Perennial Mycelium in Species of Peronosporaceae 65 



in the patch and why plants growing near each other should be infected 

 in some cases and not in others. 



As the host is a perennial, as infection by Peronospora viciae is sys- 

 temic, and as oospores are produced only sparingly, if at all, on Vicia 

 xepium,^ it seems very probable that the mycelium survives the winter in 

 the living tissues of the host. 



PLASMOPARA HALSTEDII 



In the spring of 191 1 Plasmopara halstedii was found to be very 

 abundant on some young plants of Helianthus diversicatus ^bout 6 inches 

 high. The plants were somewhat dwarfed, chlorotic, and well covered 

 with conidiophores, giving every evidence of systemic infection. The 

 location of the infected plants was 

 marked and observations made during 

 the winter and spring of 191 2. 



Fourteen of the plants that were very 

 generally infected were staked, and on 

 January 4, three of these were chopped 

 out of the ground and transplanted in the 

 greenhouse in exactly the same way as 

 were the Lepidium plants infected with 

 Peronospora parasitica. Each of these 

 rhizomes produced a chlorotic shoot 

 which was covered with spores of Plas- 

 mopara halstedii. On March 4 four more 

 were brought into the greenhouse. One 

 of these rotted in the soil, but each of 

 the others produced a shoot, which 

 showed infection as soon as it appeared 



above ground. The remaining seven of the fourteen staked were left in 

 the patch and kept under observ^ation. On May 10, when they were 3 

 to 6 inches high, all were found to be infected with Plasmopara halstedii, 

 except one plant, which was entirely free from infection, as were many 

 others in the immediate vicinity. Two of these plants were now dug 

 up, and portions of the stems at their junction with the rhizomes were 

 fixed in various strengths of Flemming's killing fluid. Paraffin sections 

 cut from this material and stained showed abundant myceUum in all 

 parts of the stem except the fibrovascular bundles, the mycelium being 

 entirely intercellular with globular haustoria extending into the cells, 

 as shown in figure i . The presence of the m3'celium in the stem at its 

 junction with the rhizome shows that the infection was systemic and 

 probably came from the rhizome in the beginning. 



The writer searched many times in the tissues of all stages of maturity for resting spores, but without 



Fig. I.— a cross section of a stem of Helian- 

 thus diversicatus which is infected with 

 Plasmopara halstedii. The mycelium is 

 shown in the cortex at the junction of the 

 stem with the rhizome of the host. 



