PHYSIOLOGY 473 



water, permits the absorption of certain rays in addition to those 

 absorbed by the chlorophyll. The ability of the purple Bacteria to 

 assimilate C0 2 is due possibly to the presence of a small amount of 

 chlorophyll mixed with the purple pigment. 



Even isolated chloroplasts can perform photosynthesis, provided 

 they are kept in a proper nutrient medium. This is demonstrated 

 by the evolution of oxygen from such isolated chloroplasts, in the 

 presence of light, indicated by the accumulation of aerobic Bacteria 

 about them. 



An excess of free oxygen is given off during photosynthesis, and 

 this is popularly confused with respiration, which is a very different 

 process. This evolution of oxygen ceases at once when the plant is 

 removed from the light. 



The evolution of gas from actively assimilating plants is easily 

 seen in any submersed aquatic exposed to the light. The foamy 

 appearance of floating masses of Spirogyra and other similar Algae 

 is due to the accumulation of bubbles of oxygen given off during 

 the process of photosynthesis. If the bubbles of gas given off by 

 a submersed water plant are collected and tested, they are found 

 to be nearly pure oxygen. 



A very delicate test for oxygen was devised by Engelmann. Cer- 

 tain Bacteria have a great affinity for oxygen, and will collect at a 

 point in the water where the amount of oxygen is greatest. Hence, 

 if green cells are present, the collecting of Bacteria at different places 

 indicates very accurately the relative amount of oxygen given off. 



The light-rays which are most effective in assimilation of plants 

 exposed to ordinary light are the red, orange, and yellow. Where 

 there are accessory pigments, the maximum assimilation occurs at a 

 different place in the spectrum. Thus, in the Rhodophyceae, it is 

 the green rays which are most efficacious ; and in the purple Bacteria, 

 the infra-red rays. The red or purple pigments, in these forms, act 

 as sensitizers, which permit the chloroplast to use rays otherwise 

 unavailable for photosynthesis. 



While the chlorophyll is the important agent in photosynthesis, 

 the yellow pigment, xanthophyll, always associated with it, also 

 takes part in the process, though much more feebly than the chloro- 

 phyll. This is indicated by the feeble assimilating power of etio- 

 lated chromatophores. 



The Products of Photosynthesis 



What the first steps in the process of photosynthesis are we do 

 not know, although it is probable that the early steps in the decom- 

 position of C0 2 and water, and the first-formed products, are the 

 same in all cases. The first visible product is usually starch ; but 



