PHILIP R. WHITE 251 



Other plants. The responses observed in these plants are cellular, gen- 

 eralized in character, and bear little apparent resemblance to the controls 

 which auxins mediate in plants in nature, although these cellular ac- 

 tivities are undoubtedly primarily responsible for the more specific 

 reactions. 



Much more specific responses can, however, be studied in tissue 

 cultures of certain plants. If roots of dandelion, chicory, or related plants 

 are cut up and the bits placed on moist sand or on a simple nutrient 

 they will form buds on the upper surface and roots on the lower. This is 

 true of almost any explant, no matter how small, if it will survive at all. 

 Addition of auxin to the substratum in progressively increasing con- 

 centrations results in a progressive blocking of bud and root formation 

 together with a considerable increase in the mass of callus formed on the 

 surface. If auxin is applied locally to the upper surface of an explant in 

 the form of an agar block, the blockage of bud formation and increase 

 in callus development can be shown to center around the appHed auxin 

 and to become weaker with distance therefrom. The same result can 

 in part be produced by implanting a preformed bud instead of using an 

 agar block. The presence of the bud not only blocks the development 

 of neighboring buds but also induces formation of a vascular strand which 

 will connect the implant either with roots that may already be present 

 or with newly formed roots. It does not, however, modify the amount 

 of callus formed. This induction of vascular strands will take place 

 through a cellophane sheet. Every indication is that it is mediated by 

 some substance or substances of the general nature of the auxins. 



This kind of approach has been extended by a double use of the tissue 

 culture approach. Kulescha has shown by direct Avena tests that plant 

 tissue cultures contain auxin in concentrations which are directly pro- 

 portional to their rate of growth in culture and their degree of resem- 

 blance to tumors. Normal tissues which will not grow in culture without 

 added auxin have little or none present. Tissues which have been 

 habituated by prolonged cultivation on an auxin medium so that they 

 are no longer dependent on external supplies show moderate levels of 

 auxin content, while sterile crown gall tissues which grow in culture at 

 a rapid rate are found to have a high auxin content. If, now, each type 

 of tissue is grafted into the surface of slices of chicory, the inhibition of 

 buds and the extent of callus formation induced parallel the observed 

 auxin content of the implanted tissues. Normal tissue implants induce 



