386 PLANT PHYSIOLOGY 



may be found on winter varieties of apples or on the early forma- 

 tion of cork tissue. Strong lignification of the vessel walls 

 inhibits the penetration of the mycelium of Fusarium causing 

 wilting. Such immunity is designated "passive" and may be 

 easily lost through mechanical injuries, for instance as when the 

 skin of fruit is pierced by insects. Another form of passive 

 immunity is shown when the chemical composition of the plant 

 does not suit the requirements of the parasite, which consequently 

 cannot develop successfully, though its spores may be able to 

 germinate. This is a very widespread form of immunity, for 

 most of the parasitic fungi as a rule show very narrow specializa- 

 tion in their nutrition and require definite nutritive substances. 

 They are adapted to certain host plants that contain the strictly 

 necessary substances. 



By ''active" immunity is understood the elaboration by the 

 plant of substances that make the development of the parasite 

 impossible. Formerly it was assumed that such an almost 

 universal mechanism of immunity was the change of the actual 

 acidity of the cell sap in a direction unfavorable to the parasite, 

 commonly by increase of the acidity, but later it was found that 

 the parasitic fungi are capable of a rather wide adaptability to 

 the reaction of the medium. Likewise it was often assumed that 

 an increase in the concentration of the cell sap, inhibiting the 

 growth of the fungal hyphae, represented one of the factors of 

 immunity, but this feature is far from being universal. More 

 detailed investigations have shown that there do not and cannot 

 exist universal causes of immunity. Plants possess various 

 factors of immunity to different parasites. 



Comparisons based on the analogy between the infection of 

 animals and plants gave rise to the idea that weak poorly nour- 

 ished plants are especially susceptible to fungous diseases, but this 

 idea has not been confirmed. This viewpoint is to a certain 

 degree correct in respect to attacks of harmful insects, for 

 instance, certain plant lice, which frequently attack weakened 

 or exhausted plants, and likewise weakly parasitic fungi, which 

 may become strongly parasitic on weakened hosts. Fungous 

 parasites, especially rust, on the contrary, attack more readily 

 the vigorous healthy plants. Fertilizing the soil with nitrogen 

 and phosphorus may increase the infection. The introduction 

 of potassium fertilizers slightly decreases rust infection. Like- 



