Phylloclades of Jacksonia (Fabaceae)—leaf-like branches as adaptation to seasonally arid environments

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Abstract
Leaves of seed plants were evolutionarily derived through syngenesis (fusion) of the photosynthetic cylindrical axes of the earliest land plants and subsequent morphological diversification. However, in some later evolved taxa leaves became very reduced or entirely lost and photosynthesis was again restricted to stems. Reduction of photosynthetic area to stems is mostly found in plants from arid environments and is generally considered disadvantageous in competition for light with plants with leaves but may be useful if water is limiting. For taxa that cannot form normal leaves on adult plants, increasing photosynthetic area is only possible by modification of other plant parts. Some taxa produce leaf-like phylloclades that are developmentally different from leaves. We investigated Jacksonia floribunda and J. anthoclada (Fabaceae) leaves and phylloclades. In all Jacksonia species true leaves are only developed in the earliest ontogenetic stages, and subsequently are reduced to minor, nonphotosynthetic brownish scales. After several nodes on the seedling, photosynthetic phylloclades, each inserted in the axil of a scale, form the foliage. Immature phylloclades have vestigial nonphotosynthetic leaves borne on small projections from the edge of the blade. These soon abscise. The phylloclades are flattened branches and when mature have a distinctly reticulate venation and a sinuous margin with alternating mucronate tips where the vestigial leaves were attached. Jacksonia species demonstrate a transformational series where in most species foliage is reduced to branchlets. In a few others branchlets are winged forming cladodes or are condensed and laterally expanded to form phylloclades. Our findings on the more derived species in Jacksonia illustrate the complexity of plant morphological responses to evolutionary pressures of seasonal water limitation.

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