326 THE FOOD OF PLANTS 



form any starch under these conditions (Schimper, 1. c., p. 787). There is a certain 

 optimal concentration at which the formation of starch is most active. Laurent 

 observed that potato shoots formed starch when supplied with from 2-5 to 15 

 per cent, solutions of dextrose, but not when fed with a solution of 20 per cent, 

 strength, although many plants can form plenty of starch at this degree of con- 

 centration ', and indeed in Iris, Galanthus, c., starch is formed only when the 

 concentration is as high or higher than this. If sufficient sugar is present in the cell 

 a further concentration produced by plasmolysis with potassium nitrate may cause 

 a formation of starch, a fact which was observed by Bohm, but wrongly interpreted 

 by him (1. c., 1889, p. 200). In many plants the fact that a strong solution of 

 sugar causes permanent plasmolysis shows that very little sugar is absorbed, so 

 that the percentages inside and outside of the cell are not necessarily the same ; 

 hence the different specific absorptive powers of different cells exercise an important 

 influence in producing the result observed. 



According to A. Meyer, the chromatophores of Cacalia suaveolens may form 

 starch when supplied with glycerine. Acton and Nadson obtained similar positive 

 results with most of the leaves examined, and confirmed Klebs' statement that 

 many Algae form starch from glycerine more readily than from cane-sugar. Meyer 

 states that cane-sugar is absorbed by the leaves of Beta without undergoing 

 inversion. On the rapid diosmosis of glycerine see Sects. 16 and 17. 



Laurent found that malic and citric acids, peptone, acetone, various amides and 

 glucosides, were all incapable of inducing any formation of starch. Bokorny - 

 observed that starchless Spirogyra filaments formed starch in dilute solutions of 

 formaldehyde, of methyl alcohol, and of sodium oxymethyl sulphonate (which 

 readily decomposes and produces formaldehyde), though only when exposed to 

 light, but sufficient precautions do not seem to have been taken to guard against the 

 production of starch by assimilation of carbon dioxide. Even if the observations 

 are correct it does not follow that formaldehyde is the primary product of carbon 

 dioxide assimilation, nor can this importance be attached to sugar or glycerine 

 simply because starch may be formed from them. It has yet to be determined 

 in each specific case whether the starch is formed directly, or only after the sub- 

 stance absorbed has undergone preparatory alterations. Certain fungi are able 

 to derive all their organic food, including carbohydrates, from formic acid and 

 from oxymethyls 3 , a fact which is of considerable importance in this connexion. 



SECTION 56. May other Carbon Compounds be Photosynthetically 



Assimilated ? 



With the exception of carbon dioxide, no carbon compounds are 

 known which can be photosynthetically assimilated. Thus neither carbon 



1 The minima, optima, and maxima in various plants are given by Schimper, Saposchnikofl, &c. 



' J I.e., 1888 and 1889; also Ber. d. Bot. Ges., 1891, p. 103. On the negative results with 

 formaldehyde and its poisonous properties, cf. Bokorny, I.e., 1889, p. 236; Laurent, I.e., 1888, 

 p. 17. On Trioxymethyl, cf. A. Meyer, I.e., 1886, p. 137. _ 



3 Reinke, Unters. a. d. Bot. Lab. in Gbltingen, 1883, Heft 3, p. 33, and ibid.,' Methylal as fungal food.' 



