THoMAS: INFECTION OF APIUM GRAVEOLENS ao 
of different ages in relation to fungus invasion is found in the 
rate of breaking down of the leaf tissue after infection becomes 
established. Except in unusual cases in the greenhouse there 
was little coalescing of spots until some days after the infection 
was evident on all of the leaves. Thus time was allowed for 
measurement of the size of spots even on very old leaves while 
they were still distinct in outline. It was found that for a given 
distance between spots, the older the leaf the more rapidly the 
intervening tissue breaks down. 
TABLE XIII shows the diameter measured to the nearest half 
millimeter of twenty-five spots on each leaf of a plant nineteen 
' days after inoculation. The marked decrease in the size of the 
spots from the oldest to the youngest leaf is at once apparent. 
In TABLE XIV the averages are shown for a number of other 
plants which were similarly studied. It has been pointed out 
that the margin of the killed area produced by Septoria is not 
identical with the limit of mycelial spread. Whether or not the 
fungus colony exceeds the margin of the spot as far in old leaves 
as in young leaves has not been determined. 
TABLE XIV 
AVERAGE SIZE OF SPOTS ON LEAVES OF THE SAME PLANT AT DIFFERENT AGES 
No. spot: Age of 
Leaf No. : = 3 + 5 6 7 | 8 9 | cukanrede | jele sti 
| j 
i | 
Plant No. 1....| 2.18] 1.22| 0.94] 0.72] 0.64 | 25.0 19 days 
fe 2....| 1.32| 0.88] 0.74) 0.74 25.0 1-5 
oF 3...-| 1-66] 1.06} 0.86; 0.80} 0.58 25.0 19 2 
St 4....| I-30] 1.10] 1.00] 0.55 10.0 ol 
é 5..-+| 1.45] 0.70) 0.70) 0.70 10.0 ae 
: 6....| 1.80] 1.40) 1.25, 1.20) 0.80) 0.60 10.0 29 * 
e oe teged We Gc T fs I.15, 0.70) sige 0.75) -77 | -62 | 10.0 2 
os 8....| 2.05] 2.00] 1.20) 1.00] 0.95/ 0.95] .80 | .75 1-75 | 10.0 29 
A possible relation between the acidity of plant juices and their 
susceptibility to fungus attack has been claimed by Comes (6). 
With this in mind two lots of leaves were collected from a number | 
of celery plants, the oldest being included in one lot and the 
youngest in another. These were put througha meat grinder and 
15 gram samples of each were extracted for thirty minutes in 
200 cc. distilled water. The extracts were filtered and 100 cc. of 
the filtrate were titrated against approximately twentieth normal 
* In four leaves less than the number of spots indicated was used. 
