Ill 
This explanation, however, when carefully considered, proves 
to be altogether inadequate. When a potato plant infected with 
the spores of FhyiopM p a bell- jar in a very 
damp atmosphere, subdued light, and high temperature— condi- 
tions most favourable to the development of the parasitt — it is 
only after a period of four or five clays that the fungus produced 
fruit on the leaves, and then only at the points of infection. On 
the other hand the fact is too well known that a field of potatoes 
or all the potato fields in a certain district which at a given 
moment appeared perfectly healthy and vigorous, have, under 
certain climatic conditions, been reduced to a blackened, decay- 
ing, foetid condition within 24 hours. Again, in the case of every 
fungus epidemic proved to be due to the diffusion of spores, the 
disease always originates from one or more primary centres of 
infection, and gradually extends, whereas in the case of potato 
disease the appearance of the epidemic is often simultaneous over 
a considerable area. 
These considerations suggested the existence of some method 
other than dissemination by means of spores as the cause of such 
sudden outbreaks of disease. The presence of mycelium can 
readily be demonstrated in the tissues of diseased potato tubers, 
and a series of experiments conducted at Kew have conclusively 
proved that such hybernating mycelium in a tuber is capable, 
under favourable conditions, of perpetuating the disease. 
Three diseased potato tubers showing rusry stain* characteristic 
of the presence of Fhyi a in the flesh were each 
cut into two equal parts. Each half tuber was planted separately 
in a plant pot ; the same kind of soil and manure, sterilized by 
steam, was used in all the experiments. Three of the pots were 
placed in a house having a temperature ranging between 70° and 
80° Fahr., in dull light, and with the moisture often at saturation 
point. Each pot was placed under a bell jar. The three remain- 
ing pots were placed in a well-lighted house, without any artificial 
heat, and with an exceptionally dry atmosphere. These pots 
were not placed under bell-jars. An equal amount of water was 
supplied to each of the six pots. The three plants grown under 
conditions oi dull light, and much moisture in 
the air, showed the first indication of Fhytophthora when the 
shoots were six weeks old, and a fortnight later the three plants 
were blackened and destroyed by the fungus. 
The three plants grown in the cool, well-lighted, dry house 
showed no trace- of disease at the end of two months, when one of 
the plants was removed to the warm house and placed under a 
bell -jar. W - plant was blackened and killed 
by the fungus. A fortnight later a second plant was removed 
from the cool to the warm house and placed under a bell -jar. 
Within a week of the removal of this plant it was also covered 
with Phytophtlwra. The third plant continued growing in the 
cool house for 13 week, and remained perfectly free from obviow 
