THE PRODUCTS OF PHOTOSYNTHESIS 207 



Maltose forms a phenylosazone (M.p.206°), which under certain con- 

 ditions, can be separated from the glucose-phenylosazone. The former is 

 more soluble in water and in acetone so that cold 50 per cent acetone can 

 be used for the separation.'^* 



Starch. We have already alluded to the classical researches of Sachs 

 on the formation of starch in the leaf and the great influence these exten- 

 sive studies had on the development of the problem of photosynthesis. 

 By use of the iodine test for starch an accurate qualitative means of identi- 

 fying this substance was provided. The accumulation of starch has proved 

 to be one of the simplest means of following in a qualitative manner the 

 photosynthetic process. 



The iodine test for starch has undergone many minor modifications. 

 The principle is, however, essentially the same as that used by Sachs. 

 The leaves are placed in boiling water for about ten minutes. They are 

 then transferred to 95 per cent alcohol at 50-60° for about one-half hour 

 until all the chlorophyll and other pigments are removed. With some 

 plant material the last traces of chlorophyll are removed with difficulty, 

 and therefore require a longer time and more alcohol. The colorless leaves 

 are placed in an iodine reagent made by dissolving one gram of iodine in 

 100 cc. of a 5 per cent potassium iodide solution. Usually the starch- 

 iodine color appears immediately but in some cases requires slight heating. 

 The excess iodine is washed ofif with water. For the microchemical de- 

 tection of starch it is advantageous to add 5 grams of chloralhydrate to 

 the iodine reagent which greatly clarifies the section. By means of this 

 method the localization of the starch can be clearly followed. Only those 

 portions of the leaves which have been illuminated form starch. In fact, 

 under the proper circumstances, the leaf is so sensitive to light of dififerent 

 intensity in its starch formation that photographic positive images can be 

 produced by means of this method. '^^ The iodine method has been applied 

 both to a study of the formation of starch in the light and to its disap- 

 pearance when the leaves are kept in the dark. The great mass of ob- 

 servational and experimental results which have been gathered by means 

 of this method indicates quite clearly, that while starch is a very common 

 product of photosynthesis, it cannot be considered as the first product 

 formed. Some plants form no starch at all though they carry on active 

 photosynthesis and starch may also be produced in leaves which are 

 artificially fed glucose, sucrose, and a variety of other organic compounds. 

 Also, the formation of starch in leaves during photosynthesis depends very 

 much upon conditions of temperature and water-content. In considering 

 starch as a product of photosynthesis, as is the case with ail other carbo- 



some source of error and special precautions must be taken to remove these sub- 

 stances. In the presence of large quantities of phosphatides it is impossible to 

 obtain the blue coloration of starch when iodine is added, until the absorption 

 capacity of the fatty acids for iodine has been satisfied. 



'* Grimbert, Jour, pharni. chim. (6), 17, 225. 



"Molisch, Sitzb. Akad. Wiss. Wien., Abt. I, 123, 923 (1914). 



