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 susceptibiHty in favor of young and vigorously growing 
shoots as against older shoots of the same plant. His observations indicated 
that 
The incubation period of tlie rust on plants of the same age and growing in the vSame 
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 original 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 (191 7, 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 
