HEMIBASIDIOMYCETES 327 



more or less perfect concentric circles, as in Aeddium 

 zonale, Uromyces sdllarum, U. colchiti, Pucdnia lychni- 

 dearum, etc. In many instances this centrifugal manner 

 of growth is interfered with by the special structure of the 

 portion of the host-plant infected. When the venation of a 

 leaf is strongly pronounced, the centrifugal method of exten- 

 sion is much modified, and in many instances completely 

 neutralised. This is especially marked in the case of 

 Uredines developed on the leaves of grasses and sedges, 

 where, owing to the presence of strongly developed parallel 

 rows of vascular bundles or veins, the sori or spore masses 

 are always elongated or linear in form, as in Pucdnia 

 gramtnis, P. rubigo-vera, etc. 



On the other hand, the mycelium is in some species by 

 no means localised, but permeates almost every part of the 

 host. Such mycelium is generally perennial, and con- 

 sequently when a plant is once infected it remains so 

 during the remainder of its life. As illustrations of such 

 perennial mycelium may be mentioned the teleutospore 

 form of species of Gymnosporangium, produced on various 

 species of juniper ; Endophyllum euphorbiae, Aeddium 

 leucospermum, A. pimctatum., A. tragopogonis, etc. 



Heteroecism, as already stated, is common in the 

 Uredineae, and this phenomenon implies the presence of 

 more than one form of spore produced by the same species 

 during different periods of its life-cycle. Five such spore- 

 forms are known to exist, but it is very unusual for all the 

 five forms to be produced by any given species. The 

 spore-forms, always produced in the following sequence, 

 are as follows : Spermatia, Aecidiospores, Uredospores, 

 Amphispores, Teleutospores. 



The spermogonia or conceptacles containing the sper- 



