i64 METABOLISM 



several new problems, and more especially lead us to inquire whether the 

 dissolution of reserves in the leaf is also effected by means of enzymes. 



From a comparison of the results of experiments conducted by Vines {1891), 

 Jentys (1892), and also by Brown and Morris (1893), the occurrence of dias- 

 tase in the foliage leaf cannot any longer be doubted. Although Wortmann 

 (1890) arrived at another conclusion, still his failure to find diastase can be 

 readily explained ; for diastase, in the first place, is present in the foliage leaf 

 only in small quantity, and further, it cannot be extracted completely by 

 water, considerable further loss taking place during filtration. The chief reason 

 for the apparent absence of diastase lies in the tannin so frequently present in 

 the leaf, which by precipitating the diastase makes it inactive. To Brown and 

 Morris we owe convincing proof of the fact, not only that diastase is present in 

 the leaf, but also that it occurs in sufficient quantity to transform into sugar all 

 the starch present there. These authors also showed that different leaves contain 

 the most varied amounts of diastase. For this purpose they estimated the 

 amount of maltose which was produced from so-called soluble starch in 

 forty-eight hours by the action of an extract of 10 g. of dried and powdered 

 plant substance, and found that 10 g. of malt gave 634 g. of maltose, 10 g. of 

 the leaf of Pisum gave 240 g., 10 g. of Lathyrus leaf gave 100 g., log. of 

 Tropaeolum leaf gave 4-10 g., and of Hydrocharis leaf, 0-3 g. 



In comparison with malt the amount of diastase present in leaves is gene- 

 rally small, although many leaves have large supplies. Definite proportions 

 evidently exist between the starch contents of a leaf and diastase, since the 

 Leguminosae which have been examined and which are next after malt in dia- 

 static power, are at the same time very rich in starch ; on the other hand it must 

 be noted that not all the numbers obtained can be accepted as an exact measure- 

 ment of the diastatic capabilities of the leaves concerned without further con- 

 firmation ; more especially we must note how the very inactive leaf of Hydro- 

 charts owes its place at the end of the series chiefly to the large amount of 

 tannin which it contains, which, as we have already said, inhibits the action 

 of diastase. Among external factors, so far as they have not been already 

 mentioned, we must note light, which has an inhibitory action on diastase 

 (Green, 1897), so that more starch is altered into sugar at night than by day. 

 Emmerling (1901) was unable, however, to confirm this observation. Again the 

 increased activity of diastase in darkness, due to carbon-dioxide (Mohr, 1902), 

 must tend to make the amount of sugar formed at night greater, since 

 at night the leaf contains much more carbon-dioxide than by day. How far 

 our previous conclusions as to the amount of material formed during the 

 process of assimilation by day are actually affected by this fact we cannot at 

 present say ; it will be remembered that we assumed that quite as much carbo- 

 hydrate was translocated by day as by night. Perhaps also Sachs's estimates 

 (p. 114) are too high. 



As we have seen, diastase, generally speaking, transforms starch into meil- 

 tose, i. e. a reducing disaccharide, nearly related to cane sugar. The maltose 

 is by hydrolysis broken up into two molecules of dextrose, while cane sugar is 

 decomposed into a molecule of dextrose and a molecule of levulose. Since 

 cane sugar has been shown to be without doubt a product of carbon assimilation, 

 at least in certain plants, but that maltose is a product of the decomposition 

 of starch, it comes to be a question whether these disaccharides undergo further 

 conversion as such or whether they must be hydrolysed first in the manner 

 described. A final decision on this question has not yet been reached. Reference 

 may first be made to the germination of beet, where there is no doubt that 

 cane sugar is transformed into invert sugar. That the presence of invertase 

 is necessary for this decomposition has been established with sufficient certainty 

 in various plant organs (Green, 1901), and Brown and Morris have proved 

 its presence in the leaves of Tropaeolum, Kosmann in buds of trees, O'Sullivan 



