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, Solanum tuberosum. The swollen tuberous stems 

 bear correlatively small scale-leaves. (After Baillon, from Strasburger.) 



stipules which become large foliar expansions, while the lamina itself 

 is linear (Fig. 139). In such cases, 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 



'-n 



FIG. 139. 



Lathyrus aphaca. s, stem ; 

 n, stipules ; b, leaf-tendril, 

 (i size.) (After Strasburger.) 





