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Oregonian"The book, studded with stunning photographs, is divided into nine botanically based but very readable chapters."
—Lynne Terry, Oregonian, May 5, 2005
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—Lynne Terry, Oregonian, May 5, 2005
—Science News, May 21, 2005
In vascular plants, water moves from the roots to the leaves through special, narrow and vertically elongated cells in the xylem tissue. These hard-walled xylem cells are dead and devoid of living contents at maturity. In one type of water-conducting cell, found in all vascular plants, the water moves from cell to cell through thin spots in the cell walls. Some xylem cells, the vessel elements, are joined end to end and have connecting walls with one or more holes in them, so each series of cells functions as a single pipe known as a vessel. This is believed to provide a more efficient means of water transport because the water meets less resistance moving through the open end walls; some water also moves laterally through thin spots joining cells. The evolution of vessels is considered to be one of the many reasons flowering plants have been so successful in terms of their diversity. Conifers lack vessels, but they still manage to transport water to considerable heights. Some of the tallest trees in the world are conifers, including the tallest of all, the coast redwood (Sequoia sempervirens) of California and southwestern Oregon.
The sugar-conducting phloem is in a zone to the outside of the xylem. It, too, has conducting cells that are elongate and joined in linear series, but they are soft-walled and remain alive. Their usually transverse end walls are perforated like a sieve, and continuous strands of protoplasm, through which sugar in solution is conveyed, pass through the holes from cell to cell in each series.
Between the xylem and phloem in most trees and shrubs there is a layer of thin-walled, delicate cells known as the cambium. Cambial cells are able to divide, forming new xylem inwardly and new phloem outwardly throughout the life of the plant. In temperate regions a new layer of wood (xylem) and a thinner layer of phloem is formed each spring. Each layer of wood is known as an annual ring, and the rings can be counted to determine the age of a tree.
Some vines, particularly in tropical forests, have stout, woody, cable-like stems, though the quantity of water-conducting wood (xylem) is much less than that of trees of comparable height. These woody vines are often known as lianes. Vines are most abundant in tropical forests but are also present in temperate regions. Among woody vines native to Europe, ivy (Hedera helix) is widespread, as are species of Clematis and honeysuckle (Lonicera). The warmer Mediterranean area is the home of the grape vine (Vitis vinifera) and species of Dutchman's-pipe (Aristolochia) and greenbrier (Smilax). There are also nonwoody vines. Some are small annuals that climb on shrubs or perennial herbs, such as the wild pea and its relatives (Pisum); others are perennials, such as bindweed (Convolvulus), which dies down in winter, and periwinkle (Vinca), which is evergreen. Temperate North America shares a number of these genera, but there are others of tropical affinity, particularly in the warmer Southeast.
Most vines, though not all, need full light to flourish, so they are most abundant and often grow rampantly in damaged or regenerating forest. As the tree canopy gradually closes, the vines diminish as many of them cannot regenerate in shade. In a tall, mature tropical forest there may be few vines apart from the more shade-tolerant, mostly subcanopy species. In some cases, vines are found in localized clusters in these tall forests. These mark the sites of former gaps in the canopy, resulting from the collapse of large, old trees.
Some of the larger woody vines, when they have reached the canopy, continue to climb, or rather spread, from tree crown to tree crown. In fact, the total leaf area of such a vine can be at least as great as that of its supporting tree, if not several times greater. This is supported by studies in Venezuela, where lianes were found to make up 4.5% of the total above-ground biomass of the forest but 19% of its total leaf area, and Gabon, where 33–39% of the leaf litter in forests comes from vines (Putz and Mooney 1991, 127).
So to supply its extensive foliage with water, a vine clearly must have a more efficient system of transport through the limited amount of water-conducting tissue in its slender stems than a tree with the more extensive xylem in its large trunk. To find why this is so, we need to examine the cellular structure of the xylem of vine stems.
When a thin transverse section of a vine stem is examined microscopically, the most striking thing in many cases is the wide diameters of the vessels. Experiments with a woody vine of the pea family (Fabaceae) and an herbaceous vine of the pumpkin family (Cucurbitaceae) show that vines have a greater water-transporting capacity than trees. Water transport in the two vines was shown to be 60–120 times more efficient than in many trees (Putz and Mooney 1991, 150). Xylem vessels of vines can also be of considerable length, from 0.5 to 7 m (19 inches to 23 feet).
