212 PHOTOSYNTHESIS 



leaf, under normal conditions passes from the leaf to other portions of 

 the plant, or is contained in the leaf as other carbohydrates besides starch. 

 This is illustrated by another experiment of Brown and Morris. Analyses 

 were made of the following three sets of leaves of Tropaeolum: 



A, Leaves picked from plant at 5 A.M. 



B, Leaves cut from plant, but insolated till 5 P.M. 



C, Leaves picked at 5 P.M. 



Results of analyses in percentages of the dry leaves : 



ABC 



Starch 1.23 3.91 4.59 



Sucrose 4.65 8.85 3.86 



Glucose 0.97 1.20 0.00 



Fructose 2.99 6.44 0.39 



Maltose 1.18 0.69 5.33 



Total sugars 9.69 17.18 9.58 



The cut leaves, B, were deprived of the capacity of having the syn- 

 thesized material move out into other portions of the plant. There is 

 therefore a striking accumulation of material. Although there is here a 

 large increase in the amount of material gained over A, the amount of 

 starch has not reached that of the leaves, C. If the total amount of ma- 

 terial synthesized is taken as that obtained in B, it is very apparent that 

 neither starch nor any other sugar as determined in leaves attached to the 

 plant, as in C, give a true measure of the rate of photosynthesis. 



Saposchnikoff ^° endeavored to determine the maximum starch ac- 

 cumulation in leaves. For excised leaves of Vitis vinifera he regards that 

 27.5 per cent of the dry material of the leaf as starch represents the limit, 

 while for Vitis labrusca the Hmit ranged between 17 and 25 per cent. The 

 amounts of starch can be considerably increased if the leaves are placed 

 in an atmosphere enriched in carbon dioxide; photosynthesis under these 

 conditions produces starch amounting to 30-35 per cent of the dry leaves. 



Of greatest importance in the production and transformation of 

 starch in leaves are the conditions of temperature and water content. In 

 general, low temperatures result in the disappearance of starch from leaves. 

 Lidforss ^^ showed that evergreen leaves in temperate latitudes are quite 

 free from starch during the winter months. With the higher temperatures 

 of spring starch again appears in the chloroplasts. It has been assumed 

 that the low eutectic point of the monosaccharide solutions in the cell sap 

 affords protection against freezing. It has been demonstrated that tem- 

 perature is actually the determining factor in the transformation of starch 

 in leaves by the fact that when evergreen leaves are brought to a higher 

 temperature during any time of the winter, starch formation takes place 

 very readily. Maximow ®® has demonstrated that an increased concentra- 



»" Saposchnikoff, Ber. hot. Ges., 9, 293 (1891) ; 11, 391 (1893). 

 "Lidforss, Bot. Centralbi, 68, 33 (1896). 



■* Maximow, Ber. bot. Ges., 30, 52 (1912). Miyake, Bot. Gas., 33, 321 (1902). 

 Mer Bull Soc. Bot. France, 23, 231 (1876). Haberlandt, Jahrb. wiss. Bot., 13, 74 



