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The 2 types of fruit (aerial and subterranean) borne by the dwarf desert annual Gymnarrhena micrantha were compared with regard to their responses to factors affecting their formation, dispersal, germination and seedling mortality. The 2 types of fruit differed markedly in several respects. In comparison with the subterranean fruits, the aerial ones are much smaller and more numerous, but the formation of the inflorescence in which they develop is more dependent on a favorable supply of soil moisture. The aerial fruits are dispersed by wind, after becoming detached by a complex series of hygroscopic movements which involve several organs and tissues, while the subterranean fruits never leave the dead parent plant, germinating right through its tissues. Germination of the subterranean fruits starts after a shorter incubation period and is less temperature‐dependent in both light and dark. Light stimulated germination of both types of fruit, increasing their germination rates and final percentages, but not affecting the duration of the incubation period. In the subterranean fruits, the rate of germination was equally stimulated by light over the entire temperature range, with a well‐defined optimum at 15 C in both light and dark. In the aerial fruits, the same optimum was found only in the light, rates in darkness increasing with decreasing temperatures. In the aerial fruits, alternations of light and dark were more favorable to germination than either continuous light or dark, the full effect being obtained with a single 8‐hr or 16‐hr light period, provided it was preceded by 16 or 8 hr of darkness, respectively. Similar reactions to combinations of light and dark were not observed in the subterranean fruits. Seedlings developing from the subterranean fruits were much larger, but grew at a relatively much slower rate than those from aerial fruits. The former were distinctly more tolerant of unfavorable soil‐moisture regimes, such as low moisture supply and drought. It was concluded that the 2 types of fruit serve 2 distinct functions in the biology of the plant. The aerial fruits are adapted to the function of increasing the distribution of the species within suitable habitats, while the subterranean fruits are adapted to increasing the probability of the survival of the species.

Abstract. Mature leaves of many plants re‐orientate their laminae photonastically in response to non‐directional light signals, and/or phototropically in response to directional light signals, by flexing of pulvini, most commonly subtending their bases. Physiological and structural specializations of the pulvinus enable it to flex, by rapidly undergoing differential and repeatedly reversible axial volume changes (expansion/contraction) in opposite sectors of its motor tissue. Light‐driven leaf movements are adaptations that contribute to the efficiency of the photosynthetic apparatus in the leaf. The phototropic response maximizes the harvesting of photosynthetically radiant energy. The photonastic response to dark‐to‐light transitions increases the interception of light by unfolding the lamina. Another photonastic response modulates the interception of radiant energy by the lamina, allowing it to evade damage by light in excess of its photosynthetic capacity when the leaf is under stress. The same unidentified blue‐absorbing pigment system appears to be involved in all these responses. Non‐directional light signals are perceived in the pulvinus. Perception of directional light signals may be localized in other parts of the leaf in different plants: for example, the pulvinus in most leguminous species, and the lamina in malvaceous and at least one leguminous species. The perception of non‐directional and directional light signals, their transduction to differential volume changes in the target cells, and their transmission between the two where the sites are separate, are discussed.

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