Walker—Nature of Disease Resistance in Plants. 
229 
Cases of Eesistance Due to External Structure of Host 
In a few instances disease resistance has been attributed to purely 
external differences in mechanical structure of the host plant. 
Hairiness of leaves and open habit of growth in certain varieties 
of potato are cited by Stuart (77) and by Appel (3) as facilitating 
more rapid evaporation of rain drops and hence as reducing lia¬ 
bility to attack by Phytophthora infestans (Mont.) DeBary. 
Darnell-Smith (25) found a correlation between resistance to 
bunt (Tilletia tritici) in certain varieties of wheat and lack of 
hair on the terminal ends of kernels. Appel (3) noted that certain 
varieties of raspberries which were covered by a thick, blue, waxy 
layer remained free from attack by Coniothyrium, while other 
varieties were severely attacked. He suggests that the waxy layer 
may possibly influence infection by preventing penetration, or 
causing drops of water to run off. Freeman (32) noted higher 
resistance to stem rust (Puccinia graminis Pers.) in barley grown 
on alkali soils where the amount of bloom’’ was increased. 
The number and structure of stomata have been regarded in 
some cases as factors in disease resistance. In a study of numer¬ 
ous varieties of wheat in relation to rust (Puccinia graminis), Cobb 
(17) notes that in general the stomata were smaller and more 
numerous in the resistant than in the susceptible forms, and to 
these and certain other anatomical differences he attributes the 
cause of resistance. Eriksson and Henning (30), the first to test 
this theory, did not uphold it, and the more accurate studies of 
Ward (88) upon brome rust (Puccinia dispersa Erikss.) failed to 
confirm it'. Norton (59) points out a correlation between small 
stomata and resistance to rust (Puccinia asparagi DC.) in the case 
of the resistant Martha Washington variety of asparagus, but the 
actual relation of this factor to penetration and infection was not 
studied. Allen (2) reports that only a very small percentage of the 
germ tubes of Puccinia graminis tritici enter the stomata of the 
highly resistant variety of wheat, Kanred, and points out that 
the stomatal slit of this variety is smaller and more slender than 
that of the very susceptible variety, Baart. Appel (3) cites a case 
in which resistance in “some Kemontant carnations is due to the 
form of the stomata, which makes it impossible for the hyphae 
to penetrate them.” Pool and McKay (63) point out that the 
immature leaves of beet are nearly immune to infection by Cer- 
cospora heticola Sacc. because the stomata are too small to permit 
