134 DISKASK IN I'LANTS. 



alkaline medium, and yeasts which prefer an acid 

 environment are mixed in a saccharine solution, it 

 depends on the reaction of the liquid which 

 organism gains the upper hand : if the liquid is 

 acid the yeast may dominate the bacteria; if alka- 

 line it may be suppressed by them. 



That a parasite may be prevented from success- 

 fully attacking a particular plant is shown by the 

 failure of Cuscuta to establish its haustoria in 

 poisonous plants such as EtipJwrbia, Aloe, etc., 

 and it has been pointed out that poisonous secre- 

 tions in the cells of the plant protect them against 

 the penetration of fungi. This cannot be taken 

 as meaning that any poison protects against any 

 parasite, however, for EupJiorbia is itself subject to 

 attacks of Uredinae, and Pangium edule, which 

 contains prussic acid and is extremely poisonous 

 to most animals, is eaten with avidity by several 

 insects, while nematode worms can live in its 

 tissues. This is no more remarkable, however, 

 than the fact that Fontaria, a myriapod, 

 .secretes prussic acid in its own tissues, or than 

 that certain glands of the stomach secrete free 

 hydrochloric acid, and Doliuni forms sulphuric 

 acid in its glands. 



There is yet a further point to notice here. It 

 has been proved that certain substances formed in 

 plant-cells, not necessarily nutritive, attract the 

 hyphae of parasitic fungi or repel them, according 

 to the kind and degree of concentration. So clear 

 has this proof been made that it was possible in 

 experiments conducted apart from a host plant, 



