192 



Journal of Agricultural Research 



Vol. VI, No. s 



Table XII shows that the total pentosan content, which includes all 

 furfurol-yielding matter, and the pentosan content, which is the total 

 pentosan content after the methyl pentosans have been extracted, are 

 higher in all but one instance in the sound portions of the tuber. There 

 is slightly more variation in methyl pentosan content; it is the same or 

 greater in the rotted as in the sound portion in all but two cases. The 

 fungi evidently use the pentosans, but do not affect the methyl pentosans 

 to any extent. It is to be remembered that these fungi build up both 

 pentosans and methyl pentosans when growing on potato extract. The 

 content of these substances, then, in the rotted portions given in Table 

 XII is undoubtedly the difference between the amount of pentosans 

 broken down by the fungi in the interior of the potato and the amount 

 built up by the fungi. The destructive processes evidently proceed 

 more rapidly than the constructive, and some of the pentosans of the 

 potato are used either in respiration or in the building up of other 

 compounds. 



From the effect of the fungi on pentosans it was considered probable 

 that enzyms which could hydrolyze these compounds were present in 

 the mycelium. Experiments were undertaken to determine this point. 



The experiments were carried out as described in a previous paper (8) , 

 except that the fungi were grown on potato extract instead of a synthetic 

 medium with gum arabic as a source of carbon. Xylan from rye straw 

 was used as a substrate. The results of these experiments are given in 

 Table XIII. 



Table XIII. — Effect of boiled and unboiled extract of mycelium upon xylan from rye 

 straw, as shown by alcohol-soluble furfurol-yielding m.aterial and substance reducing 

 Fehling's solution. (0.2 gm. of xylan in each preparation was maintained at jo° C. 

 for one week.) 



Species of Fusarium. 



Fusarium radicicola . . 

 Fusarium. oxysporum 



Quantity of cuprous oxid 

 derived from material 

 reducing Fehling's so- 

 lution. 



Unboiled. 



Mgm. 

 45-2 

 44.8 

 14. I 

 16. 4 



Boiled. 



Mgm. 

 15-4 

 14.8 



6-5 

 6-5 



Quantity of alcohol-solu- 

 ble furfuroi - yielding 

 substances as pento- 

 sans. 



Unboiled. 



Mgm. 

 13- I 

 13- I 

 18.6 

 14. 6 



Boiled. 



Mgm. 



5-7 

 5-7 

 6.8 

 6.8 



It is evident from Table XIII that the extracts of the fungi are able 

 to break down xylan prepared from rye straw to an alcohol-soluble 

 compound which reduces Fehling's solution and which forms furfuroi 

 when boiled with hydrochloric acid. The fungi then secrete an enzym 

 or enzyms which can break down xylan probably to xylose. 



The crude fiber of the potato is undoubtedly a mixture of compounds, 

 among which are some of the cell wall constituents, including whatever 



