16 
IOWA ACADEMY OF SCIENCE 
resting spores whose function seems to be to carry the fungus over 
from year to year. 
The development of the perithecium is as follows : At the point 
where two hyphse cross, or at the place where two neighboring 
hyphse touch each develops a small upright branch which is soon 
separated from the parent hypha by a partition. One of these 
branches swells into an oval shape and becomes the carpogonium. 
The other elongates and applies itself closely to the carpogonium, 
curving above so that its end lies on the apex of the carpogonium. 
The upper part is then cut oif by a septum and forms the antherid- 
ium. At the time of fertilization the wall between the two organs 
dissolves and the nucleus of the antheridium passes over and unites 
with nucleus of the carpogonium. A new wall is afterwards formed 
between the two organs. 
The development of the walls of the future perithecium begins 
when the two nuclei unite. From the stalk cell of the carpogonium 
a number of hyphal branches grow upward forming a single layer 
around the carpogonium. Later the stalk cell swells and a second 
series internal to the first grow up in a similar manner. 
When the perithecia are about half grown certain cells of the 
outer wall begin to grow out into appendages. These cells are situ- 
ated either apically, equatorially, or basally. These appendages 
vary in character and with the number of asci form the character on 
which the genera are based. They are probably concerned with the 
distribution of the perithecia. 
The perithecia when mature vary from globose to cup-shaped or 
pezzoid. They are generally blobose-depressed. The perithecia con- 
tain one to many asci each ascus containing two to eight colorless 
spores. 
With the formation of the perithecia the mycelium sometimes 
completely disappears. Frequently, however, it is persistent. 
The asci are liberated by the irregular rupture of the wall of the 
perithecium. The ascospores seem incapable of germination before 
passing through a resting stage. In a damp atmosphere or water 
they send out germ tubes, which on the epidermis of a suitable host 
plant penetrate and form a haustorium from which center the or- 
dinary vegetative mycelium is produced. 
Economically the Erysiphacese are very important. They injure 
many species of cultivated and native plants. As examples we 
need only cite the mildews infecting the grape, gooseberry, rose, 
cherry, plum, apple, and various forest and ornamental trees. They 
work injury by impairing the function of the leaf and causing a 
