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fungus to spread with amazing rapidity; when mature 
they are easily detached, and being extremely small the 
slightest current of air carries them in large numbers to 
the leaves of adjoining plants. The conidia are only 
produced when the air is moist and sultry. Alighting 
upon the healthy leaf of the potato, under such atmospheric 
conditions they germinate in one of two ways. Either, 
each conidium behaves as a spore-case and liberates motile 
spores, or it germinates by sending out a fine filament. 
By'the former method it opens at the tip and liberates 
about eight small motile spores which swim about vigor- 
ously in the thin film of water on the surface of the leaf. 
As in the case of Pythzum, the fungus causing “ Damping- 
off” of seedlings, these motile spores are each furnished 
with two hairs of protoplasm, by means of which they are 
enabled to move in the water. After a short period the 
motile spores come to rest, withdraw the minute hairs and 
give rise to a short filament which either grows through 
one of the stomata into the leaf, or dissolves a way 
through the outer skin of the leaf, and so starts a disease 
spot. In other cases the conidium simply sends out a fine 
tube and produces an infection by the same methods as 
do the motile spores. After infection the fungus, feeding 
on the cells of the, host, grows rapidly, in the form of 
branched threads, within the tissues of the leaf. The 
conidia are extremely small, being less than one- 
thousandth of an inch in length. Enormous numbers are 
produced from a single diseased area of a leaf, and, 
further, under suitable conditions the fungus grows and 
reproduces itself very rapidly. It is therefore easy to see 
why, in weather conditions favourable to its growth, the 
fungus spreads in a few days over whole fields of potatoes. 
In the earlier chapters, it has been shown that a leaf, 
such as that of the potato, is really a complicated machine 
for the manufacture of food material. Many of the cells 
contain chlorophyll, the green colouring matter which, in 
the presence of sunlight, utilises carbon-di-oxide obtained 
from the air to produce organic chemical compounds like 
sugar and starch. The complex organic substances 
elaborated in the leaves are passed on to the stem and 
ultimately stored in the potato tubers in the form of starch. 
Thus it is obvious that by injuring the leaves, the potato 
disease reduces the weight of tubers obtained from a given 
plant, and the earlier the attack occurs in the season, the 
