Shape and Form of Plants

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Shape and Form of Plants

Plants exhibit an enormous range of shape and form. Common shapes include the conical form of conifers, the vase shape of many shrubs, the linearity of scrambling vines, and the clumped form of a daylily. Ferns have a range of forms nearly as great as flowering plants, while mosses usually take the form of miniature herbs. These plant forms result from enhanced growth in one region occurring at the expense of growth in another area. Shape results from differential growth, localized cell division, and cell expansion.

New plant cells come from single embryonic cells or groups of embryonic cells called meristems. Two groups of embryonic cells are responsible for the origin of all new shoot parts in seed plants: the shoot apical meristems and the lateral meristems. Shoot apical meristems, also called primary meristems or growing points, are found at the tips of all stems. Cells from these meristems grow primarily by elongation. The meristems may be active simultaneously, with the apical meristems of branches increasing branch length while the apical meristem of the main stem increasing plant height.

The lateral meristems, which are also called secondary meristems, produce the secondary growth or the widening of plant stems. The two lateral meristems (the vascular cambium and the cork cambium) increase the diameter of the stem. The vascular cambium is a continuous circle of cells in the stem interior and when active produces the woody portions of the stem. The cork cambium, also a continuous circle of cells, lies near the stem surface and when active produces the outer bark region. The activity of the lateral meristems is responsible for the increasing girth of plants as they age. Lateral meristems are active in plant regions where primary growth has ceased. Although increases in stem girth are associated with perennial plants, lateral meristems are also active in some annual plants (e.g., soybean) and increase their girth during the season.

Contribution of the Stem

Stems contribute to overall form in five major ways: growth direction, diameter, length between leaves, branches, and branch location.

Growth Direction.

Stems are upright in most plants (such as corn or oak), growing away from gravity, but may be prostrate, as in creeping plants (e.g., creeping devil cactus), which grow at right angles to or without respect to gravity. Creeping stems of pot-grown plants may grow beyond the pot edge, down the side of the pot, and across the table the pot is on. This type of stem growth is contact dependent. The stems are not weak, but often quite stout. Stems of other shoots are lax (e.g., ivy), unable to support themselves, and their direction of growth is related to the availability of a host plant or a substrate to provide support.

The number, location, and growth angle of the branches regulate tree form. Stems growing in different orientations are frequently found on the same plant. Christmas trees (fir or spruce, usually) have a main stem, which grows upright, and many side stems (branches) that grow at a regular angle to the stem. Many branches grow out at the same location and their orientation with respect to the stem yields a highly symmetrical, regular tree. On the other hand, branching in oaks and maples produces trees with a broad crown but a less regular form. Herbaceous plants exhibit the same features, although they are not as obvious as in conifers or large trees. The branches in these examples duplicate the architecture of the main stem, sometimes with great precision, to provide additional surface area for continued vegetative growth. Profuse branching in one plant shades out neighbor plants and limits their ability to compete for sunlight.

Branches may also be specialized for propagation and for reproduction rather than photosynthetic activity. Herbaceous plants such as strawberries have a main stem with only a few branches. Each branch extends far from the parent plant but finally touches the ground to establish a new plant. The new plant becomes independent of the parent and the linking stem can be severed with no harm to the new plant. These branches are called runners or stolons. Runners do not change the form of the parent plant, but instead duplicate the entire plant at a nearby location. The length of the runner prevents both plants from competing for the same resources, and the strategy is an effective means of vegetative propagation.

Reproduction often triggers change in plant form, commonly by enormous extension of the main axis, which might be topped by a single flower (e.g., iris, amaryllis, spring bulbs). Flowering in other species results in the outgrowth of branches from the main axis; reproductive branches may perfectly replicate or produce a slight modification of the pattern of flowers on the main stem.

Stem Diameter.

Stem diameter may be the same along its entire length, either narrow (many annuals) or broad (palms). Other plants have conical stems (the main stem of a woody perennial), which result from secondary growth occurring at the base of the stem, while primary growth occurs at the top of the stem. With each new season of growth the stem base broadens. Lastly, stems may have an obconate form (upside-down cone), broader at the top than the base. Such a stem is inherently unstable and two conditions are common. In corn, the stem has roots that grow out from lower leaf positions. These stem-borne roots act as guide wires to stabilize the plant. In other species, secondary growth from an active vascular cambium stabilizes the plant and masks the obconate form.

Length Between Leaves.

The stem length between adjacent leaves, leaf pairs, or whorls of leaves varies. When it is very short, a rosette plant is produced (e.g., strawberry, lettuce, ferns) that hugs the ground with a tight cluster of leaves. Tree ferns and palms have aerial rosettes, a series of closely spaced leaves produced each growing season. The stem is exposed as the old leaves die and fall, leaving a clump of green leaves at the top of the stem. Neither of these plants grows quickly, so tall tree ferns and palms are often more than one hundred years old. At the other extreme is papyrus, which bears a single internode topped by a cluster of leaves and associated floral branches. Sweet woodruff, a common garden plant, has stems with what appear to be whorls of leaves clustered at a single point on the stem followed by a substantial internode. When studied carefully, the whorl is a spiral of leaves with very short internodes, but a long internode separates each pseudowhorl of leaves. This is an obvious example of how differential growth yields variation in plant form.

