88 
Journal of Agricultural Research 
Vol. V, No. a 
sion over the plants; and when infections developed on the stems their 
upper and lower limits were marked with india ink. The infected plants 
were kept in the greenhouse under conditions favorable for the normal 
development of the host. 
Records were made of infections occurring anywhere on the stem from 
within 6 cm. of the ground to within a few centimeters of the top. Eight 
infections within io cm. of the ground were kept under observation for 
four days. The total upward spread of infection in these during the 
four days was 30 cm., or an average of cm., and the downward 
spread was 21 cm., or an average of 2% cm. Five infections from 10 
to 20 cm. above the soil were studied. Two of these were allowed to 
continue for 48 hours, and the remaining three for only 24 hours. After 
two days the combined spread up the stem in the five cases was 11 cm., 
and down, 6 cm., the average spread up and down in each case being 
2 Vb and 1 7 b cm., respectively. Three stem infections were studied that 
were more than 20 cm. above the soil; two were between 20 and 30 
cm. and one 45 cm. After four days the total spread of infection upward 
was 23 cm. and downward 11 cm. The average upward growth was 
7% cm. and the downward cm. 
It should be noted that in every case the spread of infection was more 
rapid up than down the stem and that the fungus progresses more 
rapidly in young than in old tissues. It is thus evident that it may 
require only a short time for P. infestans to spread sufficiently in the 
potato stem to reach the surface of the soil, once it is in the basal portion 
of the shoot. It is likewise quite probable that the fungus grows down 
the stem from the surface of the soil. 
HISTOLOGICAL, STUDIES OF THE RELATION OR THE FUNGUS TO THE POTATO 
STEM 
The question arises as to which the mycelium uses when it grows up the 
infected stem, the cortex, vascular system, or central cylinder. A section 
of an infected stem always shows that the cortex is discolored, while the 
rest of the tissues are quite normal. The natural inference from this 
macroscopic evidence is that the mycelium used the cortex most exten¬ 
sively. 
In order to get more exact evidence on this point, infected shoots were 
killed in various fixatives and were later sectioned and stained. In every 
case where the cortex was discolored, the cells had collapsed and took the 
stain very heavily, as shown in figure 1. In such cases the mycelium was 
not readily seen, and in the majority of cases it was absent. It was some¬ 
times found, however, in the cells between the outer cambium layer and 
the inner cortical cells, but more often at this stage it was seen growing 
among the pith cells, as shown in figure 2. Where the cells of the cortex 
were more normal, or from % to 1 cm. above the border line between 
