26 THOMAS: INFECTION OF APIUM GRAVEONENS 
antagonistic throughout but may on the contrary become special- 
ized in such a way that infection and the development of the 
typical symptoms of the disease are directly favored by the general 
vigor of the host plant. 
In the relation of the age of different tissue on the same celery 
plant to infection, it seems that there are two entirely separate 
conditions operating, one which governs the establishment of 
infection and another which determines the rate of subsequent 
spread of the mycelium. The first stage seems closely related to 
the immediate metabolic activity of the host cells especially in 
view of the marked increase in the number of infections produced 
by the addition of fertilizers to the plant at the time of inoculation. 
That starch metabolism has no very direct relation to infection 
is indicated by the results of inoculating etiolated plants. 
Pool and McKay (28) state that the infection of Beta vulgaris 
by Cercospora beticola is closely related to, if not directly controlled 
by the movement of the stomata. According to their data 
(pp. 1019, 1031), however, heart leaves which are said not to be 
infected show an average stomatal pore width of from op to 9u 
between 10 A.M. and I P.M., only one case in ten falling below 
2.5m. Celery leaves can be infected as soon as they come into 
‘view but they frequently reach their mature height before the 
spots are visible. It is not clear whether the incubation period is 
taken into consideration in this connection by Pool and McKay. 
They do not show comparative counts on old and young leaves 
from a single infection. These authors show a close correlation 
between the maturity of the leaf and the number of stomata per 
unit area. Ensign (10a) hasshowna very definite relation between 
the size of the vein-islets and maturity of the leaf in citrus. The 
relation of infection to age of the host as determined by these 
criteria deserves further attention. 
It has been pointed out that the tissue between spots separated 
by a given distance will break down more rapidly on old leaves 
than on young ones. It has been noted furthermore that from a 
single inoculation the spots are visible on the old leaves one to 
several days before they can be seen on the younger leaves. 
The Septoria produces a vigorous growth on widely different 
culture media, as well as on cooked celery, but at the same time 