Plants often have internodes of different length along the stem. A common pattern is short internodes at the base followed by long internodes and topped by short internodes. The diameter of these internodes changes as well, with short basal internodes having a greater diameter than the short internodes at the top. Again, differential growth regulates plant form. There may be a structural advantage to this organization: the short broad intern-odes at the bottom supporting a tall stem with short terminal internodes in the reproductive region. This organization would be advantageous in flower display to pollinators and in pollen dissemination by the wind.

Vines display an entirely different growth that is linked to their life strategy. In these plants, the internodes are long, even near the shoot tip, and leaf growth is limited. However, once the vine has made contact with a support, then leaf growth occurs. Vines put their energy into extending the stem into the light and attaching themselves to the substrate, and then the leaves expand.

Branches and Branch Location.

A branch develops from a bud located where the leaf joins the stem (the axil ). Each bud has growth potential, but some buds never grow out and sometimes only buds in particular locations extend. Differential growth of the shoot apical meristems regulates overall plant form. If the buds do not develop, the stem remains unbranched. In some instances, the terminal apical meristem prevents the outgrowth of lateral buds, but the buds grow out if the meristem is removed or damaged. Gardeners often remove the main shoot apical meristem so new buds will grow out and ultimately produce more flowers. On some plant species (such as chrysanthemum), meristem removal takes place several times during the season to create a bushy plant covered with flowers.

In other species, only buds located at the base of the stem extend as branches, which results in a shrub that with each succeeding season grows more dense. In yet other plants, like the conifers, buds at a particular location (produced near the beginning or end of a growing season) will expand to give the plant a tiered appearance. Thus, the lower tiers have longer branches than those near the top because they have grown for more seasons. This gives the Christmas tree its conical shape.

Some trees take on a candelabra appearance (e.g., buckthorn, lilac) because the apical meristem on the main axis dies at the end of the year and two or more branches grow out in its place. In the following year, the apical meristem of each branch dies and two new branches grow out in place of the old one. The death and replacement strategy creates plants with highly regular forms. The same strategy is found in Philodendron and Anthurium, common houseplants, and many orchids, although it is less obvious in these species because only a single replacement branch grows out and subsequent plant growth obscures the branching pattern. The horsechestnut (Aesculus ) also has single replacement branches.

Contribution of the Leaf

Leaves contribute to overall plant form through their size, shape, and arrangement around the stem. Leaves consist of two or three parts. Corn and leek leaves have two parts, a basal ensheathing portion and a long blade region. Geranium and oak leaves have three parts, a base region, a stalk called the petiole, and a terminal blade. The blade may be simple, an entire unit, or dissected, divided into several units called leaflets. The leaflets occur in two arrangements. When leaflets lie on opposite sides of a central axis and are terminated by a leaflet, the arrangement is featherlike or pin-nate. When all leaflets are attached to the end of the petiole, the arrangement is fanlike or palmate (like the digits of a hand). Sometimes leaflets are attached around the entire circumference of the petiole, as in lupines, or the petiole is attached to the middle of the leaf blade, as in nasturtium.

Leaves are frequently similar in shape but differ in size. For example, leaves of banana and scallion or green onions are united by shape, as are those of feather palms and many common ferns. Leaves on a single plant may also differ in shape; sometimes shape change is dramatic and other times quite subtle. These changes may be related to the life history of the plant or result from a change in the direction of growth.

Leaves are present either in spirals (one leaf at each stem position) or in whorls (two or more leaves at each stem position). The most obvious spiral leaf arrangement is shown by Costus, a member of the ginger family, in which single leaves are arranged in an ascending spiral around the stem. Corn, which has leaves present in two vertical rows along the stem, also has a spiral arrangement. One can demonstrate a plant's spiral nature by winding a string from one leaf position to the next higher leaf position. In a few plants, a spiral of leaves will have little space between each leaf, making them appear whorled, and a large gap before the next spiral of leaves. These are pseudo-whorled arrangements and found in select species of Impatiens and Peperomia and in a common garden plant, sweet woodruff.

The simplest form of whorled leaf arrangement is that of two leaves at a common stem position, seen in sunflowers, plants with opposite leaf arrangements. In members of the mint family, which includes garden mints and the common houseplant coleus, leaves originate in pairs, but each successive pair is offset 90 degrees from the previous pair. There are also plants with whorls of three leaves, such as oleander, where the succeeding set of leaves is offset from the preceding set. Looking down the length of the stem, there are six leaf positions, but only three positions are filled by each individual whorl.

see also Anatomy of Plants; Meristems; Phyllotaxis; Tree Architecture.

Judith Croxdale