Here, a brief reminder is given of the principles, with some informa-tion on the mechanical attributes of some adaptations. In leaves of most land plants, the xylem has relatively little mechanical strength, and in most species the vascular bundles are accompanied by bundle sheaths or rod-like arrangements of fibres, sclerified parenchyma or collenchyma. The me-chanical tissue may be accommodated within the thickness of the leaf, so that both surfaces are smooth, or it may produce prominent ridges above or below or both. The arrangement of veins in leaves is very varied (see Chap-ter 6). In the monocotyledons and some dicotyledons, many species have strap-shaped leaves, with the main veins parallel to one another. Grasses are typical examples. The centrally placed vein may be the largest. The axial veins are connected at intervals by transverse veins, which are in general narrower than most of those of the axial system. This produces a net-like arrangement, with the softer, non-load-bearing green tissue sus-pended between the veins.
Some monocotyledons, such as many aroids, are more like the broad-leaved dicotyledons in leaf form. They have a petiole-like structure and an expanded flattened lamina. The veins in the lamina generally consist of a midrib which follows directly in line with the veins of the petiole, and a se-ries of lateral veins. The first order lateral veins may depart from the midrib in a regular pinnate manner up its length, fairly evenly spaced, and extend-ing towards the leaf margins on either side. They might all originate near the base of the lamina, and fan out towards the margins, as three, five or more branches. In some leaves, for example Gunnera and rhubarb, the rib-bing is very pronounced, and parallels can be seen with fan vaulting in buildings. Cantilevers are common in nature. Some leaves are very large, and their strengthened, ribbed venation provides mechanical support with economy of materials, cantilevering the relatively delicate green tissue out into the light.
The arrangement of mechanical tissue in petioles is of considerable in-terest. Leaf arrangement on stems usually helps minimize self-shading, but the fine tuning is carried out by the petiole. In some plants, the petiole may assist the lamina in tracking the sun. The petiole also enables the lamina to twist and partly rotate in the wind, minimizing the damaging effects of wind on the thin, sail-like structure. The design is such that the elasticity in the system allows the lamina to return to its preferred orientation for light interception, when the wind falls. This remarkable self-righting flexibility comes about through the structure and properties of the mechanical tissue. The cross section can be U-shaped (like plastic guttering, which has similar recovery properties to twisting), with some variants, or it may consist of a cylinder, with thicker and thinner parts, or a cylinder with additional rods either internally, externally, or both. It is not so clear how those with closed cylinders function. All petioles have to be able to withstand vertical loading well. Clearly all types also withstand torsional forces, because they work.
At the petiole base and sometimes at the petiolule bases of leaflets, there may be an enlarged pulvinus which contains parenchymatous cells which when turgid hold the leaf up. When the internal pressure in these cells is re-duced, the leaf and leaflets may hang down in a ‘sleep’ position. Wilting in such plants can have a similar effect, which causes the leaf and leaflets to present a reduced surface area to the sun.
In feathery, pinnate leaves, and other forms with several leaflets, wind can be spilled by movement of the individual components. Many palms have large leaves, which when immature are entire. During expansion, predetermined lines of weakness in the lamina split, and the mature leaf gives the appearance of being pinnate. In addition to strength provided by reinforced veins, palm leaves often contain fibre strands, and possessthick-walled epidermal cells. Coconut palm is a good example. This passive approach to survival is effective. The leaves may become more tattered, but they generally survive to function.
Thin leaves, with vascular bundles arranged in one row, often more or less equidistant from either surface, are often accompanied ad- and ab-axially, by mechanical girders or strands. This makes them similar to I beams and girders in buildings and other structures. In manufactured I beams, the upper and lower flanges are more robust than the upright central part between them. Indeed, it is common to find holes to have been cut in the central part or for it to take on a lattice-like form, saving materials where they are not needed for mechanical strength. Not uncommonly in nature, the real strength is supplied in the ‘flanges’, and the tissues between them serve mechanically only to hold the flanges in their respective spatial ar-rangement. Commonly soft, turgid parenchyma cells may be found in such parts of the ‘beam’, but sometimes there may even be air spaces. Here we see an excellent parallel in the economy of use of materials in plants, and effi-ciency in engineering.
Tubes or cylinders have been described above for stems, and as a developmental stage as the stem ages or increases in diameter. It is common for the mechanical tissue to be concentrated towards the periphery, where it can provide considerable mechanical support, with an economy of material. Hollow trees are often a cause for concern, but providing the outer tissues are alive and flourishing, and suffi-cient wood remains at the points of branch insertion to hold them up, there may be little to be worried about. Tubes are commonly applied in engineer-ing because of their particularly efficient use of material in relation to strength.
Climbers like Vitis vinifera, the grape vine, and Clematis species, often re-tain separate vascular strands, even though these become radially elongated as the stem thickens with age. Between these are radial bands of thin-walled cells. When the stem becomes compressed as it climbs, the thin-walled re-gions are deformed, but the main conducting cells in the vascular tissue do not compress and can continue to function effectively.