Growth Substances 405 



mechanism is by which the abscission layer is produced or inhibited is 

 not known. 



Sprays of this sort are also used to stimulate rather than inhibit 

 abscission, notably for the purpose of thinning young fruits when too 

 many have been set. How, one may ask, does the same substance act 

 in two such different ways? Evidently a normal growing and functioning 

 organ will produce enough auxin to prevent its abscission. When this 

 production ceases, the organ will drop off unless a fresh supply is avail- 

 able through external application. Anything which checks or deranges 

 normal growth, however, will tend to check auxin production and thus 

 lead to abscission. Sprays of some substances and in certain concentra- 

 tions will tend to do this, and hence their usefulness in the thinning of 

 fruit. An answer to the problem of this double effect has been proposed 

 by Jacobs ( 1955 ) , who has shown that in addition to the inhibiting 

 effect of auxin on abscission there may be a speeding effect produced 

 by auxin formed in young nearby leaves. This stimulates the abscission 

 of a petiole whenever the flow of auxin from its leaf blade is reduced. 

 Abscission is thus controlled by an "auxin-auxin balance." 



The differentiation of more specialized tissues may be stimulated by 

 auxin. Camus (1949) grafted buds of Cichorium to pieces of storage 

 tissue and found that vascular strands began to differentiate just below 

 the bud and continued to develop until they established connection with 

 the vascular tissue beneath. Buds encased in cellophane and inserted 

 into tissues cultivated in vitro produced the same effect, indicating that 

 a diffusible substance, possibly auxin, was involved (Fig. 18-23). 



Of significance here is the work of Wetmore ( 1956 ) on the induction 

 of xylem in callus tissues. Into homogeneous callus maintained in culture 

 from parenchyma cells in the cambial region of lilac, a growing lilac 

 stem apex was grafted by inserting it into a V-shaped cut. After tissue 

 union was effected, strands of xylem began to differentiate into the 

 homogeneous callus tissue below the graft. That auxin was the factor 

 responsible for this is suggested by the fact that when the cut was filled 

 with agar containing auxin but without a stem tip vascular tissue ap- 

 peared below it in the same way, the distribution of the strands depend- 

 ing on the concentration of the auxin. It is significant that only xylem 

 tissue was thus differentiated and not phloem. This was also the case 

 in the regeneration of severed vascular strands in the stem of coleus 

 (p. 242). It may be that the factors which stimulate xylem develop- 

 ment are different from the ones involved in phloem production. 



More profound effects of growth substances on the anatomy of plants 

 have been described. Much of the work here has been done by Kraus and 

 his colleagues at the University of Chicago, who have tried a variety of 

 substances in different concentrations and on many plants. Their gen- 



