424 PLANT PHOTOPERIODISM 



PRINCIPLES OF STUDY 



The features of the photoreaction are determined from measure- 

 ments of physiological response. The methods are to be contrasted with 

 the usual in vitro photochemistry. First, the functional relation of the 

 response to the degree of pigment intraconversion in the photoreaction 

 is not immediately known and need not be linear. This is like studying 

 the photoreduction of methylene blue with ferrous iron by use of a 

 buret, with a nonlinear and unknown marking for titration of the iron. 

 An approach can be made, however, to finding the function, and con- 

 siderable progress is possible without knowledge of its form. Second, 

 each biological object — the plant to flower or the seed to germinate — 

 varies in the stimulus required for the response, much as does each 

 molecule in its reactivity in vitro. In the plant the essential result and 

 the variable sensitivity affect the observations. 



The several differences between in vitro and in vivo photochemistries 

 increase the experimental labor. In work on lettuce and Lepidiwn 

 seed, germination of over a million seeds of each kind were counted 

 in lots of 100 and divided into more than a hundred experiments for 

 each seed type. With photoperiodic control of flowering, a variance 

 approximately equal to that of seed germination can be obtained with 

 1% as many plants. A given variance is attained with roughly equal 

 areas of exposure to radiation — a square centimeter of leaf area on one 

 plant or a square centimeter of seed (possibly 200 lettuce seed). This 

 is probably a result of requiring equivalent volumes of reactant systems 

 or equal numbers of responsive cells. 



REVERSIBILITY 



Before elaborating the details of the photoreaction as deduced from 

 experiments, two simple but immense consequences of the photoreac- 

 tion are to be emphasized. First, the responses potentiated by the 

 photoreaction are photoreversible. This has been illustrated by Dr. 

 Toole, Dr. Borthwick, and Dr. Downs in their discussions, and it is 

 hardly necessary for me to give further examples in detail. This photo- 

 reversibility has the exceedingly important feature of establishing the 

 equivalence of the control of many diverse and unrelated responses 

 with a certainty rarely attained in physiological work. It also allows 



