28o Journal of Agricultural Research voi. xviii, no. s 



of cell walls for the fungus to penetrate in traveling a given distance. 

 It may, however, be due to some resistant quality of the cell wall. 



That the fungus secretes a toxic substance which kills potato cells 

 was demonstrated experimentally by a method somewhat similar to that 

 followed by Brown (6, 7, 8) in his work with Botryiis cinerea. Cultures 

 of the fungus were grown for two weeks on sterilized potato mush, and 

 potato plugs and the mycelium were removed in such a way that none of 

 the culture medium adhered to the mat of mycelium. The mycelium was 

 then ground in a mortar with sand, extracted with distilled water, and 

 filtered. Cylinders about i cm. in diameter were cut from potato tubers 

 and sliced into disks 0.5 mm. in thickness. Some of these disks were 

 placed in the extract of mycelium and some in distilled water and exam- 

 ined at intervals. After three hours the disks in the fungus extract had 

 lost their turgidity so that when grasped by the edge with a pair of forceps 

 and held in a horizontal position they collapsed limply. The disks from 

 the distilled water preparation remained turgid for 1 2 hours or more. 

 Disks from the preparation of fungus extract did not resume their normal 

 turgidity when washed and placed in distilled water. The cells of the 

 potato are apparently killed by some substance extracted from the 

 ground mycelium. The loss of turgidity can not be accounted for by a 

 loss of water from the potato cells caused by a higher osmotic pressure 

 in the extract, because tests showed that the lowering of the freezing 

 point of the extract used was only about one-fifth that of the juice from 

 the potato tuber. All three varieties of potatoes used in these experi- 

 ments behaved in the same way. From these experiments it seems hardly 

 probable that resistance to fungus attack can be due directly to the living 

 protoplasm. 



The macerating effect of this extract on the potato tissue has been 

 mentioned earlier in this paper. The properties of the toxic substance 

 secreted by the fungus were not determined, though the problem is 

 well worthy of investigation. 



It has been shown in some cases that resistance to certain fungus 

 diseases was correlated with higher acidity of the host plant tissues. 

 Thus Averna-Sacca (2) has shown that the resistance to diseases of 

 grapes caused by Oidium and Peronospora was correlated with a relatively, 

 high acidity. Comes {11) has demonstrated that a variety of wheat 

 (Rieti), resistant to rust, has an acidity considerably higher than the 

 varieties in the same locality which are susceptible to this disease. 

 Further, this writer has shown that when this resistant variety is grown 

 in other localities where the environmental conditions tend to produce 

 a plant of lower acidity, the plant is susceptible to the disease. These 

 researches indicate that acidity may play a very important r61e in the 

 resistance of a plant to disease. 



There are, of course, many other factors that tend to influence the 

 resistance or susceptibility of a plant to disease. The literature on this 



