104 THE PLANT 
A primary requisite for any method for ‘measuring adjustment is that it 
be applicable 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: (1) 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 sttidy 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 will 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 
