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

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Abstract
The relationship between the morphology and the role of the hygroscopically-awned diaspores of 6 spp. of grasses [Danthonia tenuior, Dichanthium sericeum, Dichelachne micrantha, Heteropogon contortus, Schizachyrium fragile and Stipa verticillata] is investigated. There is little experimental evidence to substantiate the frequently-implied view that the hygroscopically-active awn serves to drill the diaspore into an unbroken soil surface. The hygroscopic awn causes the diaspore to move across the soil surface and to become lodged in a microsite in an attitude that promotes germination; in some species a rigid passive awn interacts mechanically with the hygroscopically-active awn to facilitate this locomotion. There is no evidence that long hairs on the body of the diaspore and a hard pointed base bearing backwardly-directed bristles assist in the movement of awned diaspores into microsites. The hygroscopic awn has an independent influence on establishment by pushing the diaspore firmly into the microsite. The backwardly-directed bristles on the base anchor the diaspore and counter the force of the radicle pushing into the soil. Species vary in their ability to locate different types of microsite, and these variations can be related to the morphology of their diaspores. While 1 soil may promote more germination than another, the proportion of seedlings that becomes established may not necessarily differ on the 2 soils. The precise pattern of behaviour with respect to germination and establishment on 4 different soil surfaces varied amongst species. Differences in the morphology of diaspores of grasses may be advantageous in reducing the competition for similar microsites among species living together in one community. The morphology of the diaspore shows a compromise between the conflicting demands of a range of selection pressures associated with its various roles (e.g., in location of microsites, germination and establishment).