RESPIRATION 197 



It is not possible to deduce from these experiments any regular dependence 

 of the quotient on the quantity or constitution of the material used up in 

 respiration, still these results are full of interest as showing how extraordinarily 

 variable the quotient is, whose value in the majority of higher plants examined 

 the exceptions we have still to study lies close to i. This fact, also established 

 byJPuRiEwiTSCH, is especially important, viz. that in an individual experiment 

 with a definite mycelium, which was placed in various nutritive solutions in 

 succession, the fluctuations in the amount of carbon-dioxide excreted run by no 

 means parallel with those in the amount of oxygen taken in (average values are 

 given above) ; while the fluctuations in the oxygen absorbed were limited (up 

 to 35 per cent.), those of the carbon-dioxide excreted varied within far wider 

 limits (28 to 120 per cent.). The two processes which in chemical combustion 

 follow each other so closely that we may consider them as simultaneous, are 

 frequently in physiological combustion widely separated. Physiological com- 

 bustion is by no means a simple process, many intermediate reactions lie between 

 the absorption of oxygen and the excretion of carbon-dioxide, and these vary 



CO 



under different conditions. As a rule, the quotient -^~ is less in value than 



unity, some oxygen being stored in the plant, so that we may conclude that the 

 final products of the combustion of respiratory material are not in this case 

 carbon-dioxide and water, but are, at least in part, other bodies as well, and 

 organic acids suggest themselves at once to us as probable products, more 

 especially as the occurrence of these bodies in Fungi has long been known. 



Oxalic acid is very frequently formed, and an exhaustive study by 

 WEHMER (1891) has made us acquainted with the details of its manufacture. 

 Among Fungi, Aspergillus niger is known to form oxalic acid in great quantity, 

 and WEHMER' s experiments were carried out mainly on this plant. The chief 

 results which he obtained are summarized in the following table : 



Nutrient. Weight of the fun. Weight of the oxalic acid formed, 



1.5 g. in each case. estimated as a calcium salt. 



Tartaric acid o-issg. o-oo 



Citric acid 0-240 g. o-oo 



Ammonium tartrate t 0-030 g. 0-767 



Potassium tartrate 0-032 g. 0-550 



Ammonium citrate 0-056 g. 0-390 



Dextrose 0-228 g. 0-278 



It appears that the formation of the acid is not necessarily connected-with 

 the growth of the fungus ; it arises only when the substratum gives no acid 

 reaction and when the fungus is cultivated in sugar, proteid, glycerine, oil, and 

 salts of organic acids. No oxalic acid is formed if the nutritive substance be 

 a free acid, and the addition of phosphoric or hydrochloric acid to the nutritive 

 solution inhibits its formation. More recently, WEHMER (1897), and also 

 EMMERLING (1903), have obtained other results with Aspergillus niger, so that 

 probably there are several physiological forms of this fungus. The formation 

 of oxalic acid in Aspergillus has probably only a biological significance. The 

 fungus grows well in an acid substratum, and if the substratum be not acid it 

 makes it so, so that associated organisms are excluded. A continuous forma- 

 tion of acid would in the long run prove in j urious to A spergillus itself ; as a matter 

 of fact, it ceases to produce any more when the substratum contains above 

 0-3 per cent., but if the acid formed be neutralized the fungus can be made to 

 produce more. In one experiment, for example, 1-253 g- of anhydrous oxalic 

 acid was produced from 1-5 g. of sugar, whilst 2-25 g. might have been produced 

 by its complete transformation ; the 1-5 g. of sugar was altered as follows : 

 0-8318 g. was oxidized into oxalic acid, 0-290 g. was employed in the archi- 

 tecture of the fungus, and the remainder (0-3782 g.) oxidized into carbon-dioxide. 



