22 
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THESE SPOTS WERE MEASURED ON THE BASAL LEAFLET PAIR 
OF EACH LEAF NINETEEN DAYS AFTER INOCULATION. LEAF No. I IS THE OLDEST 
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THOMAS: INFECTION 
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OF APIUM GRAVEOLENS 
relation between the age of the 
plant and its susceptibility. 
However, as regards the relative 
susceptibility of leaves of dif- 
ferent ages on the same plant, 
results have been obtained to 
show that there is a definite and 
constant relation between the 
age of the leaf and the number 
of spots produced by inoculation 
(TasLe XII). 
The leaves which show no 
infection are those which were 
pushed out after the inoculation 
was made. It has been deter- 
mined by tagging the young 
leaves at the time of inoculation 
that a leaf is susceptible as soon 
as it pushes out into view. It 
does not however unfold the 
leaflets so as to be exposed to the 
inoculum on all its surface until 
it has reached a height of from 
two to four inches. This ac- 
counts in part for the smaller 
count on the very young leaves., 
No method has been devised to 
determine whether or not these 
leaves are as susceptible per unit 
area as leaves which have com- 
pletely unfolded. The chief point 
of interest here is that a very 
much larger number of infections 
is established on leaves which 
are still actively growing than on 
leaves which are more mature. 
Another very interesting differ- 
ence in the behavior of leaves 
