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Grouping Plants by Form and Function
Plants are grouped together according to their form or function in a variety of ways. One of the most obvious takes into account the overall form of the plant. Habit is one way of discussing overall form.
Habit
Gardeners traditionally have grouped plants into broad categories on the basis of their overall appearance, or habit. The following is a list of the more common habits of plants:
- Trees: Woody plants with a single or few trunks near the base.
- Shrubs: Woody plants that branch near the base and therefore have several trunks.
- Lianas: Woody plants that climb on other plants and use them for support. They may climb via tendrils, hooks, pads, aerial roots, or other mechanisms.
- Herbs: Non-woody plants. There are several types of herbs, including:
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- Forbs: Broad-leaved herbs.
- Graminoids: Grass-like herbs, with very narrow leaves.
- Ferns: Herbs with broad but highly dissected leaves and no flowers.
- Herbaceous vines: Non-woody plants that climb on other plants.
Raunkiaer's Classification System
The famous Danish botanist Raunkiaer noted that the traditional classification of plants into trees, shrubs, herbs, and other categories based on habit does not take into account much of the ecology or lifestyle of the plants. He devised an alternative system, based on lifestyle and the position of buds (the points on a stem from which new shoots grow in the spring), with the following categories:
- Phanerophyte: Woody or herbaceous evergreen perennial, taller than 20 inches, whose shoots do not die back. In other words, these plants are trees and large shrubs.
- Chamaephyte: Woody or herbaceous evergreen perennial from 10 to 20 inches tall or whose shoots die back periodically. These plants are small shrubs covered by snow in the winter.
- Hemicryptophyte: Perennial (or biennial) herbaceous plant in which the stems die back to a remnant shoot system that lies on the ground. These are herbaceous plants with runners along the ground.
- Geophyte: Perennial (or biennial) herbaceous plant in which the stems die back to a remnant shoot system with storage organs that are imbedded in the soil. These are the plants gardeners call bulbs (including corms, rhizomes, and tubers as well as true bulbs).
- Therophyte: Annual; or plant that dies after seed production and completes its entire life cycle within one year.
- Liana: Plant that germinates on the ground and maintains soil contact while using another plant for support. Grape vines are typical lianas.
- Hemi-epiphyte: Plant that germinates on other plants and then establishes soil contact; or plant that germinates on the ground but later loses contact with the soil. Hemi-epiphytes use other plants for support, at least during part of their life. The strangler fig is an interesting hemi-epiphyte: it starts life as an epiphyte, growing on other trees; as it matures it sends down roots to the ground, and slowly, over many years, the fig encloses and "strangles" the tree that was its nursery.
- Epiphyte: Plant that germinates and roots on other plants, never coming in contact with the soil. Many orchids and bromeliads are epiphytes.
- Errant vascular hydrophyte: Free-moving water plant. These are the floating aquatic plants.
- Vascular semi-parasite: Green plant growing attached to other living, green plants. Many plants, such as eastern North American native gerardia (Agalinis purpurea), photosynthesize but also supplement their nutrients by parasitizing other plants.
- Vascular parasite: Non-green plant growing on living, green plants. Indian-pipe (Monotropa uniflora) is a good example of a vascular parasite.
Mode of Nutrition
Plants can be classified according to how they get their nutrition.
- Heterotrophic: Heterotrophic plants depend on other organisms for nourishment. Vascular parasites, such as Indian-pipe (Monotropa uniflora), fit into this category. Vascular semi-parasites, such as gerardia (Agalinis purpurea), are dependent to some extent on other organisms but also photosynthesize.
- Autotrophic: Autotrophic plants produce their own nourishment via photosynthesis. The vast majority of plants fall into this category; only vascular parasites and vascular saprophytes do not produce any of their own nutrients.
Plant Longevity
Another way of looking at plants is based on longevity -- how many years the plant lives.
- Annual: A plant that completes its life cycle within a single year.
- Biennial: A plant that completes its life cycle in two years.
- Perennial: A plant that lives more than two years.
Life Stages
Within broad categories such as annual and perennial, individual plants exhibit a variety of forms and functions during their life span. These stages in the life of a plant can be classified as follows:
- Seed: A period in the life of a plant in which no growth is taking place.
- Pre-reproductive: The period in the life of a plant after germination but before reproduction. This phase can be separated into several stages: seedling, juvenile, and virginile.
- Seedling: A young plant that is partly dependent on substances of the maternal plant stored in the seed (especially the first leaves, called cotyledons). The plant always has embryological structures, including cotyledons, a primary root (the embryological root), and a primary shoot (the embryological stem).
- Juvenile: Juvenile plants are structurally simple but have progressed beyond the seedling stage. They no longer have cotyledons and often exhibit the mature forms of leaves, shoots, and roots. A tree sapling is an example of a juvenile plant.
- Virginile: Virginile plants begin to show the main features of mature plants but are not reproductive. In other words, these are mature plants that have not started to flower.
- Reproductive: A mature plant that forms flowers and fruits. This long period can be separated into three sub-categories: young, mature, and old.
- Young: Young reproductive plants exhibit more new growth than death of old parts.
- Mature: Mature plants exhibit a balance between growth and death of parts. These plants usually have the greatest yearly seed production and biomass increase (increase in weight). In other words, they are at their peak.
- Old: In old plants, the death of parts prevails over the production of new parts. Reproductive activity is diminished.
- Post-reproductive: In a post-reproductive plant, flowering and fruit production practically stops, and overall vegetative structure becomes simplified. Juvenile leaves reappear and the plant's growth rate decreases.
- Senile: A plant that has reached its final stage before death.
Growth, Architecture, and Branching
Plants are often grouped according to their branching structure. The growth and branching of plants has been compared to foraging behavior in animals. In other words, plants grow in ways that enable them to seek out the best food and habitat. Several aspects of growth are particularly notable:
- Phototropic: Growing towards light. Since light is the major source of energy for plants, they often grow toward light to increase their light-gathering ability. Some plants, such as the tulip tree (Liriodendron tulipifera), however, grow away from gravity rather than towards light. Because the tulip tree does not bend towards light, its stem is very straight.
- Geotropic: Growing towards gravity. Roots often grow geotropically to reach the water table.
- Plagiotropic: Characterized by horizontal growth. The lateral branches of a tree are often plagiotropic. Pin oaks (Quercus palustris) have plagiotropic mid branches; in fact, the lower branches decline at a slight angle, a characteristic that can be used to identify the species.
- Orthotropic: Characterized by vertical growth. The main shoot of a tree is often orthotropic. Many of the upper branches of the American elm (Ulmus americana) are orthotropic, giving the tree its characteristic vase shape.
- Dichotomous: Characterized by a type of branching in which the stem tip divides into two equal parts, as in highbush blueberry (Vaccinium corymbosum).