216 COLOCASIA BLKiHT. 



of explanation. It is no longer widely believed that potato blight 

 begins in the new crop by the extension of mycelium from the 

 tubers, where it has remained in a dormant condition since the 

 previous crop was lifted, into the young shoots, where aerial sporan- 

 gia are developed. De Bary's 1 early experiments showed, no doubt, 

 that this course is sometimes followed. Recent investigators- 

 hold that it is certainly rare and insufficient to explain the normal 

 occurrence of the disease in the field. Clinton further notes that 

 the first spots develop in leaves in contact with the ground and 

 believes that the primary attack comes by direct infection of the 

 leaves from the soil. It is true that Phytopkthoras do not appear 

 to have been cultivated successfully using sterilised soil as a medium, 

 but sterilisation may injure the soil regarded as a culture medium. 

 The allied genus Pythium is quite commonly present in garden 

 soils but as the mycelium is easily overlooked, special methods are 

 required for its isolation, and the same holds good for other genera. 3 



The germ-tubes which arise from sporangia in cultures grow 

 not only from the papilla but, when more than one are present, 

 from any part of the spore wall (PI. Ill, Fig. 12). Frequently 

 secondary or even tertiary sporangia are formed after limited 

 growth of the tube (PI. Ill, Fig. 13). Sporangia that germinate 

 in this fashion are often traversed by cellulose walls dividing the 

 cavity into two or more compartments. 



Chlamydospores are frequently formed in culture. They are 

 spherical, spore-like bodies with a smooth, amber-coloured wall 

 which may be 3n thick and which takes a clear orange-yellow colour 

 when treated with Schulze's solution. They may be terminal or 

 intercalar or formed by a lateral swelling of the hypha (PI. Ill, 



1 De Bary, A. Researches into the Nature of the Potato Fungus (Phylophihom infestuns). 

 Journ. of Botany, n. s., V, 1876. 



* Clinton, C4. P. Downy mildew of potatoes. Conn. Exper. Sta. Report, 1905, p. 304. 

 Pethybridge, G. H. Considerations and experiments on the supposed infection of the potato 

 crop with the blight fungus (Phytophthora infestans) by means of mycelium derived direct ly 

 from the planted tubers. Proc. Roy. Dublin Soc. XIII, n. s., No. 2, 1911. Jones L. R,, 

 Giddings N. J., and Lutman B. F. Investigations of the Potato Fungus (Phytophthora infestans). 

 U. S. Dept. of Agri. Bur. of PI. Indus. Bull. 245, 1912. 



3 Jensen, C. N. Fungus flora of the soil. Cornell Univ. Agric. Exper. Sta, Ithaca, Bui}. 

 J15, 1912, p. 420. 



