Dec. aa; 1923 
Cytology of Wheat Stem Rust 
59i 
The death of the fungus may occur a day or two after its entry 
through the stoma, or it may be deferred for a considerable period. The 
great majority are dead before the ninth day. It is a surprising fact, how¬ 
ever, that the few that survive this long may continue for a much longer 
period. It is not known how long, but material fixed 15 days after inocu¬ 
lation still shows an occasional infection with a few feeble living hyphae. 
The fungus carries on a meager existence, barely living from day to day, 
and never rising into vigor enough for any attempt at spore formation. 
The appearance of the tissues in one of the small flecks caused by the 
fungus is represented in Plate 7, B. It is a longitudinal section of the 
leaf 15 days after inoculation. Below the stoma, at the point of entry, d , 
are several empty hyphae, and scattered threads occur elsewhere among 
the cells. One or two of them still contain a little cytoplasm. Some of 
the earliest cells attacked (/, g, and h) have shrunken and their contents 
are so transformed as to appear a homogeneous blur. The cells attacked 
later have retained their shape, except where distorted by a swollen cell 
wall. Several cells contain rounded haustoria, with thick stratified 
coverings (a, b , c, and e). Sometimes the haustorium is the only object 
left in such a cell, the nucleus and plastids having disappeared. Some 
of the near-by cells are also empty, while others contain a nucleus and 
minute plastids. This dead area is large enough to be seen as a minute 
white speck in the living leaf. 
Effect on Nuclei and Plastids 
A study of nuclei and plastids in infected areas in Mindum, correspond¬ 
ing to the studies of Baart and Kanred, is summarized in Table IV. 
In studying these records, several points should be borne in mind. 
Mycelia vary widely in length of life and in size. Some were dead before 
the fourth day and others maintained a few living hyphae to the fifteenth 
day. Some mycelia remained minute and their effect upon the host 
extended but a short distance, while others spread farther and their 
influence extended through a broad zone beyond the fungus itself. Be¬ 
sides this, healthy host nuclei vary in size more than in the other hosts, 
ranging from 85 by 64 to 108 by 77, and this makes it more difficult to 
estimate accurately the effect of the fungus. 
In spite of these irregularities, however, several points are shown fairly 
well. Extreme expansions of host nuclei have not been found in infected 
areas of any age. Nuclei in the first cells attacked by the young fungus 
are destroyed rapidly and become indistinguishable. In older infections 
the nucleus of a cell containing a haustorium also collapses soon and dies, 
but may be seen for a time as a flat, red-stained disk. 
In a narrow zone of tissue distant only one or two cells from the fungus 
the nuclei are uniformly collapsed and dead and lie flattened against the 
cell wall. An occasional cell farther out is in the same condition. This 
collapse takes place remarkably early, being quite as marked on the fourth 
day as in older material, the short diameter of these nuclei (Table IV, first 
column) being between 40 and 50 throughout the series. The dimensions 
give little evidence of expansion previous to the collapse, for the long 
diameter of these flattened nuclei is (with one exception) but little over 
100. 
