Origin and Development Morphology of Root Nodules of Pisum sativum

The anatomy of the root of Pisum sativum in the region of nodule development is described. 2. Both the protophloem and the metaphloem in the roots of plants of the variety Perfection are characterized by sieve tubes with companion cells and sieve plates. In the variety Canada, roots show conspicuous dumbbell-shaped slime bodies in the sieve tubes. 3. Branch roots originate by divisions of pericyclic cells opposite one of the xylem arms. 4. Some radial divisions occur in the endodermal cells opposite developing branch roots, but the endodermal sheath remains continuous for only a short time and is soon ruptured near the base of the elongating lateral root. 5. Root nodules originate by proliferation of the cortical parenchyma cells in the vicinity of the rhizobial infection strand, which enters the root through a root hair and penetrates through three to five layers of cortical parenchyma cells. 6. The number of cells in the nodule increases at first by divisions throughout a spherical mass of cells, then by the action of an apically localized meristematic region. 7. Branches of the infection strand penetrate many of the cells in the central region of the developing nodule, and rhizobia are released into the cytoplasm of the host cells. 8. Divisions occur in some infected cells, but no division stages were observed in cells having many rhizobia in the cytoplasm. 9. The nodule becomes differentiated into an infected or bacteroid region, adjacent to the apical meristematic region, and an uninfected cortical parenchyma. 10. The root epidermis is broken as the nodule enlarges, but the nodule does not emerge from the root cortex. 11. Two main vascular strands differentiate at the base of the nodule and branch dichotomously to give from six to ten vascular bundles near the apex of the nodule. 12. At the level of the root at which a nodule vascular strand differentiates, the pericyclic cells adjacent to a xylem arm and the endodermal cells in the same radii of the root divide first tangentially, then radially. The provascular strands become differentiated acropetally by radial divisions of cells in the uninfected cortical region of the nodule. 13. Maturation of vascular tissues in the nodule begins in the cells derived from the root pericycle and proceeds acropetally. 14. There are two endodermal layers in the nodule. An endodermis with typical Casparian bands surrounds each vascular strand and is termed the "vascular endodermis" or "bundle endodermis." An endodermal layer designated here as the "nodule endodermis" differentiates in the outermost layer of the uninfected cells of the nodule cortex and forms a cylinder adjacent to the larger stretched cells of the surrounding root cortex. The cells of this endodermal layer also have typical Casparian bands. Both endodermal layers join the endodermis of the root. 15. The number of chromosomes found in the cells of the nodule usually is twenty-eight (4n) in its meristem and fourteen (2n) in its cortical region. One cell near the center of the nodule meristem had fourteen chromosomes. Two cells in the same provascular strand had different numbers of chromosomes, one fourteen, one twenty-eight.