DOMINANCE AND PHYSIOLOGICAL ISOLATION 313 



lower plants. For example, the very definite order of appearance, the rate, 

 and the direction of growth of lateral branches of the thalli of many 

 multiaxiate algae, even when the whole thallus is a single cell, as in 

 Bryopsis, indicate presence of a definite spatial pattern involving domi- 

 nance of the apical region of the main axis and physiological isolation at 

 a certain distance from it. Development of new axes from cells that do 

 not normally give rise to buds has often been induced by removal of the 

 apical region in various algae and fungi. In the prothallia of liverworts 

 and ferns dominance of the apical region has also been experimentally 

 demonstrated. Many of these plants also afford evidence of decreasing 

 effectiveness of dominance with increase in distance from the dominant 

 region. 



The so-called "gills" of the mushroom constitute a very definite spatial 

 pattern; but when they are removed, leaving a flat surface, outgrowths 

 may arise from any part of this surface, their localization being entirely 

 irregular and apparently determined by local chance differences in activ- 

 ity. Each outgrowth dominates a certain area about itself, and an out- 

 growth somewhat in advance retards or completely inhibits development 

 of other outgrowths within that area (Magnus, 1906) ; but beyond a short 

 distance it is ineffective. Similarly, when numbers of adventitious buds 

 develop near together, any one which develops more rapidly may retard 

 or inhibit further development of others within a certain distance from 

 itself. Whether dominance in these cases depends on auxin transport 

 within the gradient system of each outgrowth or bud or on some other 

 factor is not known. 



DOMINANCE, BUDDING, AND AXIAL RELATIONS IN 

 CERTAIN COELENTERATES 



The hydra bud appears at the most proximal levels of the body capable 

 of reacting rapidly to physiological or physical isolation. Removal of the 

 apical region proximal to the tentacles accelerates development of buds. 

 In animals in good condition buds appear only after the body has at- 

 tained a certain length, and in a stock in uniform environment this length 

 is fairly constant. Under depressing conditions, in "senescent" and in sex- 

 ual animals, buds often appear at more distal levels than the usual bud- 

 ding zone, and more or less persistent "colonies" may result from delayed 

 separation. After removal of the body distal to the budding zone a bud 

 may inhibit reconstitution of a distal region and, with gradual change in 

 position, replace the part removed. Apparently a developing bud inhibits 



