Nicotiana tabacum callus studies V. Ontogeny and structure of phloem

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Summary Apart from a relative lack of organization phloem histogenesis in Nicotiana pith cultures is broadly similar to that in the intact plant. Phloem tissue in in vitro cultures arises from internal divisions of callus parenchyma — simulating the origin of internal phloem bundles in the intact stem — as well as from mother cells cut off from procambium-like initials — resembling the origin of external phloem in the intact stem. Sieve elements have end walls ranging from transverse to very oblique and bear either simple or compound sieve plates but, although in this way corresponding with primary and secondary type sieve elements, they are not associated respectively with other features of the phloem classed in the intact plant as primary and secondary.