Structure, Formation, and Dormancy of Terminal Buds of Elm

Abstract

Extension growth in red elm (Ulmus rubra Muhl.) and in white elm (U. americana L.) is normally terminated in both seedlings and mature trees by abortion and abscission of the shoot tip. Subsequent growth is from subjacent axillary buds and produces a sympodial axis. Terminal buds are formed on a few glasshouse-grown seedlings and saplings, the progeny of trees from the Lincoln, Nebraska, region, and on still fewer field-grown seedlings. Terminal buds have not been observed on saplings or mature trees in this region. Abscission of shoot tips bearing terminal buds may be similar to normal abortion of elongating shoot tips, but more commonly renewed growth produces a monopodial axis. Terminal buds differ structurally from elongating shoot tips in that the stipules of leaves at the lower nodes mature while development of the blades, petioles, and subtending internodes is arrested. Bud scales occurring one at a node are found in axillary buds but are absent from terminal buds. The paired basal stipules of terminal buds function as bud scales by inclosing the distal portions of the shoot tip. Terminal buds are formed more commonly on seedlings exposed to daily photoperiods of 8 hr or less and on seedlings that retain the opposite-leaf arrangement at the end of extension growth. Bud dormancy was studied by defoliation and chilling treatments. Terminal buds and pseudoterminal axillary buds are similar in that following the end of extension growth a state of correlated inhibition exists which is followed by a state of rest.