8 Plant Growth
- Identify common classifications of plant growth.
Water movement is the most basic classification of plant growth. All plants need water to survive and, based on how they move water, are categorized as either vascular or non-vascular. Vascular plants, such as trees, have a water conducting system, allowing them to supply aerial tissues with water and to grow well above the ground. Non-vascular plants rely on their closeness to water and their own physical absorption to support green tissues above ground. Mosses and liverworts are examples of non-vascular plants.
Reproduction is another classification of vascular plants that is based on whether they reproduce themselves asexually or sexually. Vascular plants are subdivided into two major categories, pteridophytes, and spermatophytes. Pteridophytes (Greek for “fern plant”) include ferns and horsetails that reproduce asexually by spores. Spermatophytes (Greek for “seed plant”) include conifers and flowering plants that reproduce sexually by seed.
Conifers, from the Latin for “cone-bearing” are woody plants that bear their female and male reproductive structures in separate cones or strobili rather than in flowers. Coniferous trees and shrubs typically bear both female and male cones on the same plant. Pollen produced by male cones is transported on wind currents to the female cones wherein seed development is completed. Conifers belong to the group of seed producing plants called gymnosperms. Gymnosperm literally means ‘naked seed’ as seeds are held on the surface of a cone scale or at the end of a small structure. This is the main differentiation between conifers and the flowering plants (angiosperms) which bear their seeds in an enclosed ovary of a flower that becomes the fruit.
Angiosperms are the largest and most diverse group in the plant kingdom. Some angiosperms produce flowers and fruit over many years (polycarpic), while some die after flowering and bearing fruit only once (monocarpic). In addition to the presence of flowers and fruit, angiosperms are classified according to two major groups of plants that are each derived from a common ancestor species (clade), the monocotyledons and eudicotyledons. This classification is based on the number of cotyledons or seed leaves produced at seed germination. Monocotyledons (meaning “single seed-leaf”) include grasses, lilies, orchids, and palms. They develop from a seed with a single seed leaf. Some basic recognizable patterns of monocotyledons include leaf veins arranged in parallel lines; flower parts numbered in 3’s and a herbaceous plant structure. Eudicotyledons (meaning “true dicots”), are an evolutionary line that includes plants such as maples, oaks, roses, buttercups, mints, and sunflowers that develop from seed usually with a pair of seed leaves. Some basic recognizable patterns of dicotyledonous plants include leaves with netted venation; flower parts numbered in 4’s or 5’s and woody or herbaceous plant structure.
Type of growth, such as tree versus shrub or woody versus non-woody (herbaceous), is often the first visual recognition of a plant. Plant growth may also be categorized by some aspect of their biology or ecology such as: terrestrial or aquatic habitat (e.g., duckweed), adaptations such as twining stems for climbing (wisteria) or underground storage bulbs (e.g., daffodil), or whether they exhibit seasonal loss of leaves (deciduous) or if they remain evergreen.
Plant growth varies from trees with well-defined trunks, to multi-stemmed shrubs and climbers to spreading ground covers and clumping herbaceous plants. The above ground plant structure is typically formed by stems that are either woody or herbaceous. Woody plants such as cedars, oaks, and maples produce more or less permanent structures capable of extension and annual thickening (secondary growth). Non-woody or herbaceous plants such as dandelion, (eudicot) and grasses (monocot), and ferns, (pteridophyte) are limited to only extension growth and do not produce permanent above ground structures.
The herbaceous growth habit is common among vascular plants, and many specific plant groups are distinguished on that basis. Herbaceous plants are characterized by a lack of woody tissue, such as bark. Their stems will eventually die back to a live root crown and root structures. Deciduous herbaceous perennials wither and die back to some kind of long-lived, resistant organ (a fleshy crown, bulb, tuber, rhizome, etc.) and enter a state of dormancy when conditions are not suitable for continued growth. In comparison, evergreen herbaceous plants have leaves that persist over one or more seasons of growth.
Not all herbaceous plants are seed plants; spore producing plants such as ferns and horsetails are also considered herbaceous. The ability of some perennial plants to propagate themselves non-sexually by means of vegetative reproductive structures such as underground creeping stems (rhizomes) and tubers and bulbs is a competitive advantage over sexually reproduced plants and provides an effective adaptation for spreading.
Plants with a climbing growth habit may be woody or herbaceous. Vines (herbaceous) and lianas (woody) have various specialized adaptations for climbing on, through, and over host plants and surrounding objects to gain access to light. Self-clinging climbers attach themselves to supports by aerial (adventitious) roots or by modified leaf structures called tendrils. Tendril climbers twine around or adhere themselves to supports by contact sensitive tendrils with adhesive discs at the tips. Climbers with twining stems or curling leaf stalks coil around supports in a clockwise or counterclockwise spiral habit. Scrambling (scandent) or trailing climbers with long arching stems attach loosely, if at all to supports. Some species, such as roses, are equipped with stem modifications of hooked thorns that allow them to scramble through other plants.