MODIFICATIONS OF FORM 



185 



to a large size, while the distal part of the leaf is represented only by 

 a minute apical spur between them. In Lathyrus aphaca it is the 



Fig. 138. 



Lower parts of a Potato plant, Solamiin Utberosum. The swollen tuberous stems 



bear correlath'ely small scale-Iea\"es. (.After Baillon, from Strasburger.) 



stipules which become large foliar expansions, while the lamina itself 

 is linear (Fig. 139). In such ca?cs, which might be multiplied in- 

 definitely, extra development of one part is accom- 

 panied by the correlative reduction of another, as 

 compared with normal examples. But there is 

 no exact numerical ratio that can be put upon 

 the proportions.- They suggest in general 

 terms, rather than with any exactness, that 

 the excessive expenditure "from the total 

 amount of available material on one part 

 leaves a deficiency for others. There is no 

 doubt that this principle of correlation has 

 had very wide application in determining the 

 adult proportions of parts in plants. 



Correlation is, however, merely a concomitant 

 of adaptation. It is neither a cause nor an 

 explanation of adaptability, which remains a 

 quite independent problem. It is important to see clearly what is 

 meant when the word " adaptation " is used. It is properly applied 



Fig. 139. 

 Lathyrus aphaca. s, stem; 

 stipules : h, leaf-tendril, 

 size.} (After Strasburger.) 



