6 BULLETIN 934, U. S. DEPARTMENT OP AGBICULTUBE. 
Inoculation experiments are therefore probably even more neces- 
sary in damping-off investigations than in studies of most other dis- 
eases in order to demonstrate etiological relationships. Unfortu- 
nately, most of the inoculation work with damping-off organisms 
prior to 1900 was either crudely done by placing diseased seedlings 
against healthy ones or consisted of experiments in which purity 
of cultures and validity of controls did not receive sufficient con- 
sideration. Eecent investigations not primarily directed toward 
damping-off, but which have decidedly increased our knowledge of 
the relation between Corticium and the disease, are those of Peltier 
(98) and Fred (43). The latter established a strong presumption 
that the difficulty in securing stands of various field crops having 
oily seeds in soil where green manures had been recently turned 
under is due to the killing of the sprouting seed by damping-off 
organisms. 
In tobacco, sugar beet, and pine, whose damping-off has received 
considerable attention, it has been found that the damping-off proper 
is commonly preceded by the killing of many of the sprouting seeds 
in the soil (38; 68, p. 522; 81, p. 5) and followed, after the plants 
become too large to be killed by the damping-off organisms, by root 
sickness and the death of small roots (38, p. 161 ; 64 ; 100) . This latter 
has been reported also as a serious matter in the case of Corticium 
vagum for potato (51) , a host on which damping-off is not important 
because of the lack of commercial propagation from seed. Pythium 
debaryanum further has been reported as continuing to work in the 
cortical tissues and leaves of tobacco plants which have been in- 
fected too late to result in death (81). 
The fact that a number of the damping-off fungi are able to attack 
young or soft tissues of so great a variety of plants and are much 
less able to kill older plants suggests that resistance to damping-off 
may be in part based on purely mechanical factors. Hawkins and 
Harvey (71) recently have extended to Pythium debaryanum the 
idea, developed by Blackman and Welsford (12) and Brown (16) for 
Botrytis cinerea, of the importance of mechanical penetration in the 
fungous invasion of plant tissues. While for B. cinerea mechanical 
pressure was found to be the main factor only in cuticle penetration, 
with P. debaryanum the penetration of the cell walls of all parts of 
the potato tuber was apparently largely dependent on mechanical 
puncturing by the hyphse, onl} T tubers with mechanically weak cell 
Avails being susceptible to decay by the fungus. The extreme sus- 
ceptibility to P. debaryanum and Corticium vagum of soft, thin- 
walled tissues and the resistance of older stems and root parts would 
fit in well with such a theory as to the method of wall penetration, 
as in the older tissues the thicker cell walls would obviously be a 
serious bar to the extension of a fungus dependent partly or en- 
