Electron microscopic studies of cytological changes in phloem parenchyma cells of the living bark of black locust, Robinia pseudoacacia L., indicate that seasonal augmentation in total protoplasm, including mitochondria, lipid bodies, and, particularly, membrane-bound vesicles derived from invaginations and folding of the plasmalemma are closely related to the seasonal cycle of frost resistance. The structural organization of the endoplasmic reticulum also varies seasonally. In summer it is present as long cisternae-like units, which, during autumn and winter, appear to be replaced by a vesicular form of endoplasmic reticulum. Starch content does not appear to be closely related to initiation of the hardening process, but maximum hardiness is not developed until nearly all of the starch has disappeared from the cells. The observations support the view that the total expression of frost resistance in plant cells involves participation of the following three mechanisms: (1) augmentation of total protoplasm, extending to organelles, surface membranes, and other structural components of the cell; (2) lipid transformations; and (3) starch–sugar transformations.

Pomeroy, M. K., & Siminovitch, D. (1971). Seasonal cytological changes in secondary phloem parenchyma cells in Robinia pseudoacacia in relation to cold hardiness. Canadian Journal of Botany, 49(5), 787–795.