DEVELOPMENT OF MESTOME SHEATH-CELLS IN LEAVES OF AEGILOPS-COMOSA VAR THESSALICA

The development of mestome sheath cells of Aegilops comosa var. thessalica was studied by electron microscopy. Anatomical and cytological observations show that this grass belongs to the C3 or non-Kranz plants. In the asymmetrically thickened walls of mestome sheath cells a suberized lamella is present. This lamella is deposited asynchronously. In the midrib and the large lateral bundles it appears first in the outer and inner walls and usually later in the radial walls. In the small lateral bundles its appearance is delayed in the inner walls of those cells situated on the xylem side. At maturity the suberized lamella is observed in all cell walls; however, in the small lateral bundles it is partly or totally absent from the walls of some cells situated on the xylem side. Tertiary wall formation is asynchronous as well, for it generally follows the deposition pattern of the suberized lamella. During the development of the mestome sheath cells microtubules show marked changes in their number and orientation, being fewer and longitudinal during suberin deposition. Dictyosomes are very active and may be involved in primary and tertiary wall formation. Endoplasmic reticulum cisternae are abundant and partly smooth, while plasmalemmasomes may function to reduce the plasmalemma extension. However, cytoplasmic structures that are clearly involved in suberin synthesis could not be identified. Suberized lamellae react strongly with silver hexamine. This is probably due to post-fixation with osmium tetroxide. On the basis of structural characteristics the mestome sheath may be regarded as an endodermis (cf., also Fahn 1974). The significance of this view for water and assimilate exchange between the mesophyll and the bundle is discussed. © 1979 Springer-Verlag.