Plant Life of Southwestern Australia. Adaptations for Survival by K. Groom && Byron B. Lamont

Author:K. Groom && Byron B. Lamont
Language: eng
Format: epub
Publisher: De Gruyter Open Ltd
Published: 2015-06-15T00:00:00+00:00

9.4 Leaf-like Structures

Some genera do not produce true leaves, or they are highly reduced or modified. When the leaves are highly reduced, either the stem or the petiole takes on the photosynthetic function (Fig. 9.5). It is probably no accident that all the genera noted here contain above-average levels of nutrients in their foliage because of root adaptations to enhance nitrogen uptake (see Chapter 6) or are parasitic (Chapter 5), increasing their attractiveness to herbivores. All these modifications can be considered drought adaptations where survival is at the expense of growth rates.

1. Acacia (Fabaceae: Mimosoideae) − Adult leaves in this genus are typically either bipinnately compound (rare in SouthWest), phyllodinous, or reduced to scales. Phyllodes are expanded petioles that usually grow vertically rather than twist vertically as in true leaves. In some species (e.g. A. stenoptera) the phyllode continues down the stem to give a winged structure collectively called a phylloclade. In other species these are fasciculate (clustered or aggregated) (e.g. A. erinacea). Phyllodinous species are usually heteroblastic (Fig. 9.2), possessing compound leaves as seedlings that are soon replaced by phyllodes as the plant grows.

2. Daviesia (Fabaceae: Faboideae) − Phyllodes in this genus are either well developed, often spinescent, or are reduced to scales. When reduced to scales, branchlets (cladodes) then take over the photosynthetic function. Cladodes often end in a rigid, sharp point. Some species (e.g. D. horrida) produce phyllodes and cladodes on the same plant.

3. Jacksonia (Fabaceae: Faboideae) − Photosynthetic branchlets are flattened, angular or terete (= round in cross section) and often spinescent. Whereas adult leaves are reduced to scales, juvenile leaves are laminate and unifoliate (= a compound leaf appearing to be simple) when present. Other fabaceous genera (e.g. Bossiaea, Templetonia) also have species with reduced leaves and photosynthetic stems.

4. Allocasuarina and Casuarina (Casuarinaceae) − Outermost branchlets are the principle photosynthesising structure in this genus with the leaves reduced to whorled membranous scales. The green branchlets are sometimes called a cladode. Further, stomates are confined to grooves running along the stem (Chapter 3). The grooves close over as the stem dehydrates during drought, further restricting water loss.

5. Exocarpos and Leptomeria (Santalaceae) − Both these genera are root hemiparasites, possessing cladodes and leaves reduced to scales (in Leptomeria confined to species in Section Leptomeria of the genus).

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