Histology of the development of the graft union in pea roots

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Abstract
The development of the union of pea [Pisum sativum cv. Alaska 423] roots splice grafted when 8 days old is followed with optical microscopy. Cell divisions are induced close to the wound in both partners in 1 to 2 days. Content of cytoplasm increases in some cells of the pith, cambium, pericycle, and cortex, and these cells proliferate to form both disorganized callus within the vascular cylinder and organized nodules of cells in the cortex of each partner. There is more proliferation proximal to the graft. The resulting cells invade the graft gap, completely filling it and effecting union by day 7. The first wound-repair vascular tissue differentiates on day 4 in both partners. Wound-repair xylem bridges the union on day 7, phloem on day 8, and a cambium by day 12. The necrotic layer (cells killed by cutting), initially thick, is disrupted by day 4 and largely disappears, as do its phenolic staining properties, by the time of cohesion. Growth of the whole plant resumes by day 9. During development of the union, the cortex and pith proliferate most, the pericycle less, the endodermis and cambium least, thus disproving the dogma that the cambium is the sole proliferative layer in graft formation.