CHAPTER XIX. 



AMPLIFICATION AND REDUCTION. 



WHEREVER the attempt has been made by studying plants as they are 

 seen living or fossil, to link them together into some coherent evolutionary 

 story, theories of phyletic amplification and reduction have been freely 

 employed. Sometimes greater prominence has been given to the one, 

 sometimes to the other. 



The term amplification is used to embrace all changes leading to 

 increased formal or structural complexity of the plant. It is necessary 

 to distinguish between those changes of amplification which are indivi- 

 dual and those which are phyletic. The former are the result of development 

 traceable in some degree to the direct effect of external circumstances upon 

 the individual organism : phyletic changes of amplification are those trace- 

 able as inherited from generation to generation in an advancing stock. 

 But in actual practice it is difficult to discriminate between them, for the 

 two are not different in kind : in point of fact it is only on a basis of 

 comparison that phyletic amplification can be recognised : it may indeed 

 be held to be a perpetuation of such individual amplifications as are 

 transmitted in descent. 



In the simplest cases amplification may be a consequence of mere 

 non-localised distension of the plant-body; but in all more complex 

 organisms growth is localised and continued at certain initial points, 

 which thus take the character of apical cones, and define the polarity 

 of the resulting structure. Or, furthermore, a secondary activity may 

 appear in some intermediate zone, and new tissue be there intercalated : 

 the common and obvious type of this is where increase in length or 

 in width of the whole organ is the result, and that is what is usually 

 understood as intercalary growth. But it would also include those develop- 

 ments of vascular tissue designated as secondary thickening. Closely 

 associated with apical growth, but less commonly with intercalary growth, 

 is the initiation of new apical points, which lead to the various modes 

 of branching of parts. This has also played an important role in the 

 origin of complex plants as we see them. 



