Origin and Development Morphology of Root Nodules of Pisum sativum

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Abstract
Pea plants of the vars. Canada, Alaska, and Perfection were grown in gravel culture for the study of developmental morphology of the root nodules. The root nodule originates by proliferation of cortical parenchyma cells in the vicinity of the rhizobial infection strand, which enters the root through a root hair and penetrates 3-5 layers of cortical cells. The number of cells increases by divisions at first throughout the young nodule, then in an apically localized meristem. Branches of the infection strand penetrate many cells in the central region of the developing nodule, and rhizobia are released into the cytoplasm. Some infected cells may divide, but none containing many rhizobia was found in a division stage. The nodule becomes differentiated into an infected or bacteroid region, adjacent to the apical meristem, and an uninfected cortical parenchyma. Two main vascular strands and their dichotomous branches differentiate in this nodule cortex as it matures. Six to ten vascular bundles may be found near the apex of the nodule. The enlarging nodule ruptures the root epidermis but not the root cortex. The base of each of the 2 main vascular strands is composed of cells derived from the root pericycle and endo-dermis. Opposite other levels of the nodule, the pericycle does not divide, and the root endodermis remains intact. The vascular tissues in the nodule differentiate acropetally. There are two endodermal layers in the nodule. An endodermis with typical Casparian bands surrounds each vascular strand. An endodermal layer designated here as the "nodule endodermis", also characterized by Casparian bands, differentiates between the nodule cortex and the root cortex. Both endodermal layers join the endodermis of the root.
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