598 VV. S. Hillman and IF. K. Pwves 



sucrose promotion, while the increments obtained in buffer alone, 

 or, again, with GA or lAA, are unaffected by light. 



This somewhat extended account is presented solely to indicate 

 that it may be unwise to regard the elongation of pea sections, or 

 other organs, as homogeneous and limited by a single system through 

 which all effects are to be explained. It behaves under certain condi- 

 tions as if it consisted of separate "components" which add or sub- 

 tract to give the total but frequently do not interact. This of course 

 does not establish the objective reality of such components. In this 

 connection it is worth recalling that a two-phase mechanism for root 

 cell elongation, in which one phase is promoted while another is in- 

 hibited by auxin, has been proposed by Burstrom (3). Further investi- 

 gation along analogous lines might uncover these and other phases 

 in pea sections as well, and help provide data with which to judge 

 the reality of the components mentioned here. It is well to remember 

 that the only observations made in most work of this kind are on 

 length or weight changes, with no close examination of histological 

 or cytological changes. One cannot assume, for example, that two 

 substances have the same action simply because each can cause a 2 

 mm. increase over the control. Although the additivity of various 

 effects is often imperfect in the presence of optimal lAA, and al- 

 ways in the presence of superoptimal levels, as observed elsewhere 

 (1,5,11) and as is evident in the few data presented here, it may 

 still be useful to consider possible interpretations for independent 

 elongation components on the assumption that the interactions are 

 secondary. 



Returning specifically to GA and lAA, this independence may 

 arise from their action (at least when promoting growth) in different, 

 spatially-separated systems. They may be limiting to the growth of 

 different groups or types of cells within a single section. Alternatively, 

 there may be, within a single cell, regions whose elongation, expan- 

 sion, or differentiation is limited by these two different factors, and 

 others as well. 



Some evidence for this sort of view is given by the different re- 

 sponses, mentioned earlier, of apical and subapical tissues to GA and 

 lAA. The two substances may affect different stages in cell develop- 

 ment. Strong support for such a view can be foinid in "Wareing's (15) 

 report on the effects of GA and lAA on cambium in trees. Although 

 the two synergize in promoting development, GA specifically pro- 

 motes cell division, and lAA, differentiation; together they promote 

 the formation of a large zone of normal wood, but their actions are 

 nevertheless qualitatively distinct, and there is no reason to suppose 

 that only a few biochemical steps intervene between them. Yet if 

 growl h had been measured only in some gross quaniitaiive sense sudi 



