Cell enlargement and tissue differentiation during leaf expansion in beech, Fagus grandifolia

Abstract

Cell enlargement and differentiation of leaf tissues have been studied from the stage of bud expansion to leaf maturation in beech. Fagus grandifolia Ehrh. Leaf tissue was collected at weekly intervals and observations were made on plastic-embedded, 2-μm serial sections; all measurements were made using a microprojector. At the earliest stages examined, six cell layers are present, and with the exception of a slightly elongated palisade layer, cells of the protoderm and ground meristem are similar in size, shape, and staining characteristics. The following observations have been made during maturation: (1) cell division occurs in the upper palisade layer for at least 1 week longer than in other cell layers: (2) the highest relative rates of leaf expansion are due to increase in cell number rather than to cell enlargement; (3) relative rates of cell enlargement in all cell layers peak during the same time period, but are highest and of longer duration in the upper and lower epidermis; (4) relative rates of increase in cell height are highest and of longest duration in the palisade. These and other observations demonstrate the important role that is played by duration and rates of enlargement as well as direction of expansion in the differentiation of mature leaf tissues.