16 



MASS. EXPERIMENT STATION BULLETIN 183. 



various heights. The spores are frequently borne on lateral branches 

 of this stem (Fig. 6), while the main stem is continued upward and 

 terminates in an enlarged club. The ultimate branchlets, and one or 

 two series below them, are usually in threes, as shown in Fig. 5, but 

 twos are not uncommon. In regard to the dimensions of the co- 

 nidiophore, Morgan (1892) writes: "the fertile hyphaj have a simple 

 septate stem 5 to 7 /^ in thickness, and are dissolved above into a level- 

 topped cyme of branches; their height, exclusive of the spores which 

 easily fall off, is 125 to 150 z'." Ellis and Everhart (1900) give the di- 

 mensions as 50-110 X 5-6 //. In pure 

 culture the writer has found them taller 

 than the above measurements; an average 

 of 50 conidiophores grown on potato agar 

 gave 291 ^t, and the diameter of the stalk, 

 6.6. fi. 



CONIDIA. 



The conidia are 



long, cylindrical, 



obtuse at each end, 



hyaline, divided 



into 2 cells by a 



septum at the 



center (Fig. 7). The 



contents are at first 



homogeneous, but 



later show vacu- 

 oles or oil drops 



(Fig. 8). Morgan 

 (1892) gives the dimensions as 40-50 x 4 // at the apex, and 3 /^ at the 

 base; Ellis and Everhart (1900), 40-50x4-5/^; Massey (1917), 36-55 x 

 3.3-4.51 fi, with an average of 48.3 x 4.13 /i. The writer found the aver- 

 age of 50 on a young potato agar culture to be 48.8 x 5.1^; 50 on a 

 two-months' culture, 39.2 x 4.03//; 50 produced on a pod of Gleditsia, 

 41x4.1/^. 



LIFE HISTORY OF THE FUNGUS. 



Before any measure of control could be intelligenth^ attempted it 

 was first necessary to become intimately acquainted with the life history 

 of the causal organism (the pathogene) . In the studies which are recorded 

 below most attention was directed to those points which appeared to 

 have a direct connection with control. Nevertheless, in order to become 

 familiar with the entire life cycle, certain phases of development which 

 have no obvious connection had to be investigated. For convenience in 

 discussion, the life history is treated under three heads : — 



1. Germination of the spores. 



2. Parasitic life of the fungus (pathogenesis). 



3. Saprophytic life of the fungus. 



Fig. 7. — Germinating conidia. 



Fig. 8. — Old conidia. 



