Structural positioning and orientational development in the vegetative organs of Poa pratensis with special reference to the rhizome axillary buds

Abstract

The apical meristem of P. pratensis is hemi-ellipsoidal, the plane of bilateral symmetry being readily recognizable. On each side of the apex a prominent pair of tiers of surface cells straddles the line of the plane of symmetry from the upper limit of the active promeristem and extends basipetally into the first and frequently second foliar primordia. Initiation of a new foliar primordium (phytomer) is centered in one or the other of the paired tiers. Occasionally, a single cell in a tier appears to undergo initial mitotic activity. The specific tier of initial activity determines the deviation from symmetry of the whole phytomer including that of the leaf at the top of the phytomer and of the lateral bud at its bottom. The subtending leaf does not determine the deviation from the plane of symmetry of the axillary bud. The first leaf of the axillary bud (not the prophyll) is positioned with its point of origin, hence its midrib, displaced away from the plane of symmetry of the main stem and with its larger semicircumference towards the main stem. Thus the deviation from bilateral symmetry of all the organs of the phytomer and the initial deviation from symmetry of the organs produced by the axillary bud are determined originally by the specific tier of cells in the promeristem of the main shoot that is stimulated into initial activity. This initial orientation of the axillary bud is the first of the many orientations that determines the pathway through the soil that the rhizome will ultimately follow.