -gall tumor tissue previously isolated from sunflower by 

 White and Braun (45). Subsequent culture of these tissues on a medium 

 devoid of lAA showed that the change in growth pattern was not 

 permanent. After about six months on this medium the shapeless tissue 

 masses differentiated into roots. White and Braun (45) published similar 

 findings in relation to the action of indoleoxaloacetic acid on plant tissues. 



In the experiments quoted above the application of lAA to plant 

 tissues did not result in a permanent change in their pattern of growth. 

 Gautheret (18), however, has been able to isolate from carrot tissue, 

 cultured for several years on a medium containing o.i mg. per liter of 

 lAA, a strain of tissue capable of growing on a medium free of lAA 

 without any reduction in its rate of growth. This strain of tissue was not 

 only altered as regards its reaction to lAA, it was also changed in external 

 appearance, having become friable and translucent instead of compact 

 and opaque. Gautheret called the change which had taken place in this 

 tissue "accoutomance" which can be translated as habituation. The 

 tissue which had undergone this change he referred to as habituated 

 tissue. 



The phenomenon of habituation was also studied by Morel (30). 

 Tissue cultures of Virginia creeper which had been grown on a medium 

 containing 0.3 mg. per liter of napthaleneacetic acid (NAA) were trans- 

 ferred to a nutrient devoid of this substance. They ceased growing, but 

 after a period of 14 months in one of the six cultures and on a limited 

 portion of the tissue a callus developed which grew in the absence of 

 NAA, although the rest of the culture remained unchanged. This callus 

 on transfer continued to grow on a medium devoid of NAA. 



It is diflicult to explain this observation of Morel's by assuming 

 that the NAA itself had a mutagenic action on the tissues; nor does it 

 seem likely that anything in the nature of adaptation had occurred. The 

 phenomenon can be more adequately accounted for by assuming that a 

 somatic mutation occurred spontaneously in a small part of one of the 

 tissue fragments used. This would explain why habituation occurs 

 sporadically rather than regularly when tissues are grown in the presence 



