ALBERT C. HILDEBRANDT 393 



cultures from normal artichoke failed to grow without supplementary 

 growth-regulating substance. A concentration of io~'^ of indole-3-acetic 

 acid, indolebutyric acid or a-naphthaleneacetic acid stimulated cambium 

 formation, and a large callus was formed on the part of the tissue in 

 contact with the medium. At a concentration of io~^ the tissue con- 

 tinued to form cambial layers, but root formation was induced in 

 addition. At io~^ cell division and callus formation were no longer 

 promoted, but a swelling of the cells was favored instead, and finally 

 at io~* giant cells were produced which emphasized the action of the 

 growth-regulating substance on the swelling of the cells. Gautheret (6) 

 also indicated the bud-inhibiting power of the growth-regulating sub- 

 stances. For example, a slice of chicory root incubated on a-naphthalene- 

 acetic acid at a concentration of io~^ produced roots and a large callus, 

 while a slice on media lacking the material produced mainly young 

 shoots. Skoog and Tsui (24) showed similar morphological responses 

 of normal tobacco tissue in vitro. They found that adenine induced bud 

 formation in callus and stem internode tissues. Callus growth and root 

 formation in stem segments were stimulated by a-naphthaleneacetic 

 acid, but bud formation was inhibited. Combinations of adenine and 

 a-naphthaleneacetic acid greatly stimulated cell proliferation and enlarge- 

 ment of all tissues, especially the pith, but it did not result in organ 

 formation. A few studies have dealt with the effects of these materials 

 on tissues of pathological origin, and have compared the responses of 

 such tissues with normal tissues as they grew in vitro. 



Tissues of pathological origin have been isolated from several species 

 and studied in vitro. Tissues isolated from the hybrid, Nicotiana glauca 

 Grab. 9 X M langsdorffii Weinm. cT, provided the first true callus 

 cultures capable of unlimited growth (29). This and related hybrids 

 have been of special interest because of the type of galls (evidently of 

 genetic origin) that are commonly produced at points of injury (16). 

 Such galls have the general appearance of crown galls produced on a 

 wide variety of plants by the bacterium Agrobacterium tumefaciens. 

 Tissue isolated from the apical stem of the hybrid tobacco forms un- 

 differentiated callus when incubated on solid media, but may form leaves, 

 buds, or stems when cultured in a liquid medium. Skoog (22) found that 

 the differentiation in liquid media was completely inhibited by the 

 addition of 0.2 to 10 mg. per liter of indole-3-acetic acid or a-naph- 

 thaleneacetic acid. Concentrations which did not decrease the growth 



