BLACK ROT OF CRUCIFEROUS PLANTS. 307 
firms infection by mollusks and reports successful transmissions by aphides. He recovered 
a yellow Schizomycete from the body of an aphis which had punctured a diseased spot, 
and with this organism he induced the disease on sound plants. A curculio (Contrachelus), 
which lays eggs in the stems of young cabbage plants, is alsoopen to suspicion. Anycreature 
which gnaws diseased leaves or stems and then gnaws or even crawls over healthy ones 
is very likely to transmit the infection. It is desirable that, further studies should be made, 
especially on plants in the seed-bed and soon after transplanting, particularly with reference 
to underground infection. 
Wounds are not necessary, however, for the transmission of this disease, nor do the 
majority of cases arise as a result of trauma. 
The greater part of the infections (Smith, Russell, Hecke) unquestionably take place 
through certain natural openings of 
the plants, known as water-pores. 
These are modified stomata which 
occur in groups on the teeth of the 
leaf and through which excessive 
moisture taken up by the root- 
system is extruded from the plant. 
When the air is warm this moisture 
is given off as an invisible vapor, 
but during cool nights it gathers on 
the leaf-serratures in drops like dew, 
and may persist for hours after 
sunrise. 
It was experiments with slugs 
which led the writer to the discovery 
of water-pore infections. On leaves 
which had been bitten and infected 
by Agriolimax, a few belated infec- 
tions appeared on the leaf-margins 
where no wounds could be detected. 
A study of these infections showed 
that they began in the leaf serra- 
tures. This placed the question of 
natural infection in a wholly new 
light and led to further experiments 
with the results already known 
(vide Centralb. f. Bakt. 2 Abt., 
1897, page 411). Up to the time of 
the preparation of that paper the 
writer had not studied this disease 
in the field and his conclusions were 
based only on experimental data. 
It was therefore observed with the Fig. 105.* 
greatest interest, in the summer of 
1897, how well the field observations bore out the conclusions of the laboratory and hothouse. 
In the last (second) edition of his ‘‘ Vorlesungen” Fischer has questioned the possi- 
bility of general infection through the water-pores on two a priori grounds: (1) There 
is very little nutrient material in the fluid extruded from the water-pores; and (2) it would 
*Fic. 105.—Stems of collards (Brassica oleracea f. gemmifera) from Tampa, Fla., badly affected by the black-rot. 
Pure cultures of Bact. campestre were plated fromthe interior of young shoot at point marked X (see fig.128). Photo- 
graphed Sept., 1902. Natural size. 