There is another way in which the stem anatomy of woody vines differs from that of trees. In trees, the wood or xylem, of which only the newest and outermost annual ring actually conducts water, is in the form of a solid cylinder whose rigidity is able to support large crowns of leaves and branches. Vines need to be more flexible to cope with the twists and turns of climbing or the stresses that result when they partly or completely slip away from their supports. Woody vines achieve flexibility by having a considerable amount of soft tissue as well as wood in their stems. In some, the cylinder of wood is divided into segments that alternate with soft tissue; in others, there are alternating cylinders of wood and soft tissue. Some woody vines also have flattened, ribbon-like stems to achieve greater flexibility. These are just a few of a number of patterns.
How do vines climb? They do so in a variety of ways. The scramblers are the least specialized with their weak, slender stems trailing through the foliage of shrubs and young trees. Those woody species that do not have a way to anchor their stems do not attain any great height because as their weight increases they tend to slip from their supports. In a number of species of oleaster (Elaeagnus), some anchorage is provided by short, stiff, backward-angled side branches. Elaeagnus pungens is sometimes grown as a hedge, but if not controlled it can quickly develop into an impenetrable thicket.
Many tropical species of Gnetum climb in a similar way by means of short, stiff side shoots. Gnetum belongs to an unusual, small group of plants (the Gnetales) that has some similarities to flowering seed plants but in fact are cone-bearing seed plants, along with conifers and cycads. The net-veined leaves of this genus look very similar to those of dicotyledonous flowering plants. Stems of climbing gnetums become large and woody and can attain considerable heights in the forest.
More specialized scramblers are beset with backward-curved prickles on their stems and leaves. The genus Rubus is conspicuous. In northern temperate regions the blackberry and its relatives mostly form scrambling thickets on the ground, but in the tropics and southern temperate latitudes some Rubus species are high-climbing lianes with stout stems. They get established in the low growth at forest margins or in canopy gaps, spread from support to support, anchored by their hooks, and keep pace with the growth of trees so that eventually their foliage spreads widely in the forest canopy.
The most spectacular of the scrambling vines are the rattan palms of the African and Asian tropics, although their anchorage is provided by backward-angled spinelike prickles. The tips of their compound leaves or, in some species, modified inflorescences (clusters of flowers) lengthen into barbed whips as long as 5 m (16 feet). The whips become almost thread-like toward the tips and can be quite a hazard to anyone pushing through the undergrowth. The prickles can hook into hair, nostrils, and ears, and it is unwise to try to pull free. Rather, one should stop and carefully disentangle. In Queensland, Australia, these climbing palms have the appropriate common name wait-a-while. Firmly anchored by the whips, the stems of rattans elongate rapidly, sometimes as much as 5 m (16 feet) a year, eventually reaching the forest canopy, where they form crowns of leaves in the full light. The relatively slender cane-like stems can be remarkably long, with a record of 171 m (561 feet). In Southeast Asia these slender but tough stems are used for making cane furniture and other items and are an important source of income.
The largest group of both woody and nonwoody vines are the stem twiners. In these, the stems of seedlings, or sometimes the shoots from spreading or fallen stems of already established plants, are initially self-supporting up to an average height in tropical forests of 30–40 cm (12–16 inches) and a maximum height of 2 m (6.5 feet). If no support is encountered, the shoot curves down to the ground, grows along it for a while, then turns up at the end to begin searching again. To increase the chances of encountering a support the stem tips undergo a spiraling movement that completes a circuit from a few centimeters or an inch or more to more than 1 m (39 inches) in diameter every hour or two. It is thought these circling movements result from alternating expansion and contraction of cells on opposite sides of the stems.
When a circling stem comes near a support it begins to swing closer and closer until contact is made. If the support is less than 30 cm (12 inches) in diameter, as a general rule, the vine will twine around it and grow rapidly upward. At the earlier freestanding stage, the vine stems have, like the seedling trees, a solid cylinder of wood. When climbing begins there can be quite an abrupt change to a more flexible form of stem structure.
