I04 THE TLANT 



A primary requisite I'or any method for measuring adjustment is that it 

 be appHcable to field conditions. Many instruments for measuring trans- 

 piration, for example, are valueless, not because they are inaccurate, but 

 because the plant studied is under abnormal conditions. To avoid the latter 

 is absolutely necessary, a fact which makes it peculiarly difficult to devise a 

 satisfactory field method. After the latter has been found and applied, it 

 becomes possible to check other methods by it, and to give them real value. 

 The final test of a field method is three-fold: (i) the plant must be studied 

 while functioning normally in its own habitat; (2) the method must give 

 accurate results; and (3) it must permit of extensive and fairly convenient 

 application in the field. Until methods of this character, some of which 

 are described later, have been employed for some time, it is' impossible to 

 connect definite intensities of factor stimuli with measured amounts of 

 adjustment. Ultimately, it seems certain that researches will regularly take 

 this form. 



Adaptation is primarily indicated by changes in the arrangement and 

 character of the cells of the plant. Since these determine the form of each 

 organ, morphology also furnishes important evidence in regard to the 

 course of adaptation, but form can be connected certainly with adjustment 

 only through the study of cellular adaptation. In tracing the modifications 

 of cell and of tissue, the usual methods of histology, viz., sectioning and 

 drawing, suffice for the individual. It is merely necessary to select plants 

 and organs which are as nearly typical as can be determined. The ques- 

 tion of quantity becomes paramount, however, since it often gives the clue 

 to qualitative changes, and hence it is imperative that complete and accurate 

 measurements of cells, tissues, and organs be made. These measurements, 

 when extended to a sufficiently large number of plants, serve to indicate 

 the direction of adaptation in the species. They constitute the materials for 

 determining biometrically the mean of adaptation for the species and the 

 probable evolution of the latter. In its present development, biometry con- 

 tains too much mathematics, and too little biology. This has perhaps been 

 unavoidable, but it is to be hoped that the future Avill bring about a wise 

 sifting of methods, which will make biometry the ready and invaluable 

 servant of all serious students of experimental evolution. This condition 

 does not obtain at present, and in consequence it seems unwise to consider 

 the subject of biometry in this treatise. 



149. Plasticity and fixity. As the product of accumulated responses, 

 each species is characterized by a certain ability or inability to react to 

 stimuli. Many facts seem to indicate that the degree of stability is con- 

 nected with the length of time during which the species is acted upon by 



