THE NATURE OF PHOTOSYNTHESIS 107 



drops again and attains a second and principal maximum. Thereafter 

 increased light intensity has no influence on the rate, so that the curve 

 becomes almost parallel to the abscissa on which the light intensities are 

 plotted. With the exception of the two maxima the curves bear a strong 

 resemblance to the original curves of Blackman in that they are in no 

 sense logarithmic. Miss Henrici's experiments, however, were appar- 

 ently carried out with atmospheric air, so that the limiting influence of 

 the carbon dioxide concentration is clearly evident. These investigations 

 are also discussed under the influence of temperature on photosynthesis. 



Turning now to the effect on photosynthesis of light of different wave- 

 lengths, this subject has for over a century engaged the attention of plant 

 physiologists and has resulted in a most voluminous literature. The re- 

 sults, however, are by no means commensurate with the efforts expended. 

 The problem of the effect of different wave-lengths involves experimental 

 difficulties which at first glance are not apparent and in many cases have 

 entirely escaped the notice of the experimenter. These difficulties are 

 concerned with obtaining a satisfactory source of light, the measure- 

 ment of energy in the different portions of the spectrum, methods of ob- 

 taining light of sufficient intensity of various wave-lengths either through 

 filters or a prism, the relative absorption by the leaf of light of different 

 frequency, temperature effects, as well as devising reliable methods of 

 measuring photosynthetic rates. The most common error is the disregard 

 of the intensity or more properly, the energy of the different wave-lengths 

 employed. Colored glasses and light filters rarely yield monochromatic 

 light ; reliable data on their transmission can be gained only from spectro- 

 graphs. In using prisms consideration must be given to the degree of 

 dispersion of the different spectral regions. 



A perusal of the many different investigations which have been under- 

 taken to determine the effect on photosynthesis and plant growth in general 

 of the various portions of the spectrum lead to the conclusion that the 

 cause for the disappointing outcome of many of these elaborate and costly 

 experiments lies in a disregard of some of the fundamental principles of 

 the physics of light. There is a wealth of information originating in 

 physical and chemical laboratories which has direct application to these 

 problems and cannot be disregarded if intelligible results are to be hoped 

 for. The question of the source of light used in such experiments is of 

 paramount importance. \\'hile it seems desirable often to adhere to nat- 

 ural conditions of illumination by using sunlight, for experimental work, 

 in spite of the high intensity, this source of light is not very satisfactory. 

 The cause is that sunlight, at the surface of the earth, varies both in inten- 

 sity and composition from hour to hour and from day to day. The high 

 intensity of the infra-red rays in sunlight also introduces experimental 

 difficulties. Artificial sources of light have many advantages furthermore, 

 from the viewpoint of regulating intensity, constancy and composition of 

 the light. Only it is necessary that some study be devoted to the physical 

 problems involved, so that the experimenter is thoroughly familiar with the 



