196 AMERICAN JOURNAL OF BOTANY '[Vol 9. 



Age and Maturity of Host Tissue 



In his experiments with the asparagus rust Sheldon (1903, p. 47) found 

 a great difference in susceptibility in favor of young and vigorously growing 

 shoots as against older shoots of the same plant. His observations indicated 

 that 



The incubation period of the rust on plants of the same age and growing in the same 

 pot so that conditions were practically identical, was very uniform. When the plants were 

 of the same age and growing in the same kind of soil in different pots, there was still a uni- 

 formity. But when there was a difference in maturity, as of two shoots from the same 

 root, or several plants growing in the same pot, there was a difference in the time of four 

 days in one instance, the young growing shoots showing sori first. The sori showed first 

 on the young growing shoots, and developed faster and to larger size. The more robust 

 the shoot, the larger the sori were and the more spores they produced. 



Sheldon found practically no difference between young shoots of young 

 and old plants. A few shoots from three-year-old plants growing in the 

 same pots showed rust the same day that the seedlings did. Sheldon made 

 similar observations on the carnation rust. 



An age factor of a different kind is reported by Sheldon (1905, p. 227) 

 in the susceptibility of onions to Puccinia Asparagi. Complete immunity 

 was found when seedling onions were inoculated with the rust. The inocu- 

 lations were begun as soon as the seedlings appeared above ground, and 

 were repeated at intervals until the seedlings were two months old, when 

 almost every inoculation was successful. 



Galloway (1903, p. 208) reports a maturity factor as seemingly affecting 

 the susceptibility of wheat to rust. In his extensive experiments on the 

 possibility of controlling cereal rusts by means of spraying or soil treatment, 

 Galloway found that the rust, while abundant on the primary experimental 

 plots, was absent from nearly all the duplicate plots. The latter had been 

 planted a week to ten days later than the oiiginal plots, and in point of 

 growth were at least as much behind them at the time of observation. As 

 the experiments were with a winter wheat, planted the preceding fall, it 

 cannot have been that the plants of the duplicate plots had missed a wave 

 of inoculating material. 



Miss Gibson (1904, p. 188) reports the presence of a seasonal factor in 

 the susceptibility of certain varieties of chrysanthemum to the chrysan- 

 themum rust. She noticed that certain varieties do not take the rust in 

 summer, although growing in the midst of plants thickly covered with it. 

 As the rust spores germinate well in summer and the germ tubes penetrate 

 readily, it is a problem not in the physiology of the parasite but rather in 

 that of the host. 



Stakman and Piemeisel (1917, p. 486), in their extensive inoculation 

 work with cereal and grass rusts, found the cereals and Dactylis glomerata 

 apparently equally susceptible at all ages up to ripening time. Agropyron 

 and Elymus were extremely susceptible when young and much less so when 



