250 PLANT GROWTH SUBSTANCES 



the superficial hyperhydric response is further accentuated. The sub- 

 surface cambial activity is initially not much changed, but instead of 

 continuing unmodified for long periods these dividing regions are 

 quickly organized into large numbers of well-defined root primordia. 

 There are usually no stem growing points, although Nobecourt has 

 reported the repeated, sporadic, and unexplained appearance of stem 

 growing points and leaves on his carrot cultures. They are rare or 

 entirely wanting in other laboratories and may indicate no more than 

 a difference in varieties of carrot used. Such rooted cultures are, of 

 course, quite useless as tissue cultures. They do, however, give us im- 

 portant leads in the study of auxin function. These cultures are not 

 merely differentiated, but this differentiation possesses a definitely polar 

 orientation. Moreover it is clear that here large numbers of roots have 

 arisen without the presence of leaves or similar tissues from which 

 rhizocahnes in the classic sense could arise. The function of the hypo- 

 thetical rhizocahne has been taken over, in dramatic manner, by either 

 the heteroauxin itself or, secondarily, by substances arising in undif- 

 ferentiated stem or root tissue under the influence of externally supplied 

 auxin. 



If, finally, the concentration of growth substance is still further in- 

 creased, both polarity and organization are completely lost, and there 

 remains only an exaggerated and pathological hyperhydricity. Enormous 

 superficial cellular vesicles are formed, giving rise to a rapid and isodia- 

 nietric initial enlargement of the explant. These vesicles no longer appear 

 to be capable of cell division so that the initial rapid mechanical enlarge- 

 ment is followed shortly by necrosis and death. This behavior pattern 

 at high auxin concentration is probably one of the facts responsible for 

 the weed-kiUing action of such substances as 2,4-D, although excessive 

 prohferation at lower auxin concentrations and, in the absence of in- 

 creased nutrient supplies, subsequent nutritive exhaustion is undoubt- 

 edly another major factor in weed killing. 



These matters of cell size, cell number, and cell arrangement, of 

 organization versus disorganization, of organization versus gross expan- 

 sion, of continued function versus necrosis, and similar contrasting 

 aspects of behavior can all be shown in tissue cultures to be under the 

 control of growth substances. This is most clearly defined in the Jerusalem 

 artichoke {Helianthus tuberosa) but is also demonstrable in varying 

 degrees in tissue cultures of willow, hawthorn, grape, carrot, and many 



