CHIEF ENVIRONMENTAL CONDITIONS. 137 



surely plays an important role here. Likewise, the development of 

 certain tissues, as of leaf-palisade, are apparently often dependent 

 upon the quality, intensity, and direction of luminous rays, and the 

 relative positions assumed by most ordinary leaves are largely due to 

 the asymmetrical effects of light as such. The ''leaf mosaics" of 

 plant ecology are considered as due to the operation of this condition. 

 Also, the positions assumed by many stems and other parts are 

 primarily due to light conditions. Such morphogenic activities are 

 directed, not by light in general, but only by radiant energy of certain 

 ranges of wave-length. Certain intensities within these ranges are 

 necessary for the usual development of ordinary plants. 



A third effect of light, and the one that is most fundamentally im- 

 portant for all terrestrial life, is the photochemical process called 

 photosynthesis. It is only by the action of a certain range of wave- 

 lengths of radiant energy, within certain limits of intensity, that the 

 production of carbohydrates from water and carbon dioxid may occur 

 in chlorophyll-bearing cells. Fundamental as is the photosynthetic 

 process, the conditions determining its velocity have hardly begun to 

 be studied quantitatively, and this statement is especially true with 

 regard to the light relation. In order to begin a study of the relation 

 of plants to light it will be necessary first to possess some suitable 

 method by which the light conditions may be measured. It is essen- 

 tial that both quality and intensity of the impinging light be de- 

 termined for the different hours of the day and for the different days 

 of the growing-season. The present apparent difficulty of obtaining 

 such measurements of the environmental conditions is surpassed only 

 by its fundamental importance to plant physiology and by its practical 

 bearing upon the problems of ecology and agriculture. 



3. DURATION ASPECT OF LIGHT RELATION OF ORDINARY PLANTS. 



It is obvious that the result, the amount of material change, pro- 

 duced by any of the physiological processes that are dependent upon 

 light must be determined by the duration of the process as well as 

 by the nature of the light conditions determining its velocity. It 

 seems highly probable, also, that mere fluctuation of the light condi- 

 tions, as between daylight and darkness, may have a more or less 

 definite effect upon the development of plants. We are certain that 

 many physiological rhythms depend primarily upon this sort of fluctua- 

 tions. As we have seen, however, the feature of duration is usually 

 the last one to be carefully considered in the case of any environmental 

 condition, and we need not be surprised to note that very little indeed 

 has been accomplished in this direction regarding light influence upon 

 plants. Until we are able to measure and control light conditions it 

 piust be quite hopeless to attempt any but the most superficial con- 

 sideration of this aspect of the general problem of plant control. 



