ARKIV FÖR BOTANIK. BAND. 1. 143 



has not quite grasped the import of my theory. I feel obli- 

 ged to elucidate the matter in some few words. 



If we try to make clear to ourselves the origin of the 

 outbreak of a uredo-pustule fleck of an heteroecious species 

 of rust (e. g. Piiccinia gyaminis), we have to suppose several 

 different possibilities. The fleck can arise 1) from an infec- 

 tion by uredospores (e. g. Uredo (jraminis) or 2) from an 

 infection by aecidios pores (e. g. Aecidium Berheridis). 

 Numerous ex})eriments have fully proved that both of these 

 modes of origin occur. The time that elapses between the 

 infection by spores and the outbreak of pustules, or the period 

 of incubation, as it is called, is comparatively short, — being 

 from about 8 to 10 days. 



The present position of the rust-question is, however, 

 such that we can no longer be satisfied with these two* 

 modes of origin. Willingly or unwillingly we are obliged 

 to suppose two further possibilities. It is possible that an 

 uredo-pustule fleck can also arise o) from a direct infection 

 by t eleu to spor es (e. g. Piiccinia (jr am in is) without the 

 intervention of an aecidium-stage, or 4) from a latent germ 

 of disease inherited from the parent jjlant, concealed 

 in the seed sown or, as far as concerns a perennial grass, 

 in the stolon growing forth in the spring. Both these modes 

 of origin presuppose a long period of incubation, varying 

 from about 2 to 10 months.- 



Although many reasons speak for a direct infection by 

 teleutospores,^ it must however be confessed that this question 

 is still far from being solved.* 



On the other hand there seems to me to be a sufficient 

 number of proofs which point to the mode uf origin last 

 mentioned — a latent germ of disease, inherited from the 



' The latter of these alternatives does uot, of course, exist in the ease 

 of an homoecious species of rust, which has no aecidiani-stagc (e. g. Piiccinia 

 glumarum). 



■ All of these modes of origin have heen taken into account as early as 

 1M%, when arranging the cycle of development of the different species of 

 cereal rust (J. Eriksson & E. Henning. Die Getreideroste, Stockholm. 1896, 

 p. 107. 141, 1.S4, -233, 254). 



^ I would esj)ccially remind my readers of the peculiar fact that two 

 American wheats (llurslortl's Pearl Wheat and Midiigau Bronze) and one Au- 

 stralian liarley (Skinless) are in the highest degree suscejitible to Yellow 

 Rust (Pucci)iin jilnuiaytun). although this species of rust occurs neither in 

 America nor in Australia (J. Eriksson & E. Henning, Die Getreideroste, 

 p. 207-20S. 210-211). 



* J. Eeiksson, Sur I'origine etc., T. 15, p. 141 [Extr. p. 265J. 



