THE PRODUCTS OF PHOTOSYNTHESIS 209 



have been found to vary considerably. On account of the neglect of giving 

 due regard to these influences there exist in the literature of the subject 

 many contradictory results. The fact must be kept in mind that a leaf is 

 capable not only of photosynthesis, but that at the same time there are 

 taking place migration of the substances formed, respiration and growth 

 and that each of these functions may be influenced in a different manner 

 by temperature, water-content, etc. Hence an increase or decrease of any 

 particular carbohydrate (e.g. starch) or, for that matter, of the total 

 carbohydrate-content, can give little exact information regarding the rate 

 of photosynthetic activity. 



Conclusions regarding the transformation of carbohydrates in the leaf 

 have been based upon the analysis of the leaf components. It has proven 

 to be a matter of considerable difficulty to devise methods for determining 

 the carbohydrates in leaves which give reliable results. Although the 

 sources of error are many there are two which are particularly troublesome. 

 The first of these is due to enzyme action resulting in changes in the nature 

 and amounts of carbohydrates after the leaves have been harvested. The 

 second arises in the preparation of extracts and solutions for analysis. In 

 this process it is possible that some of the carbohydrates are lost or other 

 substances are included which behave in a manner similar to the carbo- 

 hydrates and thus spurious results are obtained. The first of these diffi- 

 culties must be met in the preparation of the leaf material for analysis. 

 This is an exceedingly important step and the value of the analyses de- 

 pends more upon the proper choice of a method for this procedure than 

 has been commonly assumed. Various methods of killing and drying have 

 recently been subjected to a critical study by Link and Tottingham ^° 

 who have established valuable facts regarding the choice of methods for 

 different types of leaf material. Attention has also been given to this 

 phase of the subject by Parkin ^'^ who compared air dried leaves with those 

 immersed in liquid air. Waterman and Tollenaar also have devoted atten- 

 tion to the effect of drying on the composition of vegetable material, par- 

 ticularly on the starch content. The choice of a method of killing and 

 drying depends greatly upon the nature of the material to be analyzed. 



The methods of analysis have also recently been critically discussed 

 by Tollenaar, Cast, Link and Tottingham, Davis, and others.^- 



One limitation of the method of chemical analysis is that it gives no 

 information regarding the cellular mechanism of carbohydrate trans- 

 formation and movement. Different portions of a leaf exhibit different 

 rates of carbohydrate accumulation as well as of transformation. The 

 formation and hydrolysis of starch can be followed by microchemical tests 



80 ■ 



Link and Tottingham, Jour. Ariier. Chem. Soc, 45, 439 (1923); 47, 470 

 (1925). 



^Parkin, Biochem. four., 6, 5 (1911). Waterman, Chem. Weekblad., 12, 48, 

 924 (1915). Tollenaar, Dissertation Wageningen, 1925. 



''Cast, Zeit. physiol. Chem., 99, 1 (1917). Ahrens, Bot. Archiv., 5, 234 (1924). 

 Davis and Daish, Jour. Agri. Sci., 5, 437 (1913) ; 6, 152 (1914). Spoehr, Carnegie 

 Inst, of Washington, Pub.' No. 287 (1919). 



