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Little is known about the control of leaf size in plants, yet there must be mechanisms by which organ size is measured. Because the control of leaf size extends beyond the action of individual genes or cells, an understanding of the role of leaf cell layers in the determination of leaf size is warranted. Following the construction of graft chimeras composed of small-and large-leaf genotypes of Nicotiana , bilateral leaf blade asymmetry was observed on leaves possessing either a genetically larger or smaller epidermis on one side of the midrib. Although cell size was unaffected by the genotype of the epidermis, the rate and extent of cell division in leaf epidermis altered the rate and extent of cell division in mesophyll and affected leaf size. The data presented neither prove nor disprove whether the mesophyll impacts epidermal cell division but provide the fi rst unequivocal evidence that the extent of cell division in the leaf epidermis alters the extent of cell division in the mesophyll and is a factor regulating blade expansion and ultimate leaf size.

A method for shoot regeneration from leaf explants in two cultivars of cranberry (Vaccinium macrocarpon Ait.) is described. Modified Anderson's medium supplemented with combinations of thidiazuron (TDZ) with or without 1 luM NAA (a-naphthaleneacetic acid) was used to optimize shoot regeneration. The effect of light or dark incubation was also determined. Maximum regeneration was obtained in the light in the presence of 10 ~M TDZ and 1 IJ-M NAA. While this medium was suitable for leaf explants obtained from shoot cultures, regeneration did not occur from leaves collected from greenhouse-grown plants. Elongation of the regenerated shtSot tips did not occur until explants were transferred to growth regulator-free medium at which time only a minority of shoots elongated. Elongated shoots could be dissected away from leaf tissue, rooted easily, and acclimitized to ambient conditions.Abbreviations: NAA -a-naphthaleneacetic acid; TDZ -1-phenyl-3-(1,2,3 -thiadiazol-5-yl) urea

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SummaryUtilizing a complete set of six periclinal graft chimeras composed of Nicotiana tabacum and Nicotiana glauca (TGG, GTT, TFG, GGT, TGT, and GTG), the fete of the three apical cell layers in both vegetative and reproductive organs has been traced. An analysis of leaf phenotype indicated that only rarely did deviations from expected cell lineage occur and in only TTG did such deviations originate in the shoot apical meristem rather than during leaf development. In most plants that possess a stratified shoot apical meristem, gametes are derived from the second apical layer (L2). A phenotypic and/or DNA analysis of seed progeny following reciprocal crosses between all chimeras and their component species indicated that pollen and eggs were sometimes derived from non-L2 lineage in all but one periclinal chimera. There was no evidence for non-L2-derived gametes in 95 crosses where GT~ was a parent whereas 40 of 104 crosses with l"rG as a parent yielded some offspring that resulted from non-L2-derived gametes. Of these 40 cases, non-L2-derived pollen grains were responsible 39 times while non-1.2-derived eggs were responsible just once. Therefore, the occurrence of non-L2-derived gametes was not random. The disruption of 'normal' lineage patterns was dependent on the specific arrangement of genetically dissimilar tissue layers in the shoot apices of the chimeras and was different for different organs.