A QUARTER-CENTURY OF PLANT PHYSIOLOGY 9 



and recent advance of plant physiology has had much to do 

 with the measurement of the different members of these two 

 groups and with the study of their relations. The morphological 

 study of internal structures led to the consideration of the 

 physiological processes by which these structures came into 

 being, and it looked for a time as though a new branch of botani- 

 cal science might thus arise. Goebel's later writings, on organ- 

 ography and experimental morphology, illustrate this movement. 

 Since, however, experimental morphology is but the morphologi- 

 cal aspect of the physiology of growth and development, it 

 seems undesirable to create a new branch of science for this study, 

 which appears recently to have become merged into certain 

 aspects of ecology and physiology. Nevertheless, the relation 

 of external and internal conditions to growth, as well as to other 

 internal processes, is now occupying the time of many workers, 

 and promises to be an important part of physiology in the 

 future. 



This study of the sequence of internal conditions or states, 

 as they follow one another in plants growing under natural con- 

 ditions, now embraces all stages from the simply descriptive 

 to the rather precisely quantitative. The more quantitative 

 work along these hnes also necessarily includes careful measure- 

 ment of the controlling conditions. 



Artificial and, natural conditions. Along with this advance 

 toward a causal intei^pretation of the natural processes of plants 

 there has developed another aspect of the same kind of study, 

 in which the older reverence for natural or ''normal" phenomena 

 has largely disappeared. Not very long ago that which was 

 usual in nature as we knew it was called normal and we were led 

 to think that a knowledge of what occurred in plants under 

 so-called abnormal conditions was nearly useless or even un- 

 desirable. We have learned, however, that the range of con- 

 ditions offered by nature does not generally happen to be great 

 enough to allow adequate experimental interpretation of plant 

 processes, and many kinds of artificial conditions have come 

 to be employed in physiological experimentation. One of the 

 great advantages of artificial experimental conditions lies in 



