A primary requisite for any method for measuring adjustment is that it be applicable to field conditions. Many instruments for measuring transpiration, 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 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 question 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 contains 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.