434 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



The absorption of ultraviolet rays by tyrosine has been found by 

 Kober to be markedly increased by an alkaline reaction and, as the 

 above results show, the detoxication of the ultraviolet light by tyrosine 

 solutions is also very greatly increased by an alkaline reaction. 



The results of Harris and Hoyt are thus in harmony with the view 

 that the susceptibility of protoplasm to ultraviolet light is conditioned 

 by the selective absorption and consequent "activation of the toxic 

 rays by the aromatic ami no-acid radicals of the proteins. These 

 results have a practical as well as a theoretical bearing, for they imply 

 that fluids containing proteins would be much more difficult to sterilize 

 with ultraviolet light than water, owing to the protective action of 

 the proteins in the fluid through which the light has to pass before 

 it impinges upon the protoplasm of the infecting organisms. 



THE STORAGE OF POTENTIAL ENERGY: THE PHOTO- 

 SYNTHESIS OF CARBOHYDRATES. 



The leaves and other chlorophyll-containing organs of green plants 

 absorb Carbon Dioxide from the atmosphere and simultaneously liberate 

 an equal volume of oxygen. The carbon which is thus retained is 

 built up into the tissues or reserve-materials of the plant, appearing 

 chiefly in the form of Carbohydrates which accumulate very rapidly 

 during active assimilation. 



The process of carbon dioxide assimilation by green plants takes 

 place only in the light and in the presence of Chlorophyll or related 

 pigments. Within certain limits the rate of absorption is proportionate 

 to the intensity of the illumination of the leaf, and to the percentage 

 of carbon dioxide in the atmosphere. Not all parts of the spectrum 

 are equally efficient in promoting this process, the red rays between B 

 and C causing the most rapid -assimilation while the activity of the rays 

 between D and E on the Frauenhofer scale is a minimum, and there is a 

 second maximum in the violet, beyond R This was first shown in a 

 most ingenious manner by Engelmann. The Aerobic Bacteria require 

 the presence of free oxygen to display motility. If some green algae 

 and aerobic bacteria be imprisoned together under an air-tight cover- 

 glass and kept in the dark, the free oxygen is soon consumed and the 

 bacteria become motionless. If the cell is now exposed to light the 

 algae decompose carbon dioxide, setting free oxygen, and the bacteria 

 become motile again. Exposure of the cell in different parts of the 

 spectrum yielded the above-quoted results. 



The assimilation of carbon dioxide by green plants is the foundation 

 of the existence, not only of the plants themselves, but of the animal 

 world. The radiant energy of the sun which is in this manner stored 

 up in the tissues of the green plants, reappears at the other extremity 

 of the life-process as the heat or muscular energy and mechanical work 

 of an animal. A very striking peculiarity of living material also 

 originates in this process, for while the components of protoplasm are, 



