Further Experiments on the Biological Significance of the Morphology of Seed-Dispersal Units in Grasses

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Abstract
Experimental evidence is provided to indicate that rigid awns on the diaspore of Aristida vagans and Microlaena stipoides serve to orientate it during its fall from the parent plant thereby increasing the chances that it will land in a standing orientation with its callused end embedded in the soil surface. The proportion of diaspores landing in the standing orientation was influenced by the type of soil surface. The orientation and position of the diaspore on or in the soil surface influenced germination in both species. In both species the standing orientation conferred an advantage during the seedling''s establishment phase.The diaspore was held firmly in position by either the antrorse (backwardly directed) bristles on the callus embedded in a compacted soil surface or by backwardly (upwardly) directed barbs on the body of a diaspore standing in leaf litter. The anchorage so achieved produced a counter to the force of the radicle penetrating a compacted soil surface. Seedlings arising from unanchored horizontally orientated diaspores frequently died as the radicle was hindered in its penetration of the soil surface. On a soft soil surface the anchorage of diaspores of M. stipoides by the callus conferred no advantage as the radicle could easily penetrate the surface. The morphology of diaspores of A. vagans and M. stipoides may not encourage dispersal by wind but rather promotes establishment of seedlings in the immediate vicinity of the parent plant.