THE CAUSES AND DEVELOPMENTAL EFFECTS OF INTEGRATION AND INDEPENDENCE BETWEEN DIFFERENT PARTS OF GLECHOMA-HEDERACEA CLONES

The clonal species Glechoma hederacea shows both physiological integration and physiological independence between its ramets at different structural levels. The cause of these differences in intra-clone integration is analysed, using ( 1) acid fuchsin dye to reveal the vascular connections between different ramets through the xylem, (2) C-14 as a marker to demonstrate and quantify photoassimilate translocation patterns in the phloem, and (3) anatomical studies. Dye movement in the xylem was strictly acropetal and highly sectorial, and the sectoriality was determined by phyllotaxy. Patterns of movement of C-14 were qualitatively similar to those of xylem-based resources, a[though there was some basipetal movement of photoassimilate. The patterns of physiological integration and independence between ramets are explained from these results and shown to be governed by simple rules which depend on vascular continuity and discontinuity between ramets. The growth form of G. hederacea changes substantially as availability of resources alters. One marked change concerns stolon cross-sectional area, which decreases as nutrient supply falls. Evidence is presented suggesting that clones grown under low nutrient regimes may be able to transport soluble resources with relatively greater efficiency than clones growing under higher nutrient supply. An experiment using different defoliation intensities, and different defoliation patterns at the same overall intensity, demonstrates that the precise positions of leaves removed from a clone have unique consequences for its subsequent development Removal of diffcrent ramets and leaves also alters the predictability of growth; the C.V. of various measures of the growth of replicate plants is altered by each defoliation pattern to a different extent. Since herbivory rarely removes leaves in predictable or repeatable patterns, this makes it unlikely that general predictions of its effects upon size hierarchies in plant populations will be achievable. The vascular and physiological organization in G. hederacea seems likely to contribute to its success in biotically and abiotically heterogeneous environments. Physiological support to developing stolon apices results in rapid movement through the habitat and early acquisition of relative branch autonomy, and these attributes probably promote the proliferation of the clone in more favourable habitat patches.