230 Wisconsin Academy of Sciences, Arts, and Letters. 
penetration by the fungns hyphae. Valleau (80) suggests that 
resistance to Sclerotinia cinerea (Bon.) Wor. in certain types of 
plums may be due to the fact that the stomata early become plugged 
with small parenchymatous cells which impede penetration. 
The possible exclusion or retardation of parasites by means of 
unusual thickness of cuticle or by outer layers of corky cells has 
been suggested in a number of cases. Cobb (17) suggests that the 
thicker cuticle which is common in rust resistant varieties of wheat 
may contribute to resistance by preventing the maturation of the 
rust sori. Later work, such as that of Ward (88), indicates that 
this is not the important factor in rust resistance. An interesting 
case of correlation between thickness of cuticle in the tomato fruit 
and its resistance to Macrosporium tomato Cooke has recently 
been described by Rosenbaum and Sando (66). Young immature 
fruits are highly susceptible, while old fruits are resistant to attack. 
This difference does not appear to be due to changes within the 
host, since infection may readily be obtained in old fruits by 
first injuring the skin. There are no natural openings in the skin 
and the cuticle becomes thicker as the fruit matures. Moreover, 
the outer layer is shown to become gradually more resistant with 
age to mechanical puncture with a needle, and the authors suggest 
that resistance in the older fruits may be due to the ability of the 
thicker cuticle to resist puncture by the mycelium of the para¬ 
site. In discussing the pink disease of rubber (Corticium sal- 
monicolor B. & Br.) in India, Butler (16) states that the attack 
is most marked on the shady side of the tree where the thinner 
cork offers less resistance to invasion. Appel (3) points out the 
importance of corky layers as a factor in resistance to certain 
parasites. In the ease of the sweet potato, Weimer and Harter 
(91) have shown that wound cork layers may retard infection and 
that suberization of cell walls at the surface of the wound is often 
sufficient to prevent the entrance of microorganisms. 
Cases of Resistance Due to Internal Character of the Host 
In the foregoing cases, external differences in plant structure 
have been considered the determining factors in disease resistance. 
There is a much larger number of cases in which resistance is 
seemingly due to internal causes which have been sought' either 
in the cell membranes or cell contents. The pioneer work of 
DeBary (26) upon Sclerotinia demonstrated the secretion of a 
