The ultrastructure of Arceuthobium pusillum

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Abstract
The endophytic system (ES) of Arceuthobium pusillum in Picea mariana, a compatible host, ramified throughout branchlets composing witches' brooms, and its cells contained numerous nonmembranebound droplets composed largely of neutral fats which occupied much of the cytoplasmic volumes and which persisted in senescent cells. The ES in the less compatible Larix laricina was found to persist mostly in tissues adjacent to and in spur shoots, and, in less vigorous infections, it accumulated fewer lipid droplets. In more successful infections of larch, many droplets were observed. Although the droplets probably serve for storage of high-energy fats, their accumulation in L. laricina appears to be reflected in the degree of success which the parasite has in this host. The more successful the infection, the more abundant are lipid droplets. Rapid lipid synthesis by the parasite may help to maintain a sharp carbohydrate diffusion gradient from host to parasite. The lipid droplets in both hosts were often in physical contact with smaller, single membrane-bound, evenly staining bodies which may have been involved in synthesis or use of these lipids. Germinating seeds, aerial shoots, and ES of the parasite in both hosts contained functional chloroplasts that structurally resemble those in the Gramineae that use the C4-dicarboxylic acid pathway.

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