FORMS OF ^ECIDIA 31 



the genus Gymnosporangium, on the Pomaceas. Or it may be 

 oblong or globular, and more or less inflated : this is called a 

 Peridermium, and appears to be confined to the Coniferae (leaves 

 and stems) as hosts and to belong only to the genera allied to 

 Coleosporiam and Gronartium. None of the Peridermia have 

 been found in Australia. It is a remarkable fact that aecidia 

 are never found, upon Juncaceas and Cyperaceaa, nor upon 

 Graminese with only two exceptions — the sscidium of Uromyces 

 JJantlioniae, 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 aecidia 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 aecidium 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 intercalarv 

 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 aecidiospore- 

 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 gecidiospores, except those of Endopliyllum, 

 germinate conidially as in P. Garicis and the germ-tubes enter 

 the host through a stoma : the germ-pores are numerous and 

 almost always indistinct. 



When there are secondary gecidiospores, i.e. such as arise 

 from the germination of a previous gecidiospore, they always 

 take the place of uredospores. In such cases, only the primary 

 gecidiospores arise from fusion-cells, and are accompanied by 

 spermogones. There is said to be one case where gecidiospores 

 are uninucleate, and thus comparable with azygospores, but 

 further evidence is required before this statement can bo 

 accepted (Moreau, 1911). 



