The root endodermis in Ranunculus acris. I. Structure and ontogeny

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Abstract
The root endodermis in Ranunculus acris was investigated using light and transmission electron microscopy to obtain a more complete picture of endodermal development in a dicotyledonous species. Following the formation of a lateral bulge in anticlinal walls of proendodermal cells, portions of the plasma membrane became tenaciously associated with the primary wall material. The plasma membrane in the zone of the Casparian band appeared extremely electron dense and displayed a distinct tripartite organization. Cytoplasm of primary stage endodermal cells contained Golgi bodies with associated vesicles, lipid-like droplets, and multivesicular bodies containing membrane-enclosed vesicles. Cellulose microfibrils in the radial walls of endodermal cells became impregnated with an osmiophilic substance possibly through the incorporation of paramural bodies at a ledge-like invagination of the plasma membrane on either side of the Casparian strip. It is proposed that the intense organelle activity in the vicinity of the Casparian strip could signify an early stage of suberin lamellae synthesis. Between 8 and 10 suberin lamellae were laid down on radial and tangential walls of those endodermal cells which underwent secondary development, with the heaviest deposition occurring in the region of the Casparian strip. The parallel orientation of the suberin lamellae was frequently disrupted by electron-dense 'platelets' and small granules. A fibrillar suberin-like material was found in dilated cisternae of endoplasmic reticulum located adjacent to areas of deposition. Nuclei, membranous configurations, mitochondria, plastids, and other cytoplasmic organelles were found to persist in tertiary stage endodermal cells. Histochemistry and SEM showed that the mature endodermal wall complex of field-grown plants was comprised of alternating bands of cellulose, lignin, and possibly, suberin.