592 
Journal of Agricultural Research voi. xxvi. No. i> 
Table IV.— Comparative size of host nuclei and of plastids in infections of Puccinia 
graminis tritici form III of different ages on seedlings of Mindum durum wheat. 0 
3 cells 
3 or 4 cells 
8 to 10 cells 
Days after inoculation. 
beyond 
beyond 
beyond 
section. 
fungus. 
fungus. 
fungus. 
HOST NUCLEI 
100 X 43 
118X42 
105X42 
IO3X40 
8 lX 43 
IOIX 49 
110X77 
110X71 
109X60 
113X77 
95X53 
88x59 
108X77 
97 X 72 
100X68 
96X72 
90X62 
85X64 
11^X78 
118X55 
112X80 
112X5 2 
97X6o 
PLASTIDS 
50-60X3° 
52X31 
S0X24 
61X32 
32X15 
37X26 
41X19 
52X26 
31X13 
38X20 
36X22 
46X23 
3*Xi8 
34X18 
36X19 
52X28 
13X 8 
26X18 
55X32 
57X26 
15X12 
24X13 
30X16 
41X22 
a Each figure given represents the sum in millimeter sof the diameters of 10 nuclei or 10 plastidsXii3o. 
In the tissue three or four cells away from the fungus, and to a lesser 
extent even farther out, the nuclei are usually slightly enlarged (cf. second, 
third, and fourth columns in Table IV) and contain an open network. 
Plastid size in these same areas also is given in Table IV. The normal 
plastid is 50 to 60 by 30, varying somewhat in different tissues. In the 
cells containing haustoria, the plastids disappear. In the narrow zone 
of tissue distant only one or two cells from the fungus, where we find the 
nucleus collapsed and dead, the plastids are nearly normal in size (52 
by 31) on the fourth day, but undergo a slow and steady decrease from 
then on. The figures for these plastids on the fifteenth day (15 by 12) 
represent the plastids found, but do not represent the whole situation, 
as many of the cells are by this time quite empty. At all ages the small¬ 
est plastids are in the cells nearest the fungus, and the damage grows less 
as the distance from the fungus increases. 
Under these conditions, then, the nuclei near the fungus die, and those 
a little farther off undergo but slight expansion. The plastids are small¬ 
est nearest the fungus, and show progressive decrease with age. 
COMPARISONS OR FORMS III AND XIX ON BAART, KANRED, AND MINDUM 
Several questions arose in connection with the study of the death of 
guard cells of stomata occupied by appressoria of the rust. How marked 
is the difference in intensity and rapidity of this reaction on different hosts ? 
What connection, if any, exists between this reaction and the entry of 
the fungus, or its exclusion from the host? Is there any correlation 
between the strength of this reaction and congeniality, or lack of it, 
between the rust and its host ? Would two specialized forms of the rust 
differ in the intensity of this reaction on any given host, and would such 
a difference be correlated with resistance or susceptibility of that host 
to the rusts? 
