Development and Organization of the Primary Vascular System in the Phase II Leaf and Bud of Osmunda cinnamomea L.

Abstract

Cinnamon fern (Osmunda cinnamomea L.) leaf primordia of different physiological ages develop either as phase I (shoot only), as phase II (leaf + adaxial bud), or as phase III (leaf only) primordia when they are excised and cultured Third-youngest primordia (P3's) can develop either as phase II or phase III primordia in culture At the time of excision, an incipient procambial strand, which is in continuity with more mature procambium in the rhizome axis, is evident immediately beneath the leaf apical cell When a P3 develops as a phase III primordium, as it normally does in vivo, the procambium serving the leaf develops as a trace that is essentially of uniform diameter throughout its extent However, when a P3 develops as a phase II primordium, a new procambial strand diverges acropetally toward the prospective site of the adaxial bud This procambial strand is precocious; i e, it develops in advance of the bud it will serve Therefore, the adaxial bud is an induced bud A parenchymatic cleft, which forms between the acropetally developing leaf and bud procambial strands, clearly separates the leaf and bud primordia. Primary phloem differentiates acropetally and continuously, first in the leaf procambial strand and then in the bud procambial strand The timing of primary phloem differentiation in the bud trace suggests a causal relation with the appearance of an organized bud meristem Primary xylem differentiates after primary phloem It differentiates bidirectionally from an initiation site at the interstice of the parenchymatic cleft