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Cotyledons of Lupinus luteus were sampled from 1 to 21 days after sowing and processed for light microscopy and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Length, width, and surface area of the cotyledons increased gradually until day 10. The thickness of the cotyledons increased from day 7 to 12 and decreased thereafter. Morphometric analyses showed that the increase in length, width, and thickness of the cotyledon was due to cell expansion, and the decrease in thickness of the cotyledon was due to the decrease in the length of abaxial cells and in the total number of cells. Mesophyll development accompanied schizogenous and lysigenous air space formation. There were two structurally distinct types of protein bodies. Protein bodies in five to six layers of cells on the abaxial side did not contain globoids, while globoids were prominent in protein bodies in the center and adaxial side. Storage protein mobilization occurred first in the abaxial side of the cotyledon and proceeded toward the adaxial side. SDS-PAGE analysis showed that proteins ranged from 97 to 14 kD. High molecular weight [alpha]- and [beta]-conglutinins were more abundant in the abaxial region, whereas [gamma]-conglutinin occurred in both abaxial and adaxial regions. In addition, there were five minor bands between 97 and 43 kD unique to abaxial region and five minor bands between 43 and 14 kD unique to adaxial region in the nonreduced protein profiles. The [alpha]- and [beta]-conglutinins began to decrease after imbibition and disappeared by day 7 after sowing. At this stage the subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase and four new minor bands appeared.