A study was made of the mesophyll cell ultrastructure of early-spring ephemerals Scilla sibirica, Crocus vernus, Galanthus caucasicus, G. plicatus, Leucojum vernum, Muscari szovitsianum, Ornithogalum balansae of summer-vegetating. Scilla scilloides and of S. sibirica growing in greenhouse. The highly developed smooth endoplasmic reticulum (SER) was a distinctive feature of the early-spring ephemeral mesophyll cell during leaf growth (cell expansion). Direct connections of SER-compartments with the tonoplast were found (SER and vacuole cavities were continuous). On running through the cytosol the SER tubules were in close contact with the plasma membrane. SER was not found during the same period of cell expansion in summer-vegetating Scilla scilloides and S. sibirica growing in greenhouse, as well as in mature cells of all investigated plants. The role of SER in the adaptation of early-spring ephemerals to drastic temperature fluctuations is discussed.