DOMINANCE AND PHYSIOLOGICAL ISOLATION 311 



ated in some way with cell metabolism and so with physiological gradient 

 pattern seems to be indicated by the evidence. 



As regards dominance of a stem tip, it has been found that in Viciafaba 

 seedlings the stem tip produces the most auxin, the leaves less, and dor- 

 mant axillary buds almost none ; but these buds become producers when 

 they develop. Auxin in agar applied to the cut end of the stem after re- 

 moval of the tip is as effective in inhibiting axillary buds as the tip.^ 

 Further experiment has given similar results with many other plants 

 and with various auxins and substances showing auxin action. The mech- 

 anism of inhibition of axillary buds is still obscure. There is some evi- 

 dence that auxin effect differs according as it moves with, or against, the 

 polarity of the axis, being inhibitory when applied basally and moving 

 acropetally and having the opposite effect when applied apically. If it 

 passes from the stem into axillary bud axes, it moves acropetally in these 

 axes. Certain lines of experiment indicate that inhibiting action of auxin 

 may be indirect, through its effect on other factors.^ 



Physiological isolation of axillary buds can be brought about by inclos- 

 ing the stem tip in an atmosphere without oxygen or in plaster, the tip 

 remaining alive for some time under these conditions but auxin produc- 

 tion being presumably inhibited. A zone of low temperature about the 

 stem between tip and buds is effective in blocking dominance (Child and 

 Bellamy, 1919, 1920). On the basis of these experiments it was suggested 

 that a transmissive factor of some sort might be concerned. However, in 

 the light of the data on the role of auxin in dominance and those showing 

 that auxin transport is almost stopped by chilling a zone of stem below 

 5° C. (Cooper, 1936), blocking of dominance by low temperature appears 

 to be a blocking of auxin transport; but these experiments also suggest 

 that cell metabolism is in some way concerned in the transport. 



This dominance of stem tips and growing leaves over buds which have 

 already attained a certain developmental stage is a secondary, not a pri- 

 mary, type of dominance involving relations in a multiaxiate pattern al- 

 ready present. In the higher plants new axes, with leaf primordia as their 

 first developmental expression, are localized in an orderly spatial pat- 

 tern within the embryonic tissue of the tip itself. Each new bud ap- 

 pears at a certain distance from the apex of the tip and in a definite 

 spatial relation to other buds already present. This spatial pattern shown 



3 Thimann and Skoog, 1933, 1934; Skoog and Thimann, 1934. 



■< See the discussion in Went and Thimann, 1937, chap, xii; also Went, 1936; Le Fanu, 1936; 

 Snow, 1937. 



