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 also open to suspicion. Any creature 

 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 

 greatest interest, in the summer of 

 1897, how well thefield 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 



Fig. 105.* 



*Fig. 105. Stems of collards (Brassica oleracea f gemmifera) from Tampa, Fla., badly affected by the black-rot. 

 Pure cultures of Bad. campestre were plated from the interior of young shoot at point marked X (see fig. 128). Photo- 

 graphed Sept., 1902. Natural size. 



