MIMOSA: GALLS: PLANT PARASITES 313 



explanation exists of the limitation of the three races of the 

 mistletoe to their respective groups of host trees. We do 

 not even know whether this is a case of prevention by the 

 inappropriate hosts, or of lack of some intimate factor on 

 the part of the parasite ; these two aspects of the problem 

 are of course inter-related. We may assume that a strong 

 corky covering is unfavourable to the penetration of the 

 primary sucker. This might, for example, partially explain 

 the rarity of mistletoe on the oak. It certainly does not 

 take us far. Gertz (1915) shows that plants are immune 

 from the attacks of the dodder from a variety of causes — 

 the oak because of mechanical resistance, the wood sorrel 

 because of acid sap, the poppy because of latex, the thorn 

 apple because of alkaloids, eschscholtzia because of ethereal 

 oils. 



In the case of fungal parasites a strongly cutinised leaf 

 may resist the penetration of those numerous species which 

 pierce the epiderm. Many fungi, however, enter the leaf 

 by the stomata, and through these openings entrance must 

 be free for all. The rusts enter the leaf in this way, and 

 they are remarkable for their high degree of specialisation, 

 and for the numbers of biological races, confined to par- 

 ticular hosts, which exist within the bounds of many species. 

 The case of the rust-resistant strains of wheat may here be 

 cited. The spores of the rust germinate on their leaves 

 and hyphse penetrate the stomata ; inside the leaf the 

 hyphse die, or the leaf cells are killed ; in either case the 

 further advance of the parasite is barred. We may compare 

 this with the behaviour of orchid seeds to different strains 

 of the symbiotic fungi ; the proper strain enters into 

 symbiotic growth with the germinating seed ; weak strains 

 are killed ; strong strains kill the orchid. We are evidently 

 here in the presence of obscure protoplasmic reactions, 

 which may have an analogy in the phenomena of immunity 

 which have come into so much prominence in the study of 

 human pathology. 



