| Summary: | The relationship of the macrocyclic rust fungus PUccinia poarum with its pycnial-aecial host, Tussilago fapfaPa, and its uredial-telial host, Poa ppatensis, has been investigated, using light microscopy, electron microscopy and micro-autoradiography. Aspects of the morp- hology and ontogeny of spores and sari, which were previously disputed, have been clarified. Monokaryotic hyphae grow more densely in the intercellular spaces of Tussilago leaves than the dikaryotic intercellular hyphae on Poa. Although ultrastructurally sbnilar, monokaryotic hyphae differ from dikaryotic hyphae in their interaction with host cell walls, often growing embedded in wall material which may project into the host cells. The frequency of penetration of Poa mesophyll cells by haustoria of the dikaryon is greater than that of Tussilago cells by the relatively undifferentiated intracellular hyphae of the monokaryon. Intracellular hyphae differ from haustoria in their irregular growth, septation, lack of a neck-band or markedly constricted neck, the deposition of host wall-like material in the external matrix bounded by the invaginated host plasmalemma and in the association of callose reactions \vith intracellular hyphae and adjacent parts of host walls. Monokaryotic and dikaryotic infections differ also in the changes induced in the organization and ultrastructure of their respective host cells. Intracellular hyphae in bundle sheath, xylem parenchyma, transfer cells of phloem parenchyma and companion cells, give the monokaryon of P. poaPUm direct access to nutrients translocated in vascular tissue of Tussilago. Bundle sheath cells of Poa contain unusually long haustoria but· there is no penetration of the endodermis and vascular tissue by the dikaryon. After uptake of tritiated glycerol by infected tissue, microautoradiographic investigation shows radioactivity to be concentrated in sporulation structures, haustoria and, in the case of Poa, in host nuclei. Cells of uninfected tissues differ from those of infected tissue in accumulation of label in chloroplasts. These studies contribute to an understanding of the physiological interaction of P. poaPUm with its alternate hosts.
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