THE METABOLISM OF PLANTS. 231 



the decomposition of the fats. Finally, though Schmoeger's 

 observations (see p. 125) tend to prove that highly organised 

 plants cannot take up acids when supplied to their roots, 

 yet Naegeli has shewn that the lower Fungi will flourish 

 when carbonaceous food is supplied to them in the form of 

 various acids, acetic, succinic, and tartaric (p. 125). 



But there are certain acids which, so far as our present 

 knowledge goes, cannot be reduced by plants, namely, the 

 formic and the oxalic. Naegeli found that Fungi could not 

 assimilate either of these, and the general occurrence of 

 crystals of calcium oxalate in the tissues of plants, which, 

 according to Hilgers, undergo no alteration after they have 

 been formed, clearly suggest that plants in general are in- 

 capable of reducing oxalic acid. Even admitting that some 

 acids may be used in the constructive metabolism of plants, 

 there is a strong presumption that oxalic acid at any rate 

 and this is of importance, since oxalic acid is the starting- 

 point of Liebig's theory is of no value for this purpose. It 

 is only of the less highly oxidised acids that plants can avail 

 themselves. 



Instances of the solution of calcium oxalate in the plant are, how- 

 ever, on record. Frank has observed it in the mucilaginous cells of 

 the tubers of various Orchids : Sorauer has found that young potatoes 

 abundantly contain crystals of calcium oxalate which disappear as the 

 tuber becomes mature, an observation which has been confirmed by 

 de Vries. Whilst admitting the accuracy of these observations it must 

 not be overlooked that they do not prove that the oxalic acid eriters into 

 the constructive metabolism of the plants : they only prove that the 

 oxalate has been brought into solution, or that it has undergone decom- 

 position so as to give rise to soluble salts. 



In connexion with Liebig's view as to the reduction of acids in the 

 plant, we may recall the facts mentioned in the last lecture (p. 197) with 

 reference to succulent plants. De Saussure observed, namely, that when 

 a piece of Opuntia-stem which had been absorbing oxygen during the 

 night, was exposed to sunlight on the following day in an atmosphere 

 containing no carbon dioxide, it exhaled a volume of oxygen which 

 slightly exceeded that of the oxygen absorbed during the preceding 

 night (in one experiment the volume of O absorbed =74 c.c., the volume 

 exhaled = 79 c.c.). Mayer observed, in his experiments, that, under these 

 circumstances, starch-grains make their appearance in the chlorophyll- 

 corpuscles, care being taken to prevent any absorption of carbon dioxide 



