5«4 
Journal of Agricultural Research 
Vol. XXVI, No. i» 
In the 13- and 15-day infections recorded (Table III, 13, and 15) there 
is a secondary increase in the density of the mycelium and in the size 
of host nuclei some distance away from the main mycelium (normal). 
Here secondary uredinia are forming. 
The plastids undergo a very slow, steady decrease in size from the 
seventh to the fifteenth day, and the reduction is almost uniform 
throughout any given infected area. So far as observed, the reduction 
is never extreme. At the end of 15 days, under the conditions of these 
experiments, at least, the plastids are still far from minute (30 X 20) . 
No outer zone has been noted in which the plastids are markedly smaller, 
and no period of marked reduction in size followed by partial recovery. 
Perhaps this is correlated with the fact that expansion of the nuclei 
takes place sooner and is more widespread here than in Baart. 
MINDUM AND PUCCINIA GRAMINIS TRITICI FORM III 
MACROSCOPIC OBSERVATIONS 
Mindum inoculated with Puccinia graminis tritici form III gives an 
o-type of infection. No spores are formed. Small spots or “flecks” 
of discolored host tissues occur, but they are surprisingly late in appearing, 
sometimes not showing until the eighth day after inoculation. Moreover, 
the flecks are few in number even when an abundance of spores is applied 
to the leaf, and they differ markedly in appearance from the commoner 
types of rust flecks. The fleck here consists of a minute circular grayish- 
white area, uniform in color, and sometimes visible on only one side of 
the leaf. 
MICROSCOPIC OBSERVATIONS 
The study of prepared slides shows that the spores germinate readily 
when placed on the leaves, and appressoria are to be found over the 
stomata on the day following inoculation. By the end of two days 
many of the fungi have passed through the stomatal slit and begun the 
attack on the host tissues. 
Entrance Phenomena 
As in every combination of rust and host studied so far, some of the 
fungi do not enter. These usually are in the minority, but always are 
found. Many of the entries occur on the second day, when both the 
appressorium and the guard cells with which it lies in contact look 
normal. On the third day after inoculation the guard cell wall in contact 
with the appressorium loses stainability (PI. 4, A), and the central part 
of the inner wall of the cell is also pale. Slight, if any, changes in the 
guard cell contents are to be noted at this time. The formation of the 
central mass of nuclear material at a may be a reaction to a stimulus 
from the fungus, for host nuclei tend to move toward the fungus. There 
was some evidence of this in Kanred, but it is much more noticeable 
here. If, as sometimes happens, the appressorium lies at one end of the 
stoma, the elongated nucleus of the guard cell often will contract into a 
single lump, lying just under the fungus. 
The reaction progresses more rapidly and is more extreme than in 
other cases studied. The entire wall loses power to stain, and later the 
cell contents die and become dissolved, first in the part of the cell nearest 
the fungus and later in more remote parts. The effect may extend 
