Sept. 1896.] ROTTENNESS OF TUENIPS AND SWEDES. 



123 



giving rise to a branched and septate mycelium, and the tassel- 

 like organs of attachment described by de Bary and Marshall 

 Ward may be observed. The germination can also be readily 

 followed upon a piece of turnip. For this purpose a small 

 block of turnip was carefully cut in the manner described above, 

 thickly sown with conidia, and then placed in a sterile tube 

 plugged with cotton wool. On examining sections taken after 

 an interval of 18 hours, the conidia were found to have 

 germinated while lying upon the surface, and to have pro- 

 truded a short germ-tube, which had perforated the outer cell- 

 wall and was making its way across the cell (Fig. 2). [In his 

 paper upon the Lily disease Prof. Marshall Ward has shown 

 that the hypha perforates the cell by means of a ferment, 

 secreted at the tip of the germ-tube, which is able to soften and 

 dissolve the cellulose wall. In a particular case he describes, 

 the cell-wall was pierced in 35 minutes from the time of the 

 hypha coming in contact with the wall.] A section made at 

 the end of 24 hours after sowing showed that the hyphae 

 had penetrated more deeply into the cells and commenced to 

 branch (Fig. 3) : sections taken later showed them gradually 

 spreading throughout the piece of turnip until it became 

 thoroughly intersected by hyphae (comp. Fig. 9). 



The fungus does not confine itself to merely passing through 

 the cells in turn, but is able after softening them to work its 

 way in the thickness of the -cell-walls. Figure 9, taken from a 

 living turnip, shows the ramifications of the fungus among the 

 cells, many of the hyphae passing in the intercellular spaces. 

 In the living state the plant-cell consists of a cellulose wall 

 immediately enclosing and in close contact with the protoplasm, 

 in which numerous cavities full of cell-sap are distributed. 

 When healthy the plant-ceil is turgid, that is, it contains so 

 much cell-sap that its walls are stretched and the cell distended. 

 Besides having the power of dissolving the cell- wall, the hyphae 

 also secrete a poison in the region near the growing point ; this 

 kills the protoplasm, turning it brown ; the distended cells then 

 lose their turgid ity, and the sap escapes into the intercellular 

 spaces. We thus see how the hyphse as they spread cause the 

 brown discolouration and rottenness, the fungus, living upon the 

 cells as it destroys them, gradually extends through the whole, 

 and softening the cell-walls, which become permeated by the 

 cell-sap as it escapes, reduces the entire mass to a pulpy con- 

 sistency, rendering it completely brown and rotten. 



The development of the aerial hyphse does not take place till 

 a few days after sowing, and from these the conidia are pro- 

 duced. A hypha about to become a conidiophore branches near- 

 its apex (Fig. 4), with the formation of short branches pro- 

 jecting on all sides, which again branch (Fig. 5). At the 

 extremities of the tertiary branches numerous small projections 

 are now protruded (Fig. 6), and from each of these a conidium is 

 developed (Figs. 7, 8). The conidiophores are septate, and some- 

 times branch near the base, forming several clusters (Fig. 13) of 



