H2 METABOLISM 



taining sugar construct considerably less solid than leaves containing starch.] 

 In fact, it can be shown that many plants, normally without starch, can form 

 it provided the sugar formed during assimilation be present in sufficient con- 

 centration. Such a concentration may be reached in many ways ; by separat- 

 ing the leaves from the stem and so preventing the translocation of the 

 manufactured carbohydrate, by providing an atmosphere rich in carbon-dioxide, 

 and thereby occasioning an increase in assimilative activity, e.g. Musa and 

 Strelitzia (Godlewski, 1877), Iris (Schimper, 1885), or by adding certain 

 carbohydrates from extraneous sources. Schimper (1885) for example, showed 

 that Iris germanica, which is normally free from starch, can be made to form it if 

 provided with a 20 per cent, solution of sugar. This method, varied in detail, has 

 been used with great success by Bohm (1883), and also by A. Meyer (1886), 

 Laurent (1887), and Klebs (1888), to show that plants which have been 

 freed from starch can form it from a sugar solution in the dark. Starch 

 formation has thus nothing to do directly with the assimilation of carbon-dioxide ; 

 it occurs in all chromatophores, whether they contain chlorophyll or not, 

 whether they are in light or darkness, and always occurs when soluble carbo- 

 hydrate accumulates in large quantities in the cells. The degree of concen- 

 tration of sugar from which the starch is formed varies in different plants. 

 The authorities above named (and others) have further shown that, in addition 

 to dextrose and levulose, which form the source of starch in so many plants, other 

 carbohydrates (compare Czapek, 1902), such as mannose, galactose, saccharose," 

 as well as alcohols such as glycerine, mannite, erythrite (at least in some plants}7" 

 operate in the same way. The fact that glycerine may, in very many plants, be con- 

 verted into starch, proves to us in the clearest possible way that we cannot 

 argue backwards and say that all the substances employed in the formation 

 of starch must arise during the decomposition of carbon-dioxide, for glycerine 

 is apparently never formed as a product of assimilation. 



We may, therefore, conclude that soluble carbohydrates are the first 

 demonstrable products after the carbon of the carbon-dioxide had become 

 united with water. This conclusion is, however, not satisfactory, especially from 

 the chemist's point of view. The complicated nature of this substance is quite 

 rightly emphasized, and we must, in consequence, search for a simpler body 

 as the first product of constructive metabolism. There are many hypotheses forth- 

 coming to aid us in our search for such a ' first product of assimilation ' ; of 

 these we may refer here to one only which has proved of special significance 

 in physiology, because it has led to a number of investigations. Bayer (1870) 

 started from the similarity found to exist between the colouring matter of the 

 blood and chlorophyll. Since carbon-monoxide was able to form a compound 

 with haemoglobin he supposed that chlorophyll might show a similar capacity. 

 Under the influence of sunlight he fancied the carbon-dioxide was broken 

 down into carbon-monoxide and oxygen ; the carbon-monoxide was united 

 with water, forming, after evolution of more oxygen, formaldehyde. The 

 following formulae express these chemical changes : — 



CO3 = CO + O ; CO + HaO = CH2O + O. 



An optically inactive reducing sugar might then be simply produced from 

 formaldehyde, i. e. formose, with the formula CgHjjOg (Buttlerow, 1861 ; Loew, 

 1886 ; compare also E. Fischer, 1894). The fact that this sugar has not as 

 yet been found in the plant does not appear to us to mihtate against Bayer's 

 hypothesis ; but the criticism, that although carbon-monoxide, as Just (1879) 

 has shown, is not injurious to the plant as it is to the higher animals, it cannot 

 undergo higher constructive metabolism, is more to the point. [Recently it has 

 (Czapek, I, 428) been stated that carbon-monoxide may be assimilated even 

 though it be a poison.] One may get over this difficulty easily by assuming 



