BLACK ROT OF CRUCIFEROUS PLANTS. 



313 



The greatest contrast to this prompt destruction ever obtained by the writer was on 

 cabbage-plants dwarfed and forced to make a very slow growth by keeping them for a 

 long time in 4-inch pots. These plants, which were inoculated in the autumn, developed 

 the disease, became stunted, and then appeared to grow out of it, but on some of them it 

 reappeared the next summer at the top of the plants in young leaves which were unques- 

 tionably infected from the stem by way of the leaf -traces. Fifteen months from the date 

 of inoculation, and more than 30 cm. above the point where the needle entered, the organism 

 was recovered in pure culture from the woody stem of one of these tall spindling plants. 



There can be no doubt that the severity of the disease varies with varying seasons. In 

 moist warm seasons the disease often makes a clean sweep on fields which may yield a crop 

 the following season, provided the weather is dry enough to induce slow growth and to 

 prevent wholesale infection by way of the water-pores. 



U!,* 



Owing to its wide distribution, and the ease with which infections may be obtained on a 

 variety of plants, this is a very good disease for the use of classes in schools. 



For experimental or demonstration purposes there is a choice in the parts to be inocu- 

 lated. Young rapidly growing leaves and plants are better than old or slow growing ones, and 

 infection by needle-pricks generally succeeds best when made into the upper fleshy part 

 of stems immediately under leaves or when made into the midrib. Cabbage plants when 

 inoculated on the lower leaves often throw off these leaves before stem-infection has taken 

 place. Brenner notes that when he inoculated into the petiole of the leaf this was frequently 

 thrown off in course of a few weeks. He found inoculations on secondary veins or peripheral 

 veins less successful than those on primary veins. 



*FlG. III. — Cross-section of root of inoculated turnip plant (No.53),showing two reticulated vessels, one of which 

 is occupied by Bacterium campestre while the other is free (nearly). Surrounding parenchyma cells are free from 

 bacteria. Drawn from a photomicrograph which was made from material fixed in alcohol, infiltrated with paraffin, 

 sectioned on the microtome and stained with carbol-fuchsin. Contents in cells at right are protoplasmic, x looo. 



