 The effects of RNA interference can be both systemic and heritable in plants and "C. elegans", although not in "Drosophila" or mammals.In plants, RNAi is thought to propagate by the transfer of siRNAs between cells through plasmodesmata (channels in the cell walls that enable communication and transport).Heritability comes from methylation of promoters targeted by RNAi; the new methylation pattern is copied in each new generation of the cell.