THE PRODUCTS OF PHOTOSYNTHESIS 185 



Hyacinthus, Ornithogalmn and Iris can he induced to starch formation 

 when the leaves are kept on a 20 per cent solution of sucrose for 8 to 

 10 days. 



The question as to which of the carbohydrates is the first one to be 

 formed in the photosynthetic process has been a matter of controversy for 

 a great many years and is not yet definitely settled." That the starch in 

 leaves is to be regarded as transitory reserve material and is a condensa- 

 tion product of previously formed sugar was repeatedly suggested, while 

 others clung to the opinion that depending on the internal conditions in 

 the leaf either sugar or starch is formed. Schimper was probably the 

 first to express the idea that glucose was the precursor of starch and 

 that starch was formed only when the glucose had attained a certain 

 concentration. The concentration of glucose necessary for starch forma- 

 tion varies widely in different plants ; in some plants it is normally never 

 attained while in others it is reached after brief periods of photosyn- 

 thesis, so that in the latter plants starch is a regular component of the 

 chlorophyll bearing cells. 



From the investigations of Winkler ^ it follows that with few exceptions 

 all chloroplasts and leucoplasts which are sufficiently well developed are 

 capable of starch formation when the concentration of glucose or sucrose 

 is high enough. According to these observations the minimal concentra- 

 tion for starch formation in most cases is 0.2 per cent sucrose. The 

 optimal concentration is about a 10 per cent sucrose solution, higher con- 

 centrations are only very slightly more effective and a 30 per cent sucrose 

 solution produces no starch. The concentration of sugar which induces 

 starch formation varies with ditTerent plants and no general rule cover- 

 ing all plants can be made. Accurate determinations of this nature, with 

 due consideration of the influence of temperature, would be very desirable. 

 Most leaves contain monosaccharides, disaccharides as well as polysac- 

 charides. The proportion in which these are present is affected by various 

 factors and constitutes an equilibrium : monosaccharide ^ disaccharide ^ 

 polysaccharide. The transformation of one into another is the result of 

 reversible enzymatic reactions. The equilibrium is affected not only by con- 

 centration of one of the members of the group but also by temperature. 

 Before discussing the conversions which the different groups of car- 

 bohydrates undergo in the plant it will be necessary to describe briefly 

 the carbohydrates which have been found in plants. It may be stated 

 that the entire subject of the carbohydrates of foliage leaves would profit 

 by an extensive and critical experimental investigation. Owing to the 

 large number of members of this group, the difificulty of separating these, 

 the variety of other compounds which interfere in their purification and 

 separation and the possibility of changes being produced by enzymes and 

 other substances during the analysis, the work on this problem is ex- 



^ Pringslieim, Jahrh. zi'iss. Bnf.. 13, 441 (1882), for older literature. Mever, Bof. 

 Zeitg., 43, 505 (1885); 44, 147 (1886). Schimper. ibid., 43, 779 (1885). 

 "Winkler, Jalirb. iviss. Bot., 32, 525 (1898). 



