140 The Phenomena of Morphogenesis 



potentials in relation to morphological polarity and polar regeneration 

 must remain for the present rather hypothetical because of the uncertain- 

 ties which still exist as to the nature and origin of the potential differences 

 themselves ( p. 361 ) . The phenomena of organic polarities have so many 

 similarities to electrical ones that it is tempting to explain the former 

 entirely in terms of the latter, but there is insufficient evidence as yet for 

 such a simple solution of the problem. 



Physiological gradients of various kinds, particularly metabolic ones, 

 and their significance have been extensively discussed by Child ( 1941 ) . 

 Such gradients are along the major axes of the organism, and indeed 

 their existence is thought by some to establish these axes and to be a 

 major factor in the origin of polarity. Child believed that they arise early 

 in development as the result of some unilateral difference in the environ- 

 ment and that, once established, they persist. He points out that they 

 often can be obliterated or redirected by external differentials and infers 

 that they are of great importance in determining patterns of development. 

 The inherent properties of protoplasm, unable alone to control develop- 

 ment, produce their morphogenetic effects through specific reactions to 

 such axial gradients. Prat ( 1948, 1951 ) has reviewed the relations be- 

 tween physiological and histological gradients. 



Gradients in respiratory activity such as have so often been described 

 in animal axes are found in plants ( Wanner, 1944 ) . Ball and Boell ( 1944 ) , 

 however, have shown that in some plants the rate of respiration at the 

 meristematic tip is less rapid than in the zone immediatelv behind this 

 (p. 73). Hurd-Karrer (1926) found that in corn stalks the minimal con- 

 centration of solutes is in the basal internodes and increases upward, a 

 gradient reported by others for leaves at different levels in a tree. In plant 

 exudates there is a concentration gradient with the highest values near 

 the apex (Tingley, 1944). The proportion of ash to dry weight in herba- 

 ceous plants was shown by Edgecombe ( 1939 ) to increase toward the tip 

 of the plant. Many other examples might be cited. 



These gradients are often related to translocation of solutes and food 

 and thus to localized and differential growth. Hicks (1928a, b) found 

 that nitrogen tends to move toward the morphological tip of a stem and 

 carbohydrates toward the base, even in inverted shoots, so that a gradient 

 in C/N ratio results in the stem. She believes that this may be respon- 

 sible for the phenomena of polarity, but this may be a parallelism rather 

 than a causal relation. 



The unidirectional flow of nutrients to particular "centers of attraction" 

 in shoots, roots, leaves, and other structures has been emphasized by 

 Goebel as of particular importance in regeneration and other phenomena 

 of development. What causes the establishment of such centers and thus 

 directs the location of growth is a question closely related to that of 



