PARASITISM AND RESISTANCE 387 



groups: (1) mechanical, the prevention of penetration or of unlimited 

 spread by the structure of the host; (2) functional, the prevention of 

 penetration by stomatal action of the host; (3) physiological, chemical 

 action against the parasite or incompatible food relations. The relative 

 importance of these factors is difficult to determine, but Butler (1918) 

 states that physiological characters are much more important as a factor 

 for resistance than the anatomical characters of the host. 



Mechanical resistance might be considered as the first line of defense 

 by the host. According to Melander and Craigie (1927) resistance of 

 species of Berheris to infection by sporidia of Puccinia graminis is due 

 to the thickness of the cuticle. B. thunbergii, which is immune, has a 

 heavy layer of cutin, while in general the susceptible species have a 

 thin layer. These conclusions were reached after anatomical studies and 

 after using a mechanical device to measure the resistance of the epidermis 

 to puncture. The thickness of the cuticle increases with age, as does the 

 resistance to mechanical puncture and to infection. The same is true 

 with the apple scab fungus and powdery mildew of barley; young leaves 

 are susceptible but become more resistant with age. 



Resistance in some cases is apparently due to layers of cork cells formed 

 by the host in advance of the invading parasite. Varieties of flax resistant 

 to wilt {Fusarium lini) and of cotton to black root rot {Thielaviopsis 

 hasicola) seem to be successful in walling off the parasite by forming such 

 a layer of cork which it cannot penetrate. Varieties of potatoes resistant 

 to scab {Actinomyces scabies) form cork more quickly when wounded than 

 do susceptible varieties and are believed to owe their resistance to this 

 characteristic. Thomas (1934) found that the newly formed layer of 

 cork cells was penetrated by invading hyphae of Armillaria mellea and 

 that the cork layer did not successfully stop the advance of this fungus. 

 Brown (1936) states that there is some doubt as to whether the cork layer 

 really functions at all or whether it is formed after the fungus has been 

 stopped by some chemical means. 



Lignified tissues offer more mechanical resistance than nonlignified 

 cells. Certain varieties of wheat resistant to stem rust have a compara- 

 tively greater amount of sclerenchyma and a correspondingly lesser 

 amount of collenchyma and parenchyma in the stem, as compared with 

 susceptible varieties. The maturity of host tissue may be a factor in 

 resistance, even though the tissue does not become lignified or suberized. 

 Some of the systemic smut fungi in cereals are able to grow and penetrate 

 the cell walls in meristematic tissue but are apparently unable to pene- 

 trate the cellulose walls of mature parenchyma cells. After infection 

 in the embryo or seedling stage, the fungus must continue to grow in the 

 growing tip of the shoot if it is to reach the flower parts. Conditions 

 which favor slow growth and delay the maturity of the host favor the 



