MODIFICATIONS OF FORM 



219 



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

 stipules which become large foliar expansions, while the lamina itself 

 is linear (Fig. 148). 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 

 a very wide application in determining the 

 adult proportions of parts in plants. 



Correlation is neither a cause nor an 

 explanation of adaptabilitv, which remains a Lathyrus Aphaca. s, stem; 



r . . «, stipules ; b, leaf-tendril. 



quite independent problem. It is important (i size.) (After strasbm-ger.) 

 to see clearly what is meant when the word "adaptation" is used. 

 It has been used by some biologists to indicate those special modifica- 

 tions of the plant which arise in relation to the environment. In another 

 view all plant structures are to be regarded as the expression of the 

 specific hereditary substance during growth and development under 

 a certain set of conditions. The structures so developed may, or may 

 not, be adaptations. Other things being equal, those structures which 

 are advantageous to the plant will tend to be perpetuated by the 

 process of natural selection. Thus although many modifications in 

 plant structure appear to be closely adapted to the environment it 

 is not necessary to say that they actually arose in relation to that 

 environment. The advantage which certain features confer upon the 

 plants that show them often appears obvious enough. But it should 

 be realised that their recognition as adaptations is no more than an 

 assumption : it is rarely well founded. This applies to many of the 

 peculiarities of form discussed in this chapter. They have been 

 grouped under the heading of adaptations for convenience. The study 

 of " adaptation " is an attractive phase of biology. But it has led to 

 much facile or even sentimental writing, which has in it little of the 

 scientific spirit, and still less of true scientific method. 



