EVOLUTION OF PARASITIC FUNGI. 101 



food for the fungus. While I was able to obtain the fungus in 

 pure culture, and kept it growing in the laboratory in this con- 

 dition for more than eight months, it absolutely and invariably 

 refused to grow when an attempt was made to grow it upon 

 dead, though sterilized, potato and tomato leaves. The growth 

 of the fungus was obtained in the well-known way by cutting 

 open affected potatoes during the month of January, and placing 

 them in a moist chamber with the cut surface uppermost in 

 a warm room until the mycelium in the tubers started to new life 

 and appeared on the surface with the characteristic conidiophores 

 and conidia. Rectangular blocks were then cut from fresh pota- 

 toes, rinsed in a one-tenth per cent corrosive sublimate solution, and 

 dropped one each into a culture tube containing Avater previously 

 sterilized by heat in sufficient quantity to permit of the washing 

 off the corrosive sublimate from the upper end of tlie block, and 

 to provide sufficient moisture at the bottom of the culture tube. 

 To these blocks of potato, portions of the mycelium and conidia 

 of the Phytophthora were transplanted, and in several of them 

 a pure growth of the fungus was obtained, which was verified 

 by examination with the microscope and found to agree in all re- 

 spects with the fungus grown on the potato. When these blocks 

 were well covered with the fuiigus, transfers were made to fresh 

 tomato leaves sterilized by heat, as well as to potato blocks ster- 

 ilized in the same way. But in all cases the fungus refused to 

 grow on these heat-sterilized substances, though it continued to 

 grow readily when transferred to fresh living blocks of potato. 



If the fungus cannot grow as a saprophyte, then it is quite an 

 interesting case of wide range in parasitism, with no apparent 

 tendency to develop fixed forms on the different hosts. 



Another member of this genus might be mentioned here, with 

 which you are probably not so familiar, though it has been 

 reported in this country, the seedling rot Phytophthora, P. cac- 

 torum. This has been known in Europe since 1870, and occurs 

 on quite a wide range of hosts. Because of this de Bary gave it 

 later the name P. omyiivora, the specific name indicating, as you 

 see, that the fungus is not very choice in the selection of its 

 food. It Avas first described as producing a rot of species of 

 Cactus, but it is chiefly knoAvn as the seedling Phytophthora, 

 since it produces at times a serious rot of seedling trees, espe- 

 cially of the beech. The forms described on somewhat widely 



