THE RANGE OF DOMINANCE 159 



have already developed do not change their direction 

 of growth when the chief growing tip is cut off; only 

 those which develop after the operation react, but they 

 or some of them develop as main instead of lateral roots 

 and later themselves give rise to lateral roots. If the 

 outgrowth of new roots near the cut surface is inhibited 

 after the removal of the main growing tip by inclosing 

 this region of the main root in plaster, roots which arise 

 above the inhibited region may react by growing more 

 directly downward, provided they are not too far away 

 from the cut surface (Fig. 91). The lateral roots which 

 react in this way to the absence of the main growing 

 tip resemble more or less closely the main root in their 

 later development. When the growing tips of all roots 

 are cut off, adventitious roots arise, usually in large 

 numbers and without any definite order, on the parts 

 remaining. Evidently the relation between the con- 

 stituent parts of the root system is a relation of domi- 

 nance and subordination like that in the stem system. 



The root system as a whole seems to exert an inhibit- 

 ing influence on the development of roots in other parts 

 of the plant. When the whole root system is removed 

 or its metabolic activity inhibited, new roots usually 

 develop from the basal region of the stem if external 

 conditions permit their growth there; if not, they may 

 appear higher up on the stem. The propagation of 

 plants by cuttings depends on this ability to produce 

 roots on the stem in the absence of the root system. In 

 an experiment described by Goebel and represented 

 diagrammatically in Fig. 92, a bean seedling was placed 

 in nutritive solution, b, which was kept at low tempera- 

 ture, whereby the activity of the root system was largely 



