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AEcidium, or Clustercups, the name formerly applied to a genus of fungi parasitic upon living flowering plants, but now known to be only a stage in the life-history of what was considered a distinct group, the Uredineae. They are sometimes called AEcidiomycetes, the "rust" of wheat Puccinia graminis being the best known example of the group. Some species are autoecious, passing through all stages on one host-plant; others, such as the rust, are heteroecious, passing parts of their life-cycle on distinct hosts. The black two-celled spores (teleuto-spores) produced on straw in autumn, in the case of the rust appearing in linear clusters, germinate in spring, producing short tubes or promycelia, the branches of which terminate in sporidia. These sporidia will only germinate on the leaves of the barberry, the epidermis of which they perforate, producing "spawn" or mycelium threads in their interior. On these barberry-leaves yellow spots soon appear, which burst into cup-like structures filled with chains of spores. These clustered cups are still termed aecidia and their spores aecidiospores, but they were formerly supposed to complete the life-history of the fungus AEcidium berberidis. The aecidiospores of the barberry will only germinate on the surface of a grass such as wheat, arid in from six to ten days, burst out in linear masses of orange spores (uredo-spores), formerly known as Uredo. These uredo-spores will germinate on grass, giving rise to others like themselves; but towards the close of the season are replaced on the same spawn by the black teleuto-spores known as Puccinia. Thus three apparently distinct fungal parasites are found to be merely stages in the life of one. It is suggested that the aecidia are sexually produced within the barberry leaf. Another species of this large group causes the "witches' broom" in fir trees.

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