Growth of Plants
Development of a plant body is accomplished through growth, defined as increase in number of cells and size of a species. Rates of growth in plants are achieved in two ways: first, by geometric increase, in which all cells of the organism divide simultaneously, especially in a young embryonic plant; second, by arithmetic increase, in which only one cell undergoes division, especially in mature plants with localized growth in a region at the root and shoot apices.
Generally, plants grow by a combination of both kinds of cell division to produce variations of form that finally develop a specific habit that is unique to a particular plant species.
Evolution of Growth Habits
The primary purpose for the evolution of different growth habits in plants is adaptation for permanent survival and reproduction of new individuals, typically under changing climatic conditions.
Water availability, especially during the growing season, is the single most important environmental factor that limits plant distribution and productivity on a global basis. Competition in the past among plants for available water, nutrients, space, and light enhanced the evolution of adaptive growth forms in plants.
Some plants developed wood as a mechanism to counter act the destructive effects of wind, ice, mechanical damage, and fire. Erect and dense growth habits evolved to resist wind effects and other mechanical damages. Plants without wood adapted prostrate, mat-forming, spreading, creeping, or climbing habits.
As animals interacted with plants and in the past, both evolved simultaneously. Various plants developed prostrate and mat-forming habits in order to endure intense grazing and trampling, or erect and tall growth forms to escape browsing and grazing.
In the past, individual plants that were able to adapt, survive, and produce more off spring were selected naturally for success. Different plants with varied growth habits colonized different habitats, becoming the dominant plants (largest or most abundant), and thereby the principal contributors in characterizing and sustaining different biomes.
The various kinds of growth habits which evolved result in a variety of forms. It is not uncommon for one species of plant to exhibit growth habits among its different varieties.
Climbing plants are also called vines. The stems trail along or coil around other plants or structures as they grow upward. Examples include cucumber (Cucumis sativus), morning glory (Ipomea species), and grape vine (Vitis species). Climbers characterize moist forests and woodlands.
Dense plants grow many small, woody canes or stems very close together in an upright fashion. The majority are shrubs. Examples include Ephedra, southern arrow wood (Viburnum dentatum), mountain laurel (Kalmia latifolia), and creosote bush (Larrea tridentata). They characterize woodlands, grasslands, coastal vegetation, and deserts.
Mat-forming plants have many stolons (creeping stems) that grow in a trail along soil or water surfaces and spread out to produce a matlike cover. Examples include the grasses Cynodon and Digitaria, Kentucky bluegrass (Poa pratensis), crab grass (Digitaria sanguinalis), the aquatic ferns Salvinia and Azolla, andmosses, such as Sphagnum. They characterize grasslands, bogs, wetlands, secondary forest floors, and cultivated habitats.
Mound-forming plants grow to form a rounded shape resembling a mound or swollen bump. Examples include the barrel cactus (Ferocactus and Echinocactus), several other species of cacti (such as Gymnocalycium), and Euphorbia gymnocalycioides. They characterize deserts, grasslands, and the tundra.
The stems of prostrate plants grow flat on the soil surface or almost touching (hugging) the ground but not trailing. Examples include the herbaceous milkpurslane weed (Euphorbia supina), common mullein (Verbascum thapsus), and some species of juniper. They are common in the tundra, grasslands, wetlands, and disturbed habitats.
Scandent plants have prominent stems in a leaning position. Examples include sugarcane (Saccharumofficinarum), coconut palm (Cocos nucifera), bearberry (Arctostaphylos uva-ursi), blackberry (Rubus fruticosus), and some bamboos (Bambusa). They characterize the dwarfed, woody trees in the timberline of the tundra, savanna, forest undergrowth, and coastal habitats.
Spreading plants exhibit a sprawling type of growth, resulting fromprofuse lateral branching in mostly woody or succulent stems. Examples include common juniper (Juniperus communis), blueberries (Vaccinium), prickly-pear cactus (Opuntia), Sumacs (Rhus), and ferns (such as Adiantum). They characterize forest undergrowth, grasslands, sandy coastal areas, deserts, cultivated lands, and some areas of the tundra.
Stemless plants have no visible stem above ground and are composed mainly of leaves or leaflike structures. Examples include common dandelion (Taraxacum officinale), Aloe vera, sisal (Agave), onion (Allium cepa), and liverworts (such as Marchantia polymorpha). They characterize aquatic and wetland vegetation, deserts, some grasslands, cultivated land, and wasteland.