Cronartium ribicola . a–d. On Pinus: a. Aecia breaking through the stem bark of a young tree (Pinus cf. flexilis); b. Aecia with irregularly ruptured peridium and orange coloured spore mass; c. Peridium cells of an aecium in longitudinal section, note the ± smooth outside (arrow); d. Aeciospores; the broad smooth area on one side is also visible in LM, both in optical section and in surface view; e–h. On Ribes nigrum: e. Uredinia; f. Urediniospores; g. Teliospore columns (telia) on leaf; h. Longitudinal section through the base of a telium emerging from an uredinium; note the uredinial peridium under the host epidermis around the base of the telium; (c, d, f, h from Klebahn 1914: 722; e, g by Julia Kruse).