24 THOMAS: INFECTION OF APIUM GRAVEOLENS 
sodium hydrate solution. A very marked increase in acidity was 
thus shown for the old leaves. The ratio of the readings for 
young and old leaves was 4.6 : 6.3. 
Sorauer (36) states that acidity is higher in etiolated plants 
than in the normal green. It has been pointed out that young 
leaves developed in the dark room bear spots as large as those 
produced on very old leaves. This suggests the possibility of a 
relation between the size of the spot and the acid content of the 
leaf. Obviously, however, no more thana suggestion can be made 
from the data at hand. 
Discussion 
The relations of host and parasite are apparently as different 
for the groups of saprophyte, semi-saprophyte, and obligate 
parasite as are the modes of life of the organisms. It is possible 
to arrange an intergrading series according to the completeness 
of adaptation to the host, from a form such as Botrytis (see 
Blackman and Welsford, 2), which habitually kills the host 
cells before it reaches them and is probably never in intimate 
contact with the living cell, to a form such as the seed fungus of 
Lolium temulentum (see Freeman, 11), which has reached such 
a high degree of adjustment with the host that it is perpetuated 
entirely in the mycelial form through the seed of the host and 
perhaps never kills any of the host cells. As the adaptation to 
the host becomes more nearly complete, there is an increasing 
tendency to show some of the features of mutualism and sym- 
biosis. Fromme (13) has observed with the angular leaf spot of 
tobacco and Peltier (25) with citrus canker that infection is 
heavier under conditions which favor the growth of the host. 
Marchal (21) found that infection of lettuce by Bremia Lactucae 
was favored by nitrogen and phosphates and retarded by an excess 
of potash. The experiments described 
that Sepioria A pii, although it readily ass 
habit, has become so adapted to its host that the development of 
infection is favored by increased growth in the host, such as is 
produced by feeding the plants with nitrates, with a complete 
nutrient solution, or by top dressing the soil with sheep manure. 
The acceleration is manifested in both the number of infections 
in this paper show 
umes the saprophytic 
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