Light microscopy and transmission and scanning electron microscopy were used to study the development of uredia ofMelampsora lini. Uredia were produced 6ndash;10 days after inoculation with urediospores ofM. lini. Uredium ontogeny began with the formation of a uredium initial from a hyphal plexus in a substomatal cavity. The hyphae oriented vertically and expanded; the enlarged ends divided to form a palisade of uredial initial cells. These cells divided into basal and terminal cells. Each terminal cell divided transversely to form a peridial and an intercalary cell; the basal cell became the sporogenous cell. The intercalary cells disintegrated early in the expansion of the uredium and released their contents into the intercellular matrix. The sporogenous cell, usually swollen at one end, gave rise by budding to successive sympodially produced spore buds which elongated and divided transversely to form pedicels and immature spore cells. As the spores matured the pedicels shriveled and became separated from the urediospores. Elongated, often capitate, paraphyses formed throughout the uredium and functioned to rupture the peridium and epidermis which covered the immature uredium. The peridium and the intercalary cells formed only once during ontogeny of the uredium, this being associated with only the first generation of urediospores. The intercalary cells were disrupted during uredium ontogeny, and most of the peridial cell layer was sloughed off, along with the overlying epidermis, upon rupture of the latter. The paraphyses were permanent and remained in the uredium throughout its functional life. Successive generations of urediospores arose within the same uredium from spore buds produced sympodially from the original sporogenous cell but without forming additional peridial or intercalary cells.
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