117 
io waste in such a fashion all the diseased tubers, however de- 
sirable the process may be from the theoretical standpoint. In 
practice the diseased haulm is as a rule thrown up in heaps to 
form manure. Under the conditions obtaining in these heated 
heaps the spores appear to be—for the most part at all events 
—destroyed. Nevertheless care must be exercised even here, 
for it is an indisputable fact that the spores of many of the 
smut family will actually germinate under such conditions, giv- 
ing rise to fresh spore-forms capable of bringing about further 
infection. If the smutted material must be disposed of by this 
means then the heaps should be allowed to lie as long as pos- 
sible. Ihe advantage of this is well shown in a series of ex- 
periments carried out by Wollny in Munich. He shook up 
grains of maize—a crop not grown in that neighbourhood—with 
the spores of a smut which attacks it, and then sowed them in 
plots previously dressed with fresh or with old Manure. Con- 
trol plots received no dressing. These latter produced no 
diseased plants, those with the old manure a few, and those 
with the fresh manure a very considerable percentage. The 
fresh manure had provided the fungus spores with a medium 
well suited for their development. In other cases the diseased 
crop has to be disposed of as food for stock, ¢.g., potatoes in- 
fected with Phytophthora or swedes with the “ finger and toe 
fungus” Plasmodiophora brassicae. The plant remains then 
again find their way back to the soil. The assumption under- 
lying this procedure is that the fungus spores are killed during 
their passage through the animal’s digestive system. In the 
majority of cases this is undoubtedly true, but the fact should 
not be lost sight of that it has been conclusively proved that a 
preliminary digestion is actually necessary for the germination 
of the spores of some fungi. At present though we know of no 
such case amongst the common plant parasites, but this of 
vourse 1s no guarantee that such cases do not occur. At first 
sight the undoubted spreading of “finger and toe” might 
appear to be an example of a fungus whose spores germinate 
readily after their passage through an animal’s body. More 
careful investigation shows that this is not really the case, for 
in feeding the diseased tissues to stock fragments are sure to 
find their way into the litter and thence on to the land, where, 
sven a suitable host plant, they soon reproduce the disease. 
© Most rational way of disposing of such a crop is to feed it 
to ‘stock on grass-land where, as there are no Crucifere pre- 
sent, there is no possibility of the fungus spores coming into 
contact with the root system of a suitable host. 
© now come to what is perhaps the most practical side of 
our subject—the methods at our disposal for destroying para- 
sites without injuring the host plant. The means depend upon 

