GENERAL CONSIDERATION OF PLANT DISEASES 273 



tion. Such plants are designated as cast-iron, or hardy, while the 

 others are tender and need constant care and attention on the part of 

 the cultivator. Such weakness of constitution, of histologic structure, 

 or absence of protecting chemical bodies in the cells of the plant may be 

 looked upon, other things being equal, as predisposing causes of diseases. 

 Such depend on the hereditary character of the plant, and in case of 

 varieties susceptible to disease may be designated hereditary predis- 

 position. Immunity, on the other hand, may be hereditary, as in the 

 case of the plants of strong constitution, or acquired. Resistance on the 

 part of certain plants may be due to the hereditary resistance of the pro- 

 toplasm, it may be due to histologic structure, such as the presence of a 

 thick cuticle in the resistant form and its absence in the susceptible form, 

 for Sorauer has found that the resistance of different carnations was due 

 to the thickness of the cuticle. The habit of earliness, or lateness, may 

 be the determining factor in resistance. A late variety might be at- 

 tacked, because of its growth in relation to the life history of some insect, 

 or fungous parasite, while for this reason an early variety might remain 

 healthy. Morphologic peculiarities may be effective, for the investiga- 

 tions of Hecke and Brefeld have shown that in the varieties of wheat with 

 closed flowers, and which are close pollinated, therefore, the spores of 

 the loose smut fungus carried by the wind are unable to reach the stig- 

 mas, and hence, infection does not take place. Such varieties would be 

 smut proof for the simple morphologic reason that their stigmas are not 

 exposed to the smut spores. Osterwalder has indicated that varieties 

 of pears without an open channel from the calyx to the carpels are pro- 

 tected against infection by Fiisarium putrefaciens, while those varieties 

 with an open channel from calyx to t"he carpels are susceptible. The 

 habit of a plant, as to drying after a rain, may influence its disease 

 resistance, as shown by Appel.^ Infection of potatoes by the spores of 

 late blight, Phytophthora infestans , is due to the wind carrying the spores 

 to healthy plants where in the raindrops on the surface of the leaves 

 zoospores are formed. 



The leaves of some varieties dry within half an hour after a rain, 

 while on others the leaves do not dry for several hours. Quick-drying 

 varieties are less susceptible than the slow-drying ones. In some mem- 

 bers of the pea family, the seeds are imbedded in a woolly outgrowth of 



^ Appel, O.: Disease Resistance in Plants. Science, New ser., xli: 773-782, 

 May 28, 1915. 

 iS 



