Johnson — Chrysophlydis endobiotica and other Chytridiaccce. 133 



be seen abutting ou the host-protoplasm, and disputing with it, as it devours 

 it, occupation of the enlarging cell-cavity. The host-nucleus is first affected, 

 turning brown. The protoplasm follows, and then the cell- wall. Tliis, 

 though brown, does not, like the protoplasm and nucleus, disappear. The 

 starch-grains are the last attacked, and remain white and uninjured for some 

 time in an invaded cell. The parasitic plasmodium passes from cell to cell by 

 boring its passage through the host cell-wall. No doubt the parasite secretes 

 the cellulose-dissolving ferment cytase, as well as a toxin to kill the host 

 protoplasm before absorbing it. It is in this stage that it stimulates to active 

 cell-division the surrounding host-cells, and produces the gall or wart. 



Propagation. — A. Swann or Sx miner Zoosporangia. 

 The parasite not only penetrates through its host from cell to cell by 

 its Plasmodium ; it also spreads from tuber to tuber, and from plant to plant, 

 by the formation during the growing season of zoosporangia. These are 

 elliptical bodies, with smooth yellowish walls, and numerous zoospores 

 which escape through a hole in the wall, and attack healthy potato-tissue. 

 Schilberszky saw the escape of these spores, and states that they can bore a 

 passage for themselves into neighbouring host-cells, when their discharge is 

 internal. The spores escape readily when the sporangia are placed in water 

 or in macerations, says Schilberszky. 



B. Winter or Resting Sporangia or " Spores." 

 As the tuber ripens, the parasite replaces the summer sporangia by resting 

 ones which carry the disease through the winter, and serve to propagate it in 

 the spring. Schilberszky mentions these resting spores, but contents himself 

 by noting their thick, dark-brown walls, described by him as smooth. He 

 leaves to the future an account of their origin, structure, and fate. 



The resting sporangia, 30-70 /x in diameter, are very numerous in 

 diseased tubers, and are easily recognizable with a pocket lens (PI. IX., fig. 3). 

 Under the microscope the wall is seen to be not smooth, but ridged or 

 angular (PI. XI., fig, 3). These brown ridges or bands form part of a kind of 

 epispore which arises as the sporangium ripens, and seems to be formed from 

 the residual contents of the host-cell when not also from its cell-wall as well. 

 I find W. B. Grove (Gardeners' Chronicle, 1906) supports this view in part, 

 in that he says that " though the resting spores are smooth outside, they are 

 sometimes closely invested with the brown angular remnants of the host-cell 

 in which they had formed." The epispore is thus deposited from without, as 

 a third layer on the thickening wall of the sporangium. If this more or less 

 artificial epispore is ignored, then one may speak of the spore-wall as smooth. 



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