CHAPTER ITI 
SPORE FORMS OF THE UREDINALES 
ANCIDIUM. 
Acidia are usually of a cup-like shape, partly embedded in 
the host, and with the free protruding edge more or less 
recurved. This is the typical and presumably the most highly 
evolved form. In it the spores are at first completely enclosed 
by a firm structure, the peridiwm, the cells of which have the 
membrane thickened on the inside wall or the outside or both, 
and are arranged in very definite rows like the spores. But 
there are a number of variations on this type, most, if not all, 
of which belong to a lower stage of evolution. Sometimes the 
peridium has the cells less definitely arranged in rows, and 
therefore opening more irregularly ; at other times the peridium 
is thin-walled and delicate, and in that case it usually opens by 
a rounded pore and the edges do not roll back. In Hyalopsora 
and its ales such a peridium is formed round the uredo-sori, 
and there are reasons for believing that in these cases the pore 
arises just beneath a stoma. A still simpler stage is represented 
by those cases where there is no definite peridium at all, but 
merely a surrounding circle of paraphyses which in a few cases 
are almost or even totally non-existent. This is called a Ceoma 
and indicates a more primitive form ; it is found in Phragmidium 
and Melampsora. In the non-British Gymnoconia Peckiana 
(= Puceinia Peckiana, on Rubus) peridium and paraphyses are 
both entirely absent. 
Again, if there is a definite peridium, it need not have the 
shape of a cup. It may be elongated-cylindrical, straight or 
curved like a horn: this is called a Restelia and is confined to 
