The leaf, consisting of the leaf blade (lamina) and stalk (petiole), carries out photosynthesis, its shape and arrangement on the stem depend on the water and light energy supply in the species' habitat. The arrangement of leaves and examples in different species, along with the major differences between monocotyledons and dicotyledons, is described in Chapter 4. Leaf structure, as an organ of photosynthesis, is described in Chapter 8 (see p117).
The features of typical plants are given, but there are many variations on the basic form of the stem, root and leaves.
Adaptations to plant organs have enabled plants to compete and survive in their habitat. Plants adapted to dry areas (xerophytes), such as cacti, have leaves reduced to protective spines and stems capable of photosynthesis. Thorns, which are modified branches growing from axillary buds, also have a protective function, e.g. hawthorn (Crataegus). Prickles are specialized outgrowths of the stem epidermis, which not only protect but also assist the plant in scrambling over other vegetation, as in wild roses. Several species possess leaves modified specifically for climbing in the form of tendrils, as in many members of the Leguminosae family, and Clematis climb by means of a sensitive, elongated leaf stalk, which twists round their support. In runner beans, honeysuckle (Lonicera) and Wisteria, twining stems wind around other uprights for support. Others are able to climb with the help of adventitious roots such as ivies, and Virginia Creeper (Parthenocissus). Epiphytes are physically attached to aerial parts of other plants for support; they absorb and sometimes store water in aerial roots, as in some orchids.
To survive an environment with very low nutrient levels, such as the sphagnum peat moor (see p328), some plants have evolved methods of trapping insects and utilizing the soluble products of their decomposed prey. These insectivorous plants include the native sundew (Drosera) and butterwort (Pinguicula spp.), which trap their prey with sticky glands on their leaves. Pitcher plants (Sarracenia spp.) have leaves that
Figure 5.6 Variations in structure of plant parts as adaptations to modes of growth: (a) thorns of Berberisand Pyracantha; (b) prickles on the stem of rose, Rosa sericea pteracantha grown as an ornamental for its large red thorns; (c) tendrils of passion flower; (d) elongated leaf stalks of Clematis ; (e) adventitious root formed on stems of ivy (Hedera helix); (f) twining stems of Wisteria
Figure 5.6 Variations in structure of plant parts as adaptations to modes of growth: (a) thorns of Berberisand Pyracantha; (b) prickles on the stem of rose, Rosa sericea pteracantha grown as an ornamental for its large red thorns; (c) tendrils of passion flower; (d) elongated leaf stalks of Clematis ; (e) adventitious root formed on stems of ivy (Hedera helix); (f) twining stems of Wisteria form containers which insects are able to enter into, but are prevented from escaping by slippery surfaces or barriers of stiff hairs. The Venus flytrap (Dionaea muscpula) has leaves that are hinged so that they can snap shut on their prey when it alights on one of the trigger hairs.
Plants found growing in coastal areas have adaptations that allow them to withstand high salt levels, e.g. salt glands as found in the cord grass (Spartina spp.) or succulent tissues in 'scurvy grass' (Cochlearia), both inhabitants of coastal areas.
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