230 PHOTOSYNTHESIS 



of salts, as in ordinary tap water, and but very little nutriment, as sucrose. 

 In neutral meat extract of 0.2 to 0.5 per cent, in time products may be 

 formed which exercise an injurious effect on the bacteria or the plant 

 cells. The preparation is covered with a coverglass and is sealed with a 

 mixture of vaseline and paraffin. In the dark the oxygen which was 

 enclosed in the preparation is soon exhausted by the respiration of the 

 bacteria and the green plant and the bacteria soon come to rest. If oxygen 

 is admitted or generated in the preparation the bacteria begin to move 

 and are chemotactically attracted by a bubble of air or oxygen. If the 

 sealed preparation is illuminated the bacteria congregate and move in the 

 neighborhood of the green plant. When the preparations are kept in 

 the dark for about 24 hours the bacteria become very sensitive to minute 

 traces of oxygen. The best temperatures for these tests are between 20° 

 and 30°. Since the bacteria are sensitive to exceedingly small traces 

 of oxygen (Pfeffer states they react to a billionth part of a milligram) 

 the photosynthetic activity of a single cell and even of an isolated chloro- 

 plast can be detected. In the dark and consequent stopping of photosyn- 

 thesis, the movement of the bacteria gradually ceases and they are again 

 distributed in the field. Subsequent illumination again produces the move- 

 ment and congregation of the bacteria. 



The method is essentially a test for oxygen, and while it has been 

 applied in a number of ways it has distinct limitations in the study of 

 the factors influencing photosynthesis and its use must be subjected to 

 strict control. A number of classical investigations have been carried 

 out by means of this method and its interest is probably rather an his- 

 torical one. Thus Englemann ' endeavored to demonstrate what portions 

 of the spectrum are most effective in producing photosynthesis and was 

 also able to demonstrate that photosynthetic activity was confined to those 

 portions of the plant bearing chloroplasts, and takes place only in illumi- 

 nated chloroplasts. 



d. Optical Properties of Hemoglobin. 



The use of hemoglobin as a test for the oxygen emitted by plants 

 during photosynthesis was introduced by Hoppe-Seyler.^ Small plants 

 or a portion of a plant such as El odea are sealed into a tube with nutrient 

 solution and some dilute hemoglobin. After remaining in the dark for a 

 while the tube, when examined spectroscopically, shows the characteristic 

 absorption bands of hemoglobin. If the tube is brought into the light, 

 the oxygen produced by the photosynthesis of the plant converts the 

 hemoglobin into oxyhemoglobin and the formation of the latter can be de- 

 tected by the change in the absorption spectrum. 



'Englemann, Arch. ges. Phxsiol. PHiiqer's, 57, 375 (1894); Bot. Zeitg., 40, 419 

 (1882); 42, 81, 97 (1884). Pfeffer. Untersiichungcn cms dcm bot. Inst. Tiibingen, 

 2, 589 (1888). Ewart, Jour. Linncan Soc. Bot., 31, 364 (1897). Kny, Ber. bot. 

 Ges., 15, 388 (1897). 



" Hoppe-Seyler, Zcit. physiol. Chcm., 2, 325 (1879). 



