11] Potato Disease and Allied Diseases 19 
the end of the branch producing it; when the swelling 
has reached a certain size a transverse wall divides it 
from the rest of the branch. The branch then goes on 
growing and so the conidium falls off or remains at the 
side. The end of the branch then swells again and 
produces another conidium. In this way a branch 
may produce several conidia. We shall probably find 
some at the tips and others at the sides of branches. 
Those at the sides are the first formed, the lowest on 
the branch being the oldest. The portion of the 
mycelium bearing the conidia is known as the 
conidiophore and is rather peculiar. The tube which 
goes on growing after the conidium is produced is 
wider than the portion below, and gives a knotted 
appearance due to the variation in the bore of the 
hyphal tube (Fig. 9A). The conidiophores reach the 
exterior through the breathing pores of the leaf either 
singly, or several through one pore. Together with 
the spores they provide a means of recognizing the 
fungus. The conidia may vary somewhat in size, the 
average length being about jin. and the average 
width being about ;4, in. 
The conidia are very light and readily blown about, 
and as the plants overlap and touch each other they 
may easily fall or be washed by the rain from one plant 
to another. It is difficult to watch the development 
of these conidia after they have fallen on a potato 
leaf, but the further development may be followed 
by putting some of them in a hanging drop in a 
suitable chamber (see p. 5). Here changes take place 
similar to those which occur naturally when the 
conidia fall on moist leaves. After a time a change 
occurs in the contents of the conidium, there being a 
2—2 
