This study investigated leaf mesophyll cells of Caryophyllaceae plants growing in polar regions – Cerastium alpinum and Silene involucrata from the Hornsund region of Spitsbergen island (Svalbard Archipelago, Arctic), and Colobanthus quitensis from the Admiralty Bay region on King George Island (South Shetland Islands, West Antarctic). Ultra− structural changes were analyzed in mesophyll protoplasts of plants growing in natural Arctic and Antarctic habitats and plants grown in a greenhouse, including plants exposed to short−term cold stress under se mi−controlled conditions. Cell organelles of plants growing in natural polar habitats and greenhouse−grown plants were characterized by significant morphological plasticity. Chloroplasts of plants studied in this work formed variously shaped protrusions and invaginations that visibly increased the contact area between adjacent cell compartments and reduced the distance between organelles. S. involucrata plants grown under greenhouse conditions, tested by us in this wor k, were characterized by highly dynamic cell nuclei with single or multiple invaginations of the nuclear membrane and the presence of channels and cisternae filled with cytoplasm and organelles. Crystalline inclusion proteins were observed in the cell nuclei of C. quitensis between nuclear membranes and in the direct proximity of heterochromatin. Our study revealed significant conformational dynamics of organelles, manifested by variations in the optical density of matrices, membranes and envelopes, in particular in C. quitensis , which could suggest that the analyzed Caryophyllaceae taxa are well adapted to severe climate and changing conditions in polar regions.