242 Wisconsin Academy of Sciences, Arts, and Letters. 
factors entering into this relationship. Resistance may be due 
to a substance (or substances) in the host tissue which is either 
toxic to the parasite or is capable of neutralizing or destroying 
the enzymes or toxins produced by the parasite and hence may 
preclude or check invasion; it may be due to a lack of proper 
nutrients in the host tissue for the development of the invader; 
or it may be due to a lack of ability on the part of the parasite, 
first, to penetrate the host plant or, secondly, so to adjust itself 
to the chemical and physical complex of its new substrate as to 
make food materials, even though present, available for absorption 
and assimilation. It is entirely possible that one or all of these 
factors may enter into a given case of disease resistance. It should 
also be recognized at the outset that a clear conception of the 
nature of the parasitism occurring in any given case is essential to a 
clear understanding of resistance. Indeed, the two processes are 
merely phases of a single phenomenon which involves the inter¬ 
relation of two living organisms. Moreover, it should be made 
clear that, as Duggar (28) has pointed out, ‘‘there are all grades 
of parasitism and there must be a variety of effects induced in the 
host, including changes essentially autolytic.’’ He compares, for 
instance, the effect of Cyst opus candidus (Pers.) Lev. upon the 
host tissues with that of closely related fungi, as Phytophthora in- 
festans or Pythium deharyanum; and again, the rusts which have 
“little tendency to kill immediately the tissues which they invade” 
with the large group of parasites which have no contact with the 
living protoplasm but kill the cells in advance by means of 
secreted diffusible substances. The greatest advance in our knowl¬ 
edge of the nature of disease resistance will therefore come from a 
thoroughgoing, persistent study of individual cases, and especially 
of those which seem to offer the most accessible approach. 
Literature Cited 
(1) . Aamodt, Olaf S. The imheritance of resistance to several biologic 
forms of Puccinia graminis tritici in a cross between Kanred and 
Marquis wheats. (Abstract.) Phytopathology 12: 32. 1922. 
(2) . Allen, Buth. F. A cytological study of infection of Baart and Kan¬ 
red wheats by Puccinia graminis tritici. Jour. Agr. Eesearch 23: 
131-152. 1923. 
(3) . Appel, Otto. Disease resistance in plants. Science n. s. 41: 773-782. 
1915. 
(4) . Averna-Sacca, Bosario. acidita dei succhi delle piante in rapporto 
alia resistenza contro gli attach! dei parassiti. Staz. Sper. Agr. 
Ital. 43: 185-209. 1910. 
