Johnson — Further Ohservations on Powdery Potato-Scah. 171 



containing the organism iu a more advanced stage ol development. . . . 

 It appears highly probable that the cells are invaded by the parasite for 

 some time before the amoeboid bodies surrounding the nucleus can be seen ; 

 for when they are present, the starch has, as a rule, disappeared from the 

 cells." Sections stained with congo red are described in support of the view 

 that these bodies are the myxamoebse of other slime-fungi. I have spent 

 what has sometimes seemed to me a disproportionate amount of time in 

 the microscopic examination of Spongospora without observing any signs of 

 such amoeboid bodies as those just described. I suspected them to be, in 

 fact, an accumulation of young starch-grains around the host nucleus. 

 Such grains are spherical, with a centric hilum, which takes up a stain 

 readily and might pass for a nucleus. I have placed side by side with 

 Massee's figure of the myxamoebse a micro-photograph of a Flemming-fixed 

 and stained healthy potato-cell (PI. XIII., fig. '6). The similarity is, I tliink, 

 striking. Massee himself states that the starch-grains have disappeared 

 when the amoeboid bodies appear. Eeplace " amoeboid bodies " by young 

 " starch-grains " in the above-quoted statement, and everything becomes 

 clear. I am in accord with Massee in the view that the parasite gnaws 

 into the flesh of the tuber by its penetrating plasmodium — the more 

 easily the moister the soil. The drier the soil the more readily is the 

 parasite kept in check by the formation of protecting wound cork. The 

 Plasmodium, too, carries the disease from the seed-tuber through the 

 stoloniferous branches over into the new tubers, making them scabby, as 

 I have already shown. 



I must refer readers to my earlier paper for a general account of the 

 changes which I consider lead to the maturity and germination of the spores. 

 Massee sees in each ripe spore a single nucleus. At germination the 

 uninucleate spore-contents escape, as in slime-fungi generally, as a single 

 body, which shows for some time sluggish amoeboid movements, and then 

 becomes stationary. Its nucleus next divides into two, followed by complete 

 fission of the body. By repeated fission numerous 1-nucleated bodies are 

 formed, which ultimately coalesce to form a plasmodium. I regard the spore 

 as much more complicated, and comparable to the spore of Ceratiomyxa, 

 as recently described by Jahn (12), and Olive (13). PI. XIV., fig. 1, is a 

 microphotograph of a spore-ball in culture, in section. The details of the 

 contents of the different spores in the ball deserve examination by pocket- 

 lens. In the spore marked t there are apparently four nuclei present, 

 arranged tetrahedrally, highly suggestive of a stage observed in Ceratiomyxa, 

 The spore is so small that I cannot yet give a satisfactory connected 

 comparative account of the nuclear changes. In my earlier paper I have 



