190 Journal of Agricultural Research voi. vi. No. s 



then filtered, and the filtrate hydrolyzed with dilute hydrochloric acid. 

 Some of the mycelium of these fungi was extracted with alcohol, and 

 then dried and extracted with hot water. The extract was then filtered 

 off, treated with hydrochloric acid exactly as in the acid hydrolysis of 

 starch, neutralized and tested for reducing substances. A considerable 

 quantity was found. The filtrate did not give a qualitative test for 

 pentosans. The apparent increase in starch content in the rotted por- 

 tions of the potatoes, then, is due to compounds laid down by the fungi. 

 From the fact that only a small amount of mycelium of these fungi 

 could be present in the rotted potato it would seem probable that it 

 the starch were attacked to any extent the apparent starch content as 

 obtained by acid hydrolysis would be lowered in all cases. To obtain 

 further information on this point experiments were carried out to ascer- 

 tain whether these fungi secreted diastase and if so, whether this enzym 

 could break down the starch grains of the potato. 



Extracts of the undried, ground mycelium of the two fungi, F. oxy- 

 sporum and F. radicicola, were made with 50 per cent glycerin. These 

 extracts were filtered after 24 hours through absorbent cotton and por- 

 tions added to a 2 per cent solution of "soluble starch." Suitable con- 

 trols were prepared and all preparations allowed to stand in an incubator 

 under toluol at 30° C. for 48 hours. At the end of this time the starch 

 was practically all broken down by the extracts of both fungi. Similar 

 experiments were carried out with starch paste made from potato starch 

 with positive results. The fungi then secrete diastatic enzyms. The 

 experiments, however, did not prove that the diastases were able to 

 attack the starch grains before they were broken down. Brown and Mor- 

 ris (2) have shown that malt diastase can not act on ungelatinized potato 

 starch, though the starch grains of barley are readily eroded by it. 

 Whether the enzyms in the extracts of the myceUum could erode the 

 starch grains of the potato at room temperature was determined by 

 placing some well-washed potato starch in extracts and allowing the 

 preparations to stand under toluol. They were shaken up and examined 

 from time to time, but no sign of erosion of the starch grains was evi- 

 dent even at the end of a week. The extracts used were tested on starch 

 paste or "soluble starch" with positive results. Smith and Swingle (9) 

 mention that the starch in the potatoes rotted with F. oxysporum was 

 apparently not eroded. It is, of course, possible that the potato starch 

 grains are very slowly attacked by the diastases of these fungi or that 

 some inhibitor is present which prevents the action of the enzym on 

 the starch in this condition at the temperature at which these studies 

 were made. These points should be investigated. At present, however, 

 the conclusion seems warranted in view of the evidence that the starch 

 of the potato is not appreciably affected by the fungi. 



From the fact that these fungi penetrate the cell walls or parts of the 

 cell walls of the potato in living parasitically upon their host, their effect 



