88 Journal of AgrictUtural Research voi. 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 wdthin lo 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 2H 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 

 2V0 and 1V5 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 

 7^4 cm. and the downward 3^ 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 OP THE) RELATION OF 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 i. 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 grooving 

 among the pith cells, as shown in figure 2. Where the cells of the cortex 

 were more normal, or from X to i cm. above the border line between 



