382 
Journal of Agricultural Research 
Vol. XXVII, No. 6 
has been observed that varieties which may be susceptible in the seedling 
stage apparently are resistant in the field. That is, the rust does not 
develop on them sufficiently to do any damage. The indications are that 
either the germ tubes fail to enter the host, or, if they do, the mycelium 
for some reason can not develop extensively. Attention has been focused 
on this phase of the problem particularly since the discovery of numerous 
biologic forms of P. graminis tritici. 
The specific problems which the writer undertook to investigate, there¬ 
fore, were the following: (i) Are the morphological differences in differ¬ 
ent varieties of wheat sufficiently great to affect materially the entrance 
of germ tubes? (2) After the germ tubes have entered, are there struc¬ 
tural and physiological peculiarities which account for the inability of 
certain biologic forms to develop in certain varieties of wheat ? (3) Can 
these structural and physiological characters be altered sufficiently by 
environmental factors to change the reaction of the host to the rust 
parasite ? 
HISTORICAL REVIEW 
Much of the literature on the subject of rust resistance has been re¬ 
viewed elsewhere, so only a brief review will be given of those papers 
which bear definitely on the problems at hand. 
The relation of the morphology of the host to rust resistance has been 
studied by several investigators. Sappin-Trouffy (47) 3 stated that the 
mycelium of Puccinia graminis is entirely localized in the chlorophyll¬ 
bearing parenchyma, so that it can grow readily only longitudinally in 
the stem, being prevented from extensive radial spread by the hemispheri¬ 
cal sheath of sclerenchyma fibers. The natural result is the formation of 
linear pustules. 
Cobb (8) thought that the degree of rust resistance of wheat varieties 
was correlated with the tensile strength of the leaves, the ratio of scleren¬ 
chyma to chlorenchyma, the amount of waxy bloom, the number and 
size of stomata, and the number and length of leaf hairs. Petermann 
(40, p . 13-16) suggested that silica increased the strength of cell walls 
and their resistance to puncture, thus increasing resistance. Farrer 
(75) was of the opinion that wheat plants with narrow erect leaves and 
thick epidermis were likely to be resistant. Ward (57), Eriksson and 
Henning ( 12 ), and Biffen (6), on the other hand, concluded that external 
morphology had little effect on resistance to various rust fungi. 
Ward (57) found that Puccinia glumarum entered susceptible and 
resistant hosts equally well, but that the mycelium could not develop 
normally in the latter. The hyphae soon died in the resistant host, 
either on account of starvation or poisoning. Gibson ( 16) showed that 
germ tubes of rust fungi easily gained entrance through the stomata of 
many plants, but that the hyphae could produce haustoria and develop 
subsequently only in susceptible forms. Marryat (34) in general con¬ 
firmed the observations of Ward and Gibson and showed further that 
the cells of wheat varieties resistant to jF\ glumarum were killed by the 
rust hyphae, which then also died. Stakman (57), Allen (2), and Newton 
( 37 ) found that essentially the same thing was true of P. graminis . 
Allen (7) further showed that the size and shape of the guard cells may 
be a factor in the entrance of the germ tube. 
1 Reference is made by number (italic) to 1 ‘ Literature cited,” p. 408 . 
