Walker—Nature of Disease Resistance in Plants. 
239 
Other Cell Contents in Eelation to Resistance 
Jones, Giddings, and Lutman (44) considered that resistance 
of certain varieties of potato to the late blight fungus (Phytoph- 
thora infestans) is due to something within the tissues of the leaf 
and the tuber, rather than to difference in epidermal structures. 
Their studies on the relation of the potato cell sap to the fungus 
indicate that acidity of the former has little to do with the disease 
resistant quality. Attempts to study the relation of the extracted 
juice from susceptible and from resistant varieties to the growth 
of the fungus yielded largely negative results. In conclusion they 
say “this (disease resistant) product may, therefore, be assumed 
to be either a compound, modified or destroyed by cooking and 
weakened or removed by filtration through porcelain, or else it 
may be so intimately associated with the living protoplasm as to. 
be inseparable from it by the processes employed.’’ 
Wiltshire (95) studied the relation of apple and pear scab 
fungi to plants susceptible and plants highly resistant to their 
attack. Since these fungi penetrated the cuticle of both susceptible 
and resistant plants he concluded that immunity does not depend 
upon freedom from attack. A study of the germination and growth 
of spores of the apple scab fungus (Venturia inaequalis) in ex¬ 
pressed juice of the host showed in one experiment that germina¬ 
tion was inhibited in the sap of both susceptible and resistant 
varieties, while in the diluted sap no difference from germination 
in water was observed. In another experiment the germ tubes 
of the conidia appeared to grow better in the sap of susceptible 
than of resistant varieties. These experiments indicate that some 
chemical substance which had an inhibitive effect upon the fungus 
was present in the expressed cell sap, but Wiltshire apparently 
did not carry the investigation far enough to definitely establish 
the fact or to throw any light upon the nature of the inhibitive 
substance. 
The fungicidal effect of volatile substances within the plant cell 
has received some attention. Pasteur (62) early noted the retard¬ 
ing effect of onion juice upon yeasts. Bernard (6) noted the 
fungicidal effect of volatile substances in the tissue of an orchid 
(Loroglossum) upon the mycorhizal fungus from other closely 
related species. Walker (85, 86) noted that the volatile onion 
oil has a retarding effect upon germination and growth of the 
onion smudge fungus (Colletotrichum circinans). It was shown. 
