484 
Journal of Agricultural Research 
Vol. XXVII, No. 7 
Stakman (44) has studied the action of specific forms of stem rust 
fungi on immune and susceptible varieties of wheat, oats, and several 
other hosts and has shown that there is characteristically a very definite 
antagonism between the protoplasmic cell contents of the immune 
plant and the parasite. 
Woolman (58) has recently described the cytologic phenomena of 
infection of wheat seedlings by bunt caused by Tilletia tritici. He 
states that “Tilletia tritici enters the epidermis of the coleoptiie of both 
susceptible and resistant wheat plants grown under conditions for 
maximum infection, but that in highly resistant varieties it develops no 
further. Inhibiting factors evidently are active in or just beneath the 
epidermis.” 
The writer made a histologic examination of young rye plants which 
had been inoculated with spores of Urocystis tritici . Inoculation 
experiments have shown repeatedly that rye is resistant to infection 
by U . tritici , but there was no information on the relationship 
involved. The coleoptiles of young rye seedlings were heavily inocu¬ 
lated with germinating spores of U. tritici and placed under condi¬ 
tions of high humidity at 17 0 C. The epidermis was stripped from 
some of the seedlings at intervals up to four days, and examined for 
evidence of entrance of the organism. Portions of the coleoptiles also 
were fixed and sectioned. A typical section is that shown in figure 2, b. 
It was very evident that entrance had been effected but that, owing 
to the unfavorable conditions within the host, development of the 
organism had been checked. The mode of entrance of the germ tubes 
resembles the mode of entrance in a susceptible host. Figure 1, e, 
shows an early stage of entrance of the mycelium. The germ tube 
swells into an appresoriumlike body and penetrates first through a 
small opening, somewhat similar to that figured by Waterhouse (55) 
for entrance of sporidial germ tubes of Puccinia graminis into the bar¬ 
berry leaf, and Leach ( 26 ) for entrance of germ tubes of Colletotrichum 
lindemuthianum into beans. No further studies were made at this time 
on the subsequent history of the infection hyphae of U. tritici within 
the tissues of the immune host. 
DISCUSSION AND CONCLUSIONS 
A knowledge of the reaction of a pathogene to controlled environmental 
conditions is fundamental to an understanding of its behavior in nature. 
It is well known that the severity of many outbreaks of plant disease 
frequently may be correlated with the environmental conditions to which 
the pathogene was exposed at a particular period in its life history. 
Laboratory and greenhouse studies on Urocystis tritici have shown 
that the viability of the spores is profoundly affected by exposure to 
controlled temperatures and relative humidity. The germination of the 
spores, infection, and the subsequent development of the organism 
within the host also are greatly influenced by the conditions of the 
environment. 
In an infested area in which it is still impracticable to replace the 
wheat varieties which are susceptible to flag smut by others which are 
resistant, adequate control of the disease may be expected only by a 
system of cultural practice which reduces the amount of viable inoculum, 
which also reduces the possibilities for infection, and finally which tends 
to reduce the possibilities for serious damage in the crop after infection 
has occurred. 
