MORPHOLOGICALLY DISTINCT STAGES IN THE GROWTH AND DEVELOPMENT OF RHIZOMES OF POA PRATENSIS L. AND THEIR CORRELATION WITH SPECIFIC GEOTROPIC RESPONSES

Abstract

Three distinct stages in the growth and development of the rhizomes of Poa pratensis L. can be distinguished. The names, primary, secondary, and tertiary are proposed to identify the stages. Primary stage rhizomes produce cataphylls elliptical in cross section, and poreless, or with a very small pore. Cataphyll primordia, initiated by the apical meristem, develop disproportionately, producing a hood-like cowling enclosing the apical meristem. The opening partially or completely closes by ontogenetic fusion. The geotropic response is plagiotropic Secondary stage rhizomes produce cataphylls with a marked longitudinal invagination. They are seldom poreless, and then only early in this stage. The apices are similar to primary stage apices. The geotropic response is diageotropic. Tertiary stage rhizomes progressively exhibit characteristics of true aerial shoots. Cataphylls develop a rudimentary leaf blade, ligule, and buliform-cell leaf-closure apparatus. However, a collar between blade and sheath does not form until the rhizome reaches the soil surface. The apex progressively develops the broad shield-shaped leaf primordium characteristic of aerial shoots. The geotropic response becomes strongly negatively orthogeotropic. Both the secondary and the tertiary stages are initiated by a change in the morphology of the apex and the cataphyll that precedes changes in the geotropic response of the rhizomes.