252 PHOTOSYNTHESIS 



for as has previously been described by Osterhout. The effect of the 

 respiration of the plant on the pH of the solution is in the opposite direc- 

 tion to that of photosynthesis and must be allowed for. 



It is also essential to determine that the plant does not produce an 

 alkaline reaction in the water in which it is contained, independently of 

 its photosynthetic activity, for it has been reported that certain aquatic 

 plants in pure water when exposed to light develop an alkaline reaction 

 in the water.^^ No entirely satisfactory explanation of this phenomenon 

 has as yet been obtained. It is, none the less, of considerable importance 

 when the photosynthetic activity is to be determined from changes in the 

 pH of the water. This fact is further emphasized by the discovery of 

 Ulehla ^'^ who has found that certain cultures of Ocdogouium increase the 

 alkalinity of the water not only in the light but also in the dark, although 

 in the dark the amount of carbon dioxide in the water is increased due to 

 the respiratory activity of the alga. This remarkable situation has its 

 explanation in the fact that the Oedogonimn filaments are associated with 

 an incrustation known to the algologists as Psiclwh ovinia, in fact an iron 

 bacterium which surrounds the alga filament. The incrustation contains 

 iron and calcium carbonates and acts as a buffer system. As the carbon 

 dioxide-concentration of the water increases, a corresponding amount 

 of carbonate is dissolved as bicarbonate and depresses the hydrogen ions 

 through the formation of the hydroxyl ions formed by hydrolysis. Be- 

 cause of the buffer eft'ect of the incrustation, which virtually constitutes 

 a symbiotic relationship between the alga and the bacterium, the former 

 can exist in water which undergoes relatively large changes in pH. With- 

 out this buffer the alga can exist in only a relatively narrow range of 

 hydrogen ion concentration. 



4. Formation o£ Organic Matter 



The first extensive attempts to determine the amount of material 

 formed in photosynthesis were made by Sachs." His method is well 

 known and in spite of inherent inaccuracies, attempts have been made to 

 modify it and it still continues in use. The method consists essentially 

 of determining the dry weight of a unit area of one half or a smaller por- 

 tion of a leaf at the beginning of the experiment and comparing this with 

 the dry weight per unit area of the other half of the leaf after it has been 

 exposed to the light for a time. From the difference in dry weight, with 

 certain corrections, the weight of material formed in photosynthesis per unit 

 area and time are calculated. In the original method the leaves were cut 

 along the mid-rib, the cut half being used for the determination of the 

 weight of the leaf in its original condition. After a period of insolation 

 the other half of the leaf was cut from the mid-rib and the gain in weight 



°'Klebs, Untcrsuchungcn Bof. Inst. Tubingen, 2, 340 (1888). Hassack. ibid., 2, 

 A65 (1888). Anglestein, Cohn's Bcitraqc zur Biol, der PHansen, 10, 87 (1911). 

 ^Ulehla, Ber. bot. Ges.. 41 (20) (1924). Studia Mendeliana, Brno (1923). 

 "Sachs, Arb. bot. Inst. IViirzburg, 3, 1 (1884). 



