H. WORMALD 223 



the centre of the cell is in focus no bacteria at all are to be seen within 

 the four lines representing the walls in optical section. 



In microtome sections of fixed material from inoculated celery 

 petioles the bacteria are found between the cells and, by accumulating 

 in the space formerly occupied by the middle lamella, they tend to force 

 them apart (Fig. 10). In such sections also it sometimes appears as though 

 bacteria are within the cell-cavities ; this appearance is to be interpreted 

 as denoting that that portion of the section includes a cell-wall which 

 is parallel to the plane of the section, the bacteria occupying the plane 

 of the middle lamella (Fig. 11). These bacterial patches are not of 

 frequent occurrence and do not appear in the same place in successive 

 sections, both facts being evidence that the above interpretation is the 

 correct one. 



The material for these sections was obtained from celery petioles 

 inoculated from pure cultures. Carnoy's Fluid was found to be useful 

 for fixing the bacteria in situ ; acetic alcohol was also used but this 

 produced considerable distortion of the cell-walls. Heidenhain's 

 Haematoxylin followed (after difierentiating to remove the haematoxy- 

 lin from the cell- wall while leaving the bacteria still stained) by Bismarck 

 Brown as a counter stain gave excellent contrast, the bacilli being 

 black or bluish black and the cell-walls a yellowish brown. 



No swelling or lamination of the cell-wall has been observed in 

 celery as has been described by Potter in infected turnips and by Jones 

 in the carrot. 



Although so many species of plants are susceptible to infection by 

 the celery-rot organism it does not follow that the cultivation of all 

 these plants must be avoided on soil in which an infected crop has 

 grown in the previous season. The plants which are most liable to 

 be attacked are those producing fleshy organs underground, particularly 

 biennials whose roots when fully developed contain a high percentage of 

 water ; it would be undesirable therefore to grow root crops on infected 

 soil. Plants in which the more parenchymatous organs are raised 

 above ground {e.g. cabbage, cauliflower, etc.) are less readily affected 

 in nature. The custom of growing celery in trenches with the subse- 

 quent " earthing-up " to promote blanching renders this plant when 

 growing under those conditions less resistant to soft-rot, for not only 

 are the young inner leaves etiolated, mth a feeble development of 

 mechanical tissue, but in addition their growth takes place in an 

 atmosphere that is almost continually moist. 



Wrapping celery plants with a paper covering, as is sometimes 



