446 



SCIENCE 



[N. S. Vol. XXX. No. 770 



cism of this paper of Stahl's which may be 

 suggested is that he does not appear to take 

 this small proportion into account. 



The color of plants is due to the translucent 

 screen of chlorophyll which absorbs the less 

 and the more refractive rays in sunlight, but 

 does not absorb the green and yellow rays in 

 anything like the same proportion. " Hence 

 leaves appear yellowish-green because the 

 greater part of the red, orange, blue and vio- 

 let are absorbed by the pigments of the 

 chromatophores." Greater absorption would 

 increase the risk of injuring the leaves by 

 overheating. Under present conditions the 

 absorption of energy from direct sunlight ex- 

 ceeds the amount used in food manufacture. 

 If, however, the proportion and the amount of 

 CO, available were greater, a larger propor- 

 tion of the energy absorbed from direct light 

 would doubtless be used, converted into work, 

 in the manufacture of food, and the possibility 

 of overheating would be less. In diiluse light, 

 on the other hand, the available energy is less 

 while the supply of the food materials re- 

 mains the same. That the energy supply may 

 be disproportionately small is obvious. Stahl 

 sees, therefore, in the chlorophyll pigments a 

 means of absorbing a due proportion of the 

 energy available in diffuse light. He then 

 proceeds to consider the effect of the atmos- 

 phere on sunlight, both the absorption of rays 

 of certain sorts and also the diffusion of what 

 remains. 



The majority of botanists live in an atmos- 

 phere to which, besides the natural addition 

 of water-vapor, unnatural additions are con- 

 stantly made, namely, smokes and dusts of 

 various kinds. These three additions, water- 

 vapor, smokes and dusts, increase the amounts 

 and somewhat change the proportions of sun- 

 light naturally absorbed by the atmosphere. 

 One need only mentally contrast the atmos- 

 phere of Pittsburgh, London and Leipzig with 

 that of Italy, Arizona and California to realize 

 how true this is. The quantity and the quality 

 of the light reaching the earth's surface in 

 these different places is affected accordingly. 

 Natural air absorbs qualitatively and quanti- 

 tatively less than unnatural air. Stahl claims 

 that plants have adapted their color, their ab- 



sorbing agents, to light naturally impoverished 

 in its passage through pure air. The color of 

 leaves is due to a mixture of yellow and 

 green pigments, complementary to the domi- 

 nant colors of the lights in nature. The yel- 

 low and orange components, consisting mainly 

 of carotin, are complementary to the blue 

 light of the sky; the green components are 

 complementary to the red and orange which 

 impress us as predominant only when the sun 

 is low, early or late, and its light traverses the 

 atmosphere.' This is Stahl's main thesis, to 

 which he recurs again and again. 



After this study of the relations of the 

 chlorophyll pigments to the composition of 

 ordinary sunlight there follows a discussion 

 of the adaptations of plants, aquatic as well 

 as terrestrial, to the illumination. These 

 adaptations or adjustments are to the physical 

 as well as chemical properties of sunlight, to 

 heating as well as to food manufacture. It is 

 pointed out that the physical effect of intense 

 illumination may consist in overheating the 

 protoplasm itself or in producing excessive 

 evaporation, which Stahl calls transpiration. 

 These are guarded against in a variety of 

 ways interestingly described. The reaction of 

 the chromatophores themselves to various in- 

 tensities of light is shown, by reference to 

 Stahl's own earlier work and to the work of 

 others, to consist in changes in the position of 

 the chromatophores and in a " regulation " of 

 the quantities and kinds of pigments in them. 

 Thus Stahl describes the changes of shade or 

 color in insolated parts of plants — in the ordi- 

 nary green land plants, Fuciis and other brown 

 sea-weeds living between the tide-marks, the 

 green algae of fresh and salt water, of the sur- 

 face or below, the peculiar blue-green algae 

 which live on mud, etc. 



Then comes a study of etiolation, the turn- 

 ing white or the remaining white of plants or 

 plant-parts in the dark. This phenomenon has 

 so frequently been the subject of observation 

 and reflection that, each time it is mentioned, 

 it becomes clearer how little is really known 



' Stahl's words, " durch das triihe Medium der 

 Atmosphere," I find myself unable to translate 

 exactly. 



