Ir] Potato Disease and Allied Diseases 21 
suspicion of a division, but not a very marked one. 
Then suddenly the whole of its contents come pouring 
out through a small hole at the pointed end. The 
whole process may take only about 10 seconds. The 
contents do not pass out whole but in about ten separate 
masses. Each mass is considerably larger than the 
hole through which it emerges and consequently is 
much constricted on passing out (see Fig. 9D). On 
arriving outside the conidium these masses start 
swimming about very rapidly. It is from this motile 
power in which they resemble small animals that they 
are called Zoospores. When one of them becomes 
motionless the presence of two small threads is revealed 
(Fig. 9# and F). By means of these the zoospore 
moves; they lash the water and force it along. After 
a time the zoospore ceases its movements and the threads 
disappear ; it now loses its former shape and becomes 
spherical (Fig. 9G). Later on a small protuberance is 
formed ; this lengthens and forms a tube. The contents 
of the zoospore pass into this tube which grows and 
may give off branches (Fig. 9H, J and J). Eventually 
growth stops and the young fungus dies of starvation. 
All these changes take place when a conidium falls 
on the surface of a moist potato leaf, but the hyphal 
tube formed does not die of starvation; it enters the 
leaf by means of a breathing pore or by dissolving 
away the outer wall of the leaf. Growth now goes on 
rapidly as the hypha has abundant food. A mycelium 
forms which kills the leaf cells and changes them to 
a brown colour causing the characteristic appearance 
which first shows the presence of the disease. Both 
young and old leaves may become infected in this 
way. 
