FORMS OF ACIDIA 31 
the genus Gymnosporangium, on the Pomacee. Or it may be 
oblong or globular, and more or less inflated: this is called a 
Peridermium, and appears to be confined to the Conifers: (leaves 
and stems) as hosts and to belong only to the genera allied to 
Coleosporium and Cronartium. None of the Peridermia have 
been found in Australia. It is a remarkable fact that ecidia 
are never found upon Juncacee and Cyperacez, nor upon 
Graminee with only two exceptions—the xcidium of Uromyces 
Danthoniae, on Danthonia in Australia, and of Puccinia grami- 
nella, on Stipa in North and South America. In all other 
Uredines parasitic on these families, if ecidia, enter into the 
life-cycle of the fungus at all, they are formed upon some broad- 
and thin-leaved Monocotyledon, or upon a Dicotyledon, usually 
though not invariably belonging to one of the more specialised 
orders and above all to the Composite. 
The essential characteristic of the «cidium is that its 
spores are produced in chains from a fusion-cell, as described 
in Chapter I. The spores themselves are always unicellular, 
mostly with orange contents, and separated by intercalary 
cells. Their customary polygonal shape arises entirely from 
crowding and their verruculose sculpture presents a remark- 
able similarity through all the group. The cells of the peridium 
of the higher types are homologous with the ecidiospore- 
mother-cells, and represent a division of labour for the sake of 
protection: the paraphyses and the lower forms of peridium 
are not of the same character, and may have had a somewhat 
‘different origin. All ecidiospores, except those of Endophyllum, 
germinate conidially as in P. Caricis and the germ-tubes enter 
the host through a stoma: the germ-pores are numerous and 
almost always indistinct. 
When there are secondary ezcidiospores, i.e. such as arise 
from the germination of a previous xcidiospore, they always 
take the place of uredospores. In such cases, only the primary 
eecidiospores arise from fusion-cells, and are accompanied by 
spermogones. There is said to be one case where ecidiospores 
are uninucleate, and thus comparable with azygospores, but 
further evidence is required before this statement gan be 
accepted (Moreau, 1911). 
