Plant Structure, Growth, and Development
Overview: Plastic Plants?
• The fanwort, an aquatic weed, demonstrates the great developmental plasticity that is characteristic of plants. o The fanwort has feathery underwater leaves and large, flat, floating surface leaves. o Both leaf types have genetically identical cells, but the dissimilar environments in which they develop cause different genes involved in leaf formation to be turned on or off.
• In addition to plastic structural responses of individual plants to specific environments, plant species have adaptations in morphology that benefit them in their specific environments. o For example, cacti have leaves that are reduced to spines and a stem that serves as the primary site of photosynthesis. These adaptations minimize water loss in desert environments.
• The form of any plant is controlled by environmental and genetic factors. As a result, no two plants are identical.
• Angiosperms make up 90% of plant species and are at the base of the food web of nearly every terrestrial ecosystem.
• Most land animals, including humans, depend on angiosperms directly or indirectly for sustenance.
Concept 35.1 The plant body has a hierarchy of organs, tissues, and cells.
• Plants, like multicellular animals, have organs that are composed of different tissues, and tissues that are composed of different cell types. o A tissue is a group of cells with common structure and function. o An organ consists of several types of tissues that work together to carry out particular functions.
Vascular plants have three basic organs: roots, stems, and leaves.
• The basic morphology of vascular plants reflects their evolutionary history as terrestrial organisms that inhabit and draw resources from two very different environments. o Vascular plants obtain water and minerals from the soil. o Vascular plants obtain CO2 and light above-ground.
• To obtain the resources they need, vascular plants have evolved two systems: a subterranean root system and an aerial shoot system of stems and leaves.
• Each system depends on the other.
o Lacking chloroplasts and living in the dark, roots would starve without photosynthates, the sugar and other carbohydrates imported from the shoot system. o Conversely, the shoot system depends on water and minerals that roots absorb from the soil. Roots provide anchorage, absorption, and storage.
• A root is an organ that anchors a vascular plant in the soil, absorbs minerals and water, and stores sugars and starches.
• Most eudicots and gymnosperms have a taproot system, consisting of one large vertical root (the taproot) that develops from an embryonic root. • The taproot produces many small lateral, or branch, roots. o In angiosperms, taproots often store sugars and starches that later support flowering and fruit production. o Taproot systems generally penetrate deeply and are well adapted to deep soils.
• In seedless vascular plants and most monocots, including grasses, the embryonic root dies and does not form a main root. • Instead, many small roots grow from the stem. Such roots are adventitious, a terms describing a plant organ that grows in an unusual location. • Each small root forms its own lateral roots, giving rise to a fibrous root system—a mat of thin roots that spread out below the soil surface. o A fibrous root system is usually shallower than a taproot system and is best adapted to shallow soils with light rainfall. o Grass roots are concentrated in the upper few centimeters of soil. As a result, grasses make excellent ground cover for preventing erosion.
• The root system helps anchor a plant.
• In both taproot and fibrous root systems, absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs enormously increase the surface area. o Root hairs are...