210 PHOTOSYNTHESIS 



and considerable information regarding these processes has been gained in 

 this manner. These methods are, however, only of a quahtative value. 

 In general, the carbohydrates which are of importance in relation to 

 the photosynthetic process, as far as our present knowledge goes, are: 

 d-glucose, d-fructose, pentoses (probably 1-arabinose and 1-xylose) sucrose, 

 maltose, starch and pentosans. Not all leaves contain all of these carbo- 

 hydrates ; some contain no maltose nor free pentoses, in some starch is 

 replaced by inulin and in some neither of these polysaccharides are nor- 

 mally present. From the fact that the amounts of these carbohydrates in 

 leaves show very decided changes when the latter are exposed to light in 

 an atmosphere containing carbon dioxide (i.e. during photosynthesis) or 

 when they are kept in the dark, it has been concluded that these sugars 

 are of importance in the photosynthetic and respiratory processes of the 

 leaves. Under these conditions, it should be stated, the pentoses and 

 pentosans are normally but slightly afifected. 



a. Starch. 



Probably the most easily detected product of photosynthesis is starch ; 

 this is accomplished by means of the iodine reaction. That there is a 

 certain relationship between the soluble carbohydrates of the leaf and 

 the starch follows from the familiar fact that starch is formed in the 

 dark when leaves are placed in sugar solutions. The amount of starch 

 thus formed is, within certain limits, proportional to the concentration of 

 the sugar solution. Under natural conditions many leaves accumulate 

 relatively large quantities of starch during days of bright illumination 

 and sufficiently high temperature. In many cases this starch disappears 

 entirely again during the night. This accumulation of starch during illumi- 

 nation has been interpreted as being a case of the formation of reserve 

 material during photosynthesis which is consumed during the period 

 the plant is in darkness. The questions to be discussed are what are the 

 conditions which influence the formation and resorption of starch and in 

 what manner does the starch disappear. 



Different species of plants vary greatly in the rate at which starch is 

 formed and disappears from the leaves. From the compilation of Miss 

 Eckerson ®^ an idea can be gained of these rates which are given in 

 Table 28. 



The quantity of starch found in leaves varies widely with the species, 

 the time of day and conditions of temperature, water-content, etc. Brown 

 and Morris, in Tropacolnm found 1.23 per cent of the dry leaves as starch 

 when picked at 5 A.M. and 4.59 i^r cent at 5 P. M. Table 29 is compiled 

 from the results of Gast.** 



Brown and Morris ^^ have shown that although leaves may produce 

 large amounts of carbohydrates in the photosynthetic process only a small 



'«' Eckerson, Bot. Gaz., 48, 224 (1909). 



"Cast, Zeit. pliysiol. Chem., 99, 1 (1917). 



«' Brown and Morris, Jour. Chem. Soc, 63, 669 (1893). 



