170 THE CAUSE OF SPECIFIC SHAPE 



and the outermost segment-cell of Cladophora and Sphacelaria may take on the 

 functions and character of the apical cell when this has been destroyed or 

 removed : . 



The formation of new xylem on the inner side of a strip of bark and the 

 replacement of a separated strip of the latter are directly due to the activity of the 

 cambium 2 , which is in fact under normal conditions continually producing wood and 

 phloem tissue as the older layers die. No sharp line of distinction can be drawn 

 between regeneration and reproduction, and a particular activity may be regarded 

 as an instance of the one or of the other according to the point of view adopted. 



Regeneration seems to be suppressed in certain cases owing to the 

 power of producing new organs possessed by the plant, for it appears to 

 be more economical and preferable in many cases to produce a new 

 organ than to repair an old one. The character of the regeneration 

 depends also upon the nature of the tissue, and it is produced most easily 

 in highly meristematic tissues, and not at all in fully adult ones. In 

 partially embryonic tissues small wounds are readily repaired by secondary 

 callus-formation, which sometimes results in abnormal growths when the 

 wound is very large. 



Roux 3 applies the term ' post-regeneration ' to cases in which a wound 

 begins to heal some time after the injury has been produced, and in which 

 the process of healing often follows an irregular course. The reacting 

 cells here also are embryonic in character, and possibly a certain time is 

 required to awaken them from a labile condition of dormancy induced 

 in them by the previous conditions. 



PART III 



SYMBIOTIC REACTIONS 



SECTION 48. Changes of Shape due to Symbiotic Interaction. 



The most varied influences may be exercised by symbiotic organisms 

 on one another, according to the nature of the respective organisms and 

 the character of their relationship 4 . In the case of disjunctive symbionts, 

 the metabolic products of the one may interfere with or suppress the other 

 competing organisms, while in those organisms which survive, pronounced 

 changes of shape may result, such as are produced in fungi and bacteria by 

 the presence of various substances in the nutrient medium. 



1 Cf. Magnus, Morphol. d. Sphacelarien, 1873, pp. 13, 18. 



2 Frank, Krankheiten d. Pflanzen, 2. Aufl., 1894, I, p. 70. 



3 Roux, Biol. Centralbl., 1893, Bd. xin, p. 656; Gesammelte Abhandl., 1895, II, p. 894; 

 Driesch, Analyt. Th^orie d. organ. Entwickelung, 1894, p. i seq. 



4 On various types of symbiosis see Ward, Annals of Botany, 1899, Vol. xiu, p. 549. 



