246 TRANSLOCATION IN PLANTS 



streaming in cells leading backward or forward to possible 

 sources of supply, one can readily formulate a hypothetical 

 scheme that seems to offer a fairly simple basis for partly 

 explaining phenomena of dominance and regeneration. 

 If, for example, the chain of active cells leading to a 

 dominant shoot is severed, then other growing points are 

 no longer in competition with the dominant shoot. The 

 materials moving to the dominant region are stopped in 

 their movement and accumulate in sufficient quantity to 

 initiate the activity of one or more other growing points 

 and again one of these, becoming for one reason or another 

 slightly more active than its competitors, soon gains the 

 dominance. 



On the same assumption, one might explain an increased 

 activity of two complementary tissues such as root and 

 shoot. If a given shoot gains supremacy and is drawing 

 upon the major part of the supply of soil nutrients through 

 increased protoplasmic streaming in the connecting phloem 

 cells, this same increased activity should not only favor 

 the movement of solutes to this particular shoot but should 

 also favor the transfer of carbohydrate from the leaves of 

 that shoot to the roots. Those particular roots, therefore, 

 that are perhaps in a rich pocket in the soil not only would 

 favor the growth of the particular shoots with which they 

 are connected but should also in turn be favored them- 

 selves, for the quickened activity of the conducting cells 

 should carry the solutes to the tops faster, and should also 

 return more rapidly the carbohydrates which favor root 

 growth. 



Loeb (1915) observed that, if any part of the root of a 

 Bryophyllum plant was in water, root development of all 

 other parts ceased, whereas if the entire plant was sus- 

 pended in moist air, roots would develop at many points. 

 He explained this lack of widespread root development as 

 due to an inhibiting effect of root pressure developed by 

 the roots in water. The same explanation had also been 

 given by others. A better explanation would seem to be 

 that when one set of roots touches water, their growth 



