THE FOOD OF PLANTS. 49 



day have been favourable. While the leaves are assimilating, the 

 whole of the material produced does not collect in the leaf. A 

 portion of the carbohydrates migrates and is used up in respira- 

 tion. This quantity must not be neglected. We may estimate 

 it approximately by removing portions in the evening from leaves 

 very similar to those used for the experiments already mentioned, 

 and testing them for sugar and starch. The portions left behind 

 are not cut till eight hours later, and these are also submitted to 

 examination. Taking into account the loss by migration, we find, 

 e.g., that 1 sq. m. area of Helianthus leaf, under favourable con- 

 ditions for assimilation, produces about 2 gr. of carbohydrate 

 material per hour. 4 The quantity of carbohydrate which has 

 migrated must naturally be added in the calculation to that pro- 

 duced in the daytime. It is noteworthy, as recent observations 

 have taught, that even in Helianthus only a fraction (about J) of 

 the carbohydrate produced in assimilation consists of starch. 



The total quantity of the material formed by assimilation we 

 can also discover approximately by taking, e.g., from Helianthus 

 plants, in the early morning and towards evening respectively 

 500 sq. cm. of leaf area, and thoroughly drying, first rapidly at 

 about 80 C., and then, after powdering, at 100 C., proceeding in 

 the same way with other portions removed in the evening and 

 eight hours later respectively. The values for the gain in weight 

 by day per 500 sq. cm., and for the loss in weight per 500 sq. cm. 

 during the night are to be added. In this way we get a rough 

 measure of the assimilatory activity of the leaves. These experi- 

 ments, as also those previously described, are to be made on very 

 bright sunny days and very warm nights. 



We may to-day adopt the view that, even in very starchy leaves, 

 starch is not directly produced in the green cells from the Carlxm 

 dioxide and water as the result of assimilation, but that first of 

 all glucose is formed. (See my Lehrbuch der Pflanzenpliysiologie, 

 1883, pp. 38 and 198.) In leaves rich in amylum this glucose, 

 owing to specific peculiarities of their chlorophyll bodies, can 

 easily be converted into amylum, while the formation of starch 

 from the glucose developed in assimilation is more or less difficult 

 in the leaf cells of other plants. In this connection it is naturally 

 a fact of great importance that leaves have been caused to form 

 starch at the expense of glucose conveyed to them from without. 5 

 I have not specially investigated the matter, but have satisfied 

 myself that leaves can likewise form starch from cane-sugar 



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