MEASURING PHOTOSYNTHETIC ACTIVITY 231 



Englemann ^ adapted this method to microscopical demonstration o f 

 the production of oxygen. A filament of an alga, as spirogyra, is sealed 

 under a coverglass with a drop of hemoglobin solution which has pre- 

 viously been reduced. On illumination of the preparation the forma- 

 tion of the bright scarlet oxyhemoglobin surrounding the plant can be 

 clearly followed under the microscope. In the dark hemoglobin is formed 

 again and the transition of the oxyhemoglobin into hemoglobin can be 

 followed in the immediate neighborhood of the plant. With the aid of a 

 spectralocular it is possible to detect these changes taking place within 

 a few seconds after illumination. If the preparation is illuminated with 

 a microspectrum differences in the rate of change of color can be followed 

 in different portions of the spectrum. 



e. The Bubble Counting Method. 



This method depends upon the fact that when cut leaves or other parts 

 of aquatic plants are placed in water containing dissolved carbon dioxide, 

 on illumination gas bubbles are liberated at the cut ends or surfaces. 

 The method does not yield satisfactory results with land plants. The 

 solubility of carbon dioxide in water is relatively high; during photo- 

 synthesis oxygen is formed, the water is already saturated with this gas 

 which is very sparingly soluble in water and the oxygen escapes in the 

 form of small bubbles. Sachs ^" showed that the rate at which these 

 bubbles are liberated may be taken as a measure of the relative rate of 

 photosynthesis. With the proper cut surface such plants as Elodea, 

 potamogeton, ceratophyUum, hydrilla liberate bubbles at a rate which can 

 be easily counted. On account of its simplicity and ease of manipulation 

 this method has been used not only for demonstration purposes but also 

 to investigate the rate of photosynthesis under various conditions. The 

 method possesses the following advantages: (1) the execution of a large 

 number of observations of short duration (a few minutes). As against 

 the determination of gas volumes this may have some distinct advan- 

 tages, for such small volumes of gas as are liberated in a jjeriod of obser- 

 vation of gas bubbles can hardly be determined accurately by volumetric 

 analysis. (2) Natural illumination can be used and can be taken as of 

 constant intensity for such short periods. (3) The short duration of 

 the periods of observation largely excludes the errors caused by changes 

 within the plant. It is, of course, essential that comparative experi- 

 ments be made with one and the same plant, as it is impossible to get 

 two parts of a plant that are absolutely alike. It is also of importance 

 that the relative position of the plant to the light should not change, as 

 otherwise the amount of illuminated surface may change. The cut sur- 

 face should not be submerged too deeply in the water and the distance 



•Englemann, Biol. Zentralbl, XX, 8, H (1888). 

 "Sachs, J., Bolan. Zcitung., 22, 363 (1864). 



