264 BOTANICAL GAZETTE [october 



and not fully developed, growth of the parts above may be retarded 

 slightly for two or three days, but soon proceeds normally. These 

 facts do not support the view which Loeb has advanced that the 

 inhibiting action of the growing tip and of other parts on buds is 

 due to the transportation of inhibiting substances through the 

 regular channels of transportation in the plant. In these experi- 

 ments such transportation is not appreciably or only very slightly 

 affected, yet the zone of low temperature is much more effective 

 as a means of physiological isolation than mechanical compres- 

 sion or partial section of the petiole, except when these involve the 

 greater part of the petiole tissues. Unless we assume that the 

 hypothetical inhibiting substance in some way is rendered inactive 

 by the short cooled zone, we must conclude that the dominance 

 of the growing tip and of other regions over a particular leaf is 

 not dependent upon the flow of substances through the vascular 

 bundles to the leaf, but rather upon some sort of action which is 

 dependent upon the physiological activity of the cells. When this 

 activity is inhibited by the low temperature, the action is blocked, 

 unless and until some degree of acclimation of the cooled zone 

 occurs. Such acclimation occurs very readily in the bean seedling, 

 and in many cases a temperature which at first serves as a block 

 becomes ineffective after a few days. In short, the experiments 



■ 



indicate that the physiological dominance of one region over 

 another in these plants is dependent on some sort of effect trans- 

 mitted physiologically through the living active protoplasm, rather 

 than upon substances transported by the flow of fluids. 



Loeb appears not to distinguish clearly two different aspects of 

 the relations of parts: the one which is concerned with the condi- 

 tions that prevent or permit the initiation of development and 

 growth in a subordinate part; the other which is concerned with 

 the amount of growth or development of the part which may occur 

 after its initiation. Nutritive factors may play a large part in 

 determining the amount of growth of buds, but there are no 

 reasons for and many against maintaining, as Loeb did in earlier 

 papers, that they initiate it. Again, the mass of shoots and roots 

 developing from an isolated Bryophyllum leaf may show a certain 

 proportion to the size of the leaf, since the amount of certain 













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