The mesophyll cells of four species of Poaceae flowering plants growing in polar regions were studied—Deschampsia antarctica Desv. from the region of the Admiralty Bay on King George Island (West Antarctica) and D. alpina (L.) Roem. Sch., Poa alpina L. var. vivipara and P. arctica R. Br. var. vivipara from the Hornsund region of Spitsbergen island (Arctic). Ultrastructural changes were analyzed in the organelles of plants growing in Arctic and Antarctic habitats and plants grown in greenhouse, including plants exposed to short-term cold stress. The cell organelles were characterized by structural dynamics. Their morphological plasticity was manifested by elongation, formation of protrusions in the direction of adjacent organelles, as well as cytoplasm-filled pockets and invaginations that increase the contact area and reduce the distance between cell compartments. D. antarctica and P. alpina var. vivipara plants were characterized by highly dynamic cell nuclei with invaginations of the nuclear membrane filled with cytoplasm and organelles, high morphological plasticity, and conformational dynamics of chloroplasts and mitochondria, manifested by variations in the electron-optical density of matrix, membranes, and envelopes. The above could suggest that the studied taxa and their metabolic mechanisms had adapted to severe climates and changing conditions of the polar regions.