PAPERS OX BOTAXY 147 



safranin stain was employed to differentiate the euticular 

 layer of tlie liost, which lies just below the fungus. 



The Thallus. The vegetative thallus of sooty blotch is 

 made up of a mycelium of j)rofusely branched hyphal 

 threads. The mycelium is composed of cells, olivaceous 

 in color, according to Saccardo's * * Chromotaxia ' ' (1891), 

 slightly constricted at the septa, usually isodiametrical 

 in shape (Fig. 12). There is considerable variation in 

 cell dimensions, measurements of width varying from 

 2-5m, and of length from 2-8fi. Individual cells, groups 

 and chains are often found with walls relatively thicker 

 than usual, and darker in color than is tyi3ical. During 

 the early growth of the mycelium, all the hyiDhal threads 

 appear to extend in the same plane. Lateral branching 

 is initiated veiy soon, however, which may result in such 

 a profuse interlacing and crossing, that a mycelial crust 

 results. 



Several variations in the form of the thallus have been 

 observed. In one, which appeared commonly on Ehode 

 Island Greening apples, and which is illustrated in Fig. 

 2. the thallus starts from a single mycelial cell, from 

 which, by division, three or more cells are cut off. and in- 

 itiate profuse branching in many directions. Most of 

 the cells so produced, continue to divide in their turns, at 

 several points on their periferies. resulting in a much 

 branched proliferation. The cells making up the main 

 branches are prominently set off, under the microscope, 

 by their thick walls and septa, and regular shape. They, 

 in turn, branch laterally in both directions, often produc- 

 ing cells of peculiar shape (Fig. 24). Constant enlarge- 

 ment of the thallus by terminal growth, as well as thick- 

 ening, by filling in of spaces, at first open, between the 

 branches, results in a dense plate of closely packed, some- 

 times angled cells. This cell plate may occupy a small 

 area in the center of the thallus. and measure less than 

 20/1 in diameter. Its growth continues, however, in jDro- 

 portion to the proliferation of the thallus, and numerous 

 plates have been found to measure over 720u in diameter. 

 The branching hyjDhae hi all cases observed, extend out 

 from the cell plate, the whole giving the appearance of 



