Walker—Nature of Disease Resistance in Plants. 
235 
that under certain conditions the cabbage yellows fungus (Fusar- 
ium conglntinans) would invade the root hairs of flax, but was 
incapable of progressing farther and producing a wilt similar to 
that caused by Fusarium Uni. In a comparative study of two 
closely related organisms causing the leaf spots of alfalfa and red 
clover, respectively, Jones (41) showed that when sown under 
proper conditions upon clover leaves, the ascospores of Pseudo- 
peziza medicaginis (Lib.) Sacc. germinated and the hyphae pene¬ 
trated the epidermal cells, but the progress of the fungus was 
checked at this point; likewise when ascospores of the red clover 
organism, Pseudopeziza trifolii (Bernh.) FcL, were sown upon 
alfalfa leaves, the germ tube penetrated but did not advance beyond 
the epidermal cell. Salmon (67) noted that the haustoria of the 
wheat mildew (Erysiphe graminis) penetrated the epidermal cells 
of the barley leaf, but that they eventually shriveled and died with¬ 
out producing the characteristic disease symptoms. Explanation 
of the cases just mentioned on the basis of the absence of positively 
chemotropie substances in the host cell is unsatisfactory, since 
several other possible explanations might be offered, such as the 
presence of inhibitive substances in the host cell, or the absence 
in the parasite of proper enzymes or toxins to bring about the 
chemical or physical changes in the host cell necessary to provide 
food for the growth of the invader. 
Osmotic Pressure as a Factor in Resistance 
From his earlier experimental studies upon phanerogamic para¬ 
sites, McDougal (48) concluded that an osmotic pressure of the 
parasite higher than that of the host was essential. In the light 
of more recent researches he believes that osmotic pressure may be 
of minor importance in the establishment of the haustorium, and 
that absorption by imbibition and the force of expansion of the 
invading protoplast are important factors at this early stage. In 
this connection the work of Hawkins (38) is significant. Using 
two potato-rotting fungi, and one strawberry-decaying fungus, 
he found that they would all grow on solutions of glucose, sucrose, 
potassium nitrate or calcium nitrate at diffusion tensions much 
higher than the total diffusion tensions of the dissolved substances 
in the juices of their respective host plants. Vavilov (82), after 
examining many species and varieties, was unable to establish 
any correlation between osmotic pressure and resistance. 
