Immunity in Multicellular Plants 33 



petunias, dahlias, etc. The mycelium of this Sclerotinia develops in 

 the stems of herbaceous plants and produces sclerotia inside them, 

 forms of resistance, which in this instance are black and resemble 

 small particles of mouse excrement. 



The spores of the Sclerotinia germinate and form mycelial threads 

 on the surface of the plants. In order that they may penetrate into 

 the tissues these threads must attack the cell-membrane and for this 

 purpose they secrete a fluid, which contains both a digestive ferment 

 and oxalic acid, the latter being essential for the action of the ferment. 



The presence of this " toxin " has been demonstrated by de Bary 

 by macerating the mycelium of the Sclerotinia. The resultant extract 

 has a well-marked action on the tissues of many plants (carrot, 

 Jerusalem artichoke, chicory, etc.). Under its influence the proto- 

 plasm of the cells contracts, a genuine plasmolysis is set up, the cell- 

 membrane swells and its layers between the cells are dissolved. As 

 the result of this digestive action, the cells become separated and 

 the tissue softens. This extract, when heated to 52 C., loses its 

 digestive action on the cellulose membrane, but still retains its power 

 of setting up plasmolysis. This reaction to temperature confirms the 

 view that the juice of the Fungus contains a soluble ferment. The 

 results of de Bary's researches have been confirmed and in part 

 supplemented by the experiments of Laurent 1 . 



It is a fact of common observation that the Sclerotinia libertiana 

 invades for the most part young plants. It may therefore be asserted 

 that the disease produced by this Fungus is, like scarlatina or measles 

 in the human subject, an "infantile" disease. De Bary suggested 

 that the immunity of adult plants must depend on the greater 

 resistance which their cell-membranes offer to the fluid secreted by 

 the mycelial filaments. Direct experiments have shown the accuracy 

 of his suggestion. Whilst the fluid extracted from the Sclerotinia 

 readily digests the tissue of young plants it leaves intact that of adult [36] 

 plants of the same species. 



In the course of this disease we have a struggle going on between 

 two plants. The parasite brings into play toxic and digestive secre- 

 tions with which it seeks to impregnate its host. The attacked plant 

 defends itself by the secretion of membranes capable of resisting the 

 action of the secretions of the Fungus. This struggle by means of 

 chemical substances is, however, directed by the activity of the living 



1 Ann. de Flnst. Pasteur, Paris, 1899, t. xm, p. 44. 