In many species of stem twiners the leaves are reduced or absent during the rapid climbing phase. When the well-lit forest canopy is reached, lateral, generally nonclimbing branches form, bearing fully functional leaves and eventually flowers and fruits.
As already noted, some twining vine stems do not derive directly from seedlings but from a range of prostrate stems both above and below ground, and sometimes from roots. Such later-formed climbing stems have the advantage that they can get their energy for growth from food reserves in the structures that bear them. These food reserves derive from the canopy foliage of older stems in the same cluster.
Stems also arise directly from seedlings, but this is a much slower process as the only food reserves at first are those stored in the seed. The general sequence of events following germination can be illustrated by the closely studied New Zealand monocotyledonous vine Ripogonum scandens (Macmillan 1973). This twining vine is common in some New Zealand forests, where its often almost black, cane-like stems sometimes form impenetrable thickets. The seedling stem is about 1 mm (1/32 inch) in diameter and grows to a height of 6–9 cm (2.5–3.5 inches), when the apex aborts. Most of the nodes (stem joints where one or more leaves are attached) bear pairs of small dry scales, but adjacent to the aborted apex, one broad green leaf forms. Later in this first season a short branch develops near the tip and forms a few more leaves. In the second season a branch develops at the base of the stem below the ground, forms a short horizontal stem (rhizome), then turns up at the end to form an upright stem similar to the first though it is about 2 mm (1/16 inch) in diameter and attains a height of 15–20 cm (6–8 inches) before a few leaves appear and its apex aborts. In the third season a new stem forms at the base of the second, first forming a somewhat swollen and tuberous rhizome segment before turning up into another but stouter stem, with a few leaves at the top, that can reach a height 50 cm (19 inches) or more before growth ceases. The first three seasons' growth now resembles a small shrub. None of the stems exhibits any tendency to twine. Despite the dim light, the chlorophyll in the leaves and stems provides sufficient food reserves to support a more vigorous upright stem in the fourth season. This bears pairs of scales in place of leaves, and at about 50 cm (19 inches) the apex begins to make spiraling movements. If a support is encountered, the stem twines about it, and at about 2 m (6.5 feet) above the ground the tip aborts and a few side shoots with a few leaves are formed.
In subsequent years, new twining stems attain the maximum diameter of 1.5–2 cm (5/8–3/4 inch).At this stage the tips look very like Asparagus except for the dark, sometimes black, coloration that masks the green of the chlorophyll. These stems are now able to reach the forest canopy where, in the full light, lateral, nontwining, wiry stems are produced. These may also branch several times and bear two rows of quite large leaves, and eventually flowers and berries. Sometimes the twining stems on a sapling can extend into the crown of a canopy tree when the sapling grows into the latter's lower branches, and sometimes newer stems twine around older stems to form multistranded cables 10 cm (4 inches) or more in diameter.
About 95% of twining vines twine clockwise. Most of the rest twine counterclockwise, with a few switching from one mode to the other. Sometimes one encounters a tree whose trunk bears a helical scar caused by a twining vine when the tree was young. As the young trunk steadily increased in diameter, it stretched the vine until it broke.
Most climbing ferns have roots that attach to their support, but species of the mostly tropical genus Lygodium twine and can extend into the forest canopy. The climbing organ here is strictly a frond with compound leaflets, though the fact that it has indefinite growth is usually characteristic of a stem rather than a leaf. The true stems of the fern are relatively short rhizomes growing close to the ground.
|Ch. 1||The freeloaders - plants using plants||13|
|Ch. 2||Not enough water - the plants of deserts and seasonally arid places||67|
|Ch. 3||Rising from the ashes - plants and fire||125|
|Ch. 4||Serpentine and salt - coping with toxic soils||143|
|Ch. 5||Too much water - plants of rivers, lakes, swamps, and margins of the sea||155|
|Ch. 6||Too cold for trees - mountain and arctic plants||179|
|Ch. 7||A love-hate relationship - plants and animals||207|
|Ch. 8||Mostly hidden relationships - plants, fungi, and bacteria||267|
|Ch. 9||Plant evolution through the ages - an overview||279|