PLASMODIOPHORA. 



525 



decay, so that not only is the root itself worthless, but the 

 aerial shoot is badly developed. The destruction is greatly 

 favoured by moist rainy years. 



The malformations of the root are the result of hypertrophy 

 of the host-cells due to a stimulus exerted by the plasmodium 

 of Plasmodiophora, not only on the contents of cells inhabited 

 by it, but also extending into the cells of the whole neigh- 

 bouring tissue. The cells so influenced enlarge in size and 



Fig. 315. — Plasmodiophora brassicae. Effects on Turnips grown in Scotland, 

 (v. Tubenf phot.) 



become divided up by new cell-walls. The plasmodium makes 

 its way from cell to cell by means of the wall-pits, and 

 by absorbing the contents it grows and fills the whole cell. 

 On exhaustion of food, and without previous enclosure in a 

 membrane, the plasmodium forms itself into spores, so that the 

 tissues of attacked roots become completely filled with thick- 

 walled spores, which are set free only after decay of the 

 surrounding tissues and cell-membranes. The spores hibernate, 

 and in spring myxamoebae slip out, capable of infecting 



