ÆCIDIACEI.--This group differs from the foregoing three groups prominently in the presence of a cellular peridium, which encloses the spores; hence some mycologists have not hesitated to propose their association with the Gasteromycetes, although every other feature in their structure seems to indicate a close affinity with the _Cæomacei_. The pretty cups in the genus _Æcidium_ are sometimes scattered and sometimes collected in clusters, either with spermogonia in the centre or on the opposite surface. The cups are usually white, composed of regularly arranged bordered cells at length bursting at the apex, with the margins turned back and split into radiating teeth. The spores are commonly of a bright orange or golden yellow, sometimes white or brownish, and are produced in chains, or moniliform strings, slightly attached to each other,[j] and breaking off at the summit at the same time that they continue to be produced at the base, so that for some time there is a successive production of spores. The spermogonia are not always readily detected, as they are much smaller than the peridia, and sometimes precede them. The spermatia are expelled from the lacerated and fringed apices, and are very minute and colourless. In _Roestelia_ the peridia are large, growing in company, and splitting longitudinally in many cases, or by a lacerated mouth. In most instances, the spores are brownish, but in a splendid species from North America (_Roestelia aurantiaca_, Peck), recently characterized, they are of a bright orange. If Oersted is correct in his observations, which await confirmation, these species are all related to species of _Podisoma_ as a secondary form of fruit.[k] In the _Roestelia_ of the pear-tree, as well as in that of the mountain ash, the spermogonia will be found either in separate tufts on discoloured spots, or associated with the _Roestelia_, In _Peridermium_ there is very little structural difference from _Roestelia_, and the species are all found on coniferous trees. In _Endophyllum_, the peridia are immersed in the succulent substance of the matrix; whilst in _Graphiola_, there is a tougher and withal double peridium, the inner of which forms a tuft of erect threads resembling a small brush.[l] [Illustration: FIG. 25.--_Helminthosporium molle._]