STRUCTURES RESEMBLING ORGANIC GROWTHS. 163 



metal with that of the sense-organs in a regenerating portion of 

 a medusa. The influence of the nucleus in cell-regeneration 

 and cell-growth is another possible parallel of a more general kind, 

 Cases of reciprocal influence, where two or more different 

 organs exercise mutual control upon one another's growth or 

 development are also frequent in organisms. The development 

 of roots and shoots from the leaves and stem of Bryophyttum, as 

 recently described by Loeb, 1 furnishes very clear illustrations of 

 this phenomenon; the growth of a bud on a piece of stem inhibits 

 the growth of roots on an attached leaf; if the bud is removed, 

 roots grow out; similarly roots growing from the stem inhibit 

 the formation of shoots from a leaf. Such cases resemble the 

 above combination where zinc in contact with a copper wire 

 inhibits the development of filaments from iron also in contact 

 with the wire; this influence is also reciprocal, although zinc, 

 because of its greater solution-tension, has the predominant 

 effect. 2 In the inorganic model the direction and intensity of 

 the electrical current flowing between the precipitate-forming 

 metal and the solution, and hence the rate and the character 

 of the structural development, depend upon the special char- 

 acteristics of the two intersecting circuits. The conditions in 

 the living organism are probably similar in a general sense; 

 and compensations and reinforcements occur, varying according 

 to the character of the connections, the distances and other 

 space-relations, the electrical resistances, etc. Other factors, 

 however, including probably the flow of dissolved materials, 

 almost certainly enter in actual cases of growth and regeneration 

 in organisms. Nevertheless it appears probable that the most 



1 J. Loeb, Botanical Gazette, 1915, Vol. 60, p. 249; 1916, Vol. 62, p. 293; 1917, 

 Vol. 63, p. 25. 



2 Loeb enunciates the rule: "If an organ a inhibits the regeneration or growth in 

 an organ b, the organ b often accelerates and favors the regeneration in a." (loc. 

 cit., 1915, p. 276). This kind of relationship is also exemplified by the mutual 

 influence of zinc ( = a) and iron ( = b) in direct contact with each other. Other 

 well-known general facts of regeneration may be simulated by arrangements of 

 the above kind. Thus the development of a polyp on a tubularian stem inhibits 

 the development of polyps at other regions of the same stem, just as the outgrowth 

 of a shoot on a plant stem inhibits growths from neighboring axils, etc. Similarly 

 the formation of precipitation-filaments from a piece of zinc in contact with a 

 copper wire is checked by the contact of another piece of zinc (or iron, etc.) with 

 the same wire at not too great a distance. 



