34.5:
Basic Plant Anatomy: Roots, Stems, and Leaves
Plants are often perceived as less complex than animals, but much like animals, they are made up of organs and organ systems that perform specialized functions.
The vast majority of plants have two organ systems: the shoot and root systems. Roots typically grow underground. They anchor the plant and take up water and nutrients from the environment. The two major types of root systems are taproot systems and fibrous root systems.
In a taproot system, a primary root emerges from the seed upon germination and grows deep into the soil to anchor the plant. Much smaller lateral roots then branch off of the primary root.
In a fibrous root system, however, the primary root supports only seedling development. Later, many thin roots branch from the stem underground. These so-called adventitious roots form a shallow network in the soil that supports the mature plant.
The organs that comprise the shoot system can be classified as either reproductive or vegetative organs. The reproductive organs include flowers, fruit, and cones. There is considerable diversity in how plants reproduce and consequently in the reproductive organs that they possess. The vegetative organs, however, always include stems and leaves.
The stem of a plant provides structure, supporting the leaves, buds, and flowers. The stem also orients the leaves to maximize photosynthesis. Stems are composed of nodes, points at which leaves and branches attach, and internodes, the regions of stem between the nodes.
A stalk called a petiole anchors each leaf at the node. The terminal part of the leaf is called the blade or lamina. Simple leaves have one blade per petiole, while compound leaves have multiple blades attached to each petiole. Still other plants, such as grasses, lack a petiole. Instead, the blade directly envelops part of the stem.
The leaves conduct the majority of photosynthesis. The blades are often thin and flattened in shape to maximize the surface area exposed to sunlight.
34.5:
Basic Plant Anatomy: Roots, Stems, and Leaves
The primary organs of vascular plants are roots, stems, and leaves, but these structures can be highly variable, adapted for the specific needs and environment of different plant species.
Roots
While roots are most often found underground, this is not universally the case. Aerial roots are any roots that emerge aboveground. Epiphytic plants, such as orchids, can live their entire lives without touching soil. Other types of aerial roots, such as those of the strangler fig or banyan, germinate aboveground but grow downward, eventually penetrating the soil below and appearing to “strangle” their host plant.
Some plants have roots that are adapted to provide additional support. Buttress roots are one such modification. Often found on trees growing in tropical areas where the soil is nutrient-poor, and there is plenty of water available near the surface from rainfall, buttress roots provide structural support to large trees with shallow roots. Brace roots or prop roots are another type of modified root that supports tall, shallow-rooted plants such as corn. They grow as adventitious roots—those that do not develop from the primary root—from nodes near the bottom of the stem, reaching the ground at an angle to provide structural support.
Stems
Plant stems have several functions. These include providing structure, supporting the leaves, buds, and flowers. Additionally, the stem will aid in orienting the leaves to maximize photosynthesis. Stems are composed of nodes, points at which leaves and branches attach, and internodes, the regions of stem between the nodes.
The petiole is a stalk that anchors each leaf at the node.
Leaves
The leaves conduct the majority of photosynthesis. While leaves can come in a wide variety of shapes and sizes, the blades are often thin and flattened in shape to maximize the surface area exposed to sunlight. The terminal part of the leaf is called the blade, or lamina. Simple leaves have one blade per petiole, while compound leaves have multiple blades attached to each petiole. Some plants, such as grasses, lack a petiole. Instead, the blade directly envelops part of the stem.
Roots, Leaves, and Stems as Storage Vessels
Roots can be modified for food storage. Taproots such as carrots and beets are enlarged to store starches. Stems and leaves can also be modified to store starches – tubers such as potatoes are actually modified stems, even though they grow underground. Additionally, onions are a modified starch-storing leaf.
Suggested Reading
Atkinson, Jonathan A., Amanda Rasmussen, Richard Traini, Ute Voß, Craig Sturrock, Sacha J. Mooney, Darren M. Wells, and Malcolm J. Bennett. “Branching Out in Roots: Uncovering Form, Function, and Regulation.” Plant Physiology 166, no. 2 (October 1, 2014): 538–50. [Source]