Abstract
Microscopic studies were conducted on weather fleck, a non-parasitic plant disorder of flue-cured tobacco in Ontario. Within a few hours of the initial appearance of the flecks, damage was restricted to the palisade parenchyma cells; after about one day some of the spongy parenchyma cells also were affected. There was no apparent difference between flecked and healthy tissue with respect to thickness or structure of the cell wall. In affected tissue, the nuclei of the palisade cells were shrunken to spherical, ellipsoid, or irregularly ellipsoid shapes with average measurements of 6.0 × 5.1 μ; in adjacent undamaged cells, they averaged 9.1 × 7.3 μ. The nuclei of damaged cells stained more heavily than nuclei of normal cells and their structural details were lost. Chloroplasts of flecked and healthy palisade cells were almost of an equivalent size, the average lengths being 4.3 μ and 4.4 μ, respectively. However, chloroplasts in flecked cells had often been disrupted. Evidence is presented to support the hypothesis that the unknown agent or agents causing weather fleck first affect the cytoplasm and nucleus, then the chloroplasts.
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