Plastid ultrastructural features in the various tissues of sunflower leaves.

In young sunflower leaves a cell differentiation gradient has been observed along the major leaf where the leaf basal cells are less differentiated than the apical ones. The tip-base gradient also affects plastid differentiation. Moreover, diverse differentiation speeds are observed among the plastids of the various leaf tissues. The first plastids to take on a well-differentiated appearance are those of the mesophyll. In both the epidermis and vein regions (i.e. in a monocellular sheath round the bundle) one can observe plastids with electron-dense intrathylakoid compartments and lightly stained membranes which, however, are numerous and well-arranged in the grana. The latter membranes have a definite appearance for some plastids. It is assumed that the cell's internal environment is able to control the rate of plastid differentiation.