368 BOTANICAL GAZETTE [APRIL 
appear in December and January. The infection, however, is not sufficiently 
severe to cause perceptible damage to the crop. 
In the second paper, dealing with the influence of external conditions on 
the occurrence of rust infection, the author points out that in dealing with 
problems of this kind it is necessary to take into consideration the effect of the 
state of development of the plant itself. In regard to this question he finds, as 
others had noted, that, within wide limits, the age of plant organs has little 
to do with their susceptibility to infection by uredospores and aecidiospores, 
but that there is, nevertheless, an age limit beyond which infection does not 
take place. is limit GassNER finds coincides with that stage of develop- 
ment of an organ at which teleutospore formation begins. Leaves and stems 
infection. This period varies with different rusts. For example, leaves which 
are producing teleutospores of Puccinia triticina, and hence no longer capable 
of infection by that fungus, can still be infected by P. graminis, since teleuto- 
spores of P. graminis are produced on leaves which have reached a more 
advanced stage of maturity than those on which teleutospores of P. trilicina 
are produced. A peculiar condition of immunity of seedlings of wheat, rye, 
and oats to the attacks of P. graminis was observed. Seedlings of these plants 
are infected only from January to April. For P. ériticina and P. coronifera 
and P. Maydis no such immunity for the young stages of the host plants was 
observed. These facts make it imperative that in a study of the influence of 
seasonal and climatic conditions on the occurrence of rust, only plants of the 
same state of development should be compared. This condition was met by 
+1 
the author by sowing the intervals throughout the year, 
so that practically all stages were available for observation at all seasons. . 
The results of this long series of observations can barely be mentioned. It 
should be stated, however, that the indefiniteness of the results indicates that 
the problem cannot be settled by observation alone, and that an experimental 
analysis with control of all the factors involved is necessary before the effect 
of the individual constituents of the environment can be determined. In 
general GASSNER believes that the environment acts not directly on the fungus 
itself, but indirectly through the effect on the host. He finds that the yearly 
seasonal changes do not affect the occurrence of these 4 grain rusts alike. 
P. graminis is found from the beginning of summer to the beginning of winter; 
P. triticina and P. coronifera are to be found producing new infections at all 
seasons; while P. Maydis occurs from midsummer until autumn. A favorable 
effect of high relative humidity for rust development could not be observed, 
for the period of highest relative humidity, the winter, was also the period of 
least rust development. It is, of course, a question to what extent the effect 
of humidity was obscured by other factors, especially low temperature. In 
general, high temperatures appear to influence the host plant in such a way as 
to favor rust development, but isothermal periods in spring and in autumn are 
not characterized by equal i satan of rust development. It may be a matter 
