Tissue Mixtures 259 



from stock to scion, or in a mutual exchange between the two do the 

 specific qualities of one become transferred, in any measure, to the other? 

 There has been much disagreement about these matters in the past, and 

 final answers are not yet available. 



There are several means by which the two graft partners may affect 

 each other. Of most importance, probably, is nutrition. If a root into 

 which a scion is grafted has less capacity to absorb, store, or utilize nu- 

 trients than do the roots of the plant from which the scion was taken, 

 reduction of size and vigor of the shoot system produced by the scion 

 will result. Many cases have been found where there are such physio- 

 logical differences between stock and scion clones which modify the 

 effect of one on the other, particularly of root on scion. Second, there 

 may be differences between the two partners in the ease with which they 

 translocate water and nutrients. The wood of dwarf rootstocks usually 

 has a much smaller proportion of vessels than normal roots. Differences 

 in phloem transport are probably even more important, as is shown by 

 the dwarfing effect of inverted rings of bark and in other ways. Finally, 

 there may be differences between partners in the amount of auxin in 

 each or in the rate at which it is inactivated. In several herbaceous plants 

 it has been shown that dwarf types are relatively poor in auxin. This 

 may be the case in woody forms, for dwarf types of fruit trees are 

 much branched, an indication that auxin-induced bud inhibition is weak 

 in them. Other growth substances may pass from one partner to the other. 

 It is clear that water and salts from the soil pass from stock to scion 

 and that carbohydrates also pass across a graft union, and in either 

 direction. Certain nutritional changes are thus produced by one on the 

 other. A common practice in producing dwarf fruit trees is to graft 

 scions of standard varieties on stocks which are genetically small and 

 thus have small roots. This reduces the amount of top growth. Most of 

 this dwarfing results from the reduced water supply available from the 

 roots (Colby, 1935), and dwarfing rootstocks also tends to cause earlier 

 cessation of growth in the fall (Swarbrick, 1928). Dwarfing may be pro- 

 duced in other ways than by reduction in root size. Tukey and Brase 

 ( 1933 ) found that where a dwarf variety was used either as a rootstock, 

 intermediate stem piece, or top scion its effect was to dwarf the whole 

 plant. Some dwarfing may also be attributed to defective graft unions 

 and the consequent failure to transfer materials readily (Bradford and 

 Sitton, 1929 ) . Dickson and Samuels ( 1956 ) have studied translocation 

 across a graft union by means of radioactive tracers and find that there 

 is a high concentration of the isotope at the junction of stock and scion, 

 suggesting that the dwarfing effect may be due to a block in the flow 

 of nutrients to the roots. 



But there seem also to be more subtle factors involved. A special kind 



