Angiosperms are the most diverse and widespread group of plants. There are over 280,000 known species of flowering plants.
Plant Adaptations to the Environment
Like other organisms, plants have evolved over time, often reflecting the environment in which they live. For example, the cactus that has reduced its leaf size and uses its stem for photosynthesis as a way of reducing water loss. Or plants that live in water that have adapted feathery leaves to increase surface area for photosynthesis.
For most plants however, conditions are not that extreme, and could vary on a daily, weekly or seasonal basis. Because of this, plants have developed physiological adaptations.
Plants produce a hormone that closes stomata when there is not much rainfall or water in soil. Stomata are pores in the plant leaves through which water is lost, or released. In wetter conditions, plants will open their stomata to excrete extra water.
The Difference Between Monocots and Dicots
Monocots:
- One cotyledon (embryo)
- Veins in leaves usually parallel
- Stems have vascular bundles, complexly arranged
- Fibrous root system
- Floral parts usually in multiples of threes
Examples of monocots include grasses (wheat, rice, corn), cattails, lilies, palms trees, orchids, bamboos and yuccas.
Dicots:
- Two cotyledons
- Leaf veins are usually netlike
- Stems have vascular bundles arranged in a ring
- Taproot usually present
- Floral parts usually in multiples of four or five
Examples of dicots include many trees, and most ornamental and crop plants such as roses, sunflowers or beans.
Plant Structure
The three basic organs of a plant are:
- Roots
- Stems
- Leaves
Plants are multi-cellular organisms. They have organs composed of different tissues, and tissues composed of different cells.
Plant Roots
A plant’s roots are what anchors it to the soil and how the plant takes up nutrients. Monocots have fibrous root systems that expand a mat of thin roots below the surface of the soil to increase the plants exposure to water and minerals.
Dicots have a taproot, which is one large root, which produces smaller lateral roots. These taproots often store food for the plant to consume during flowering and fruit production.
One both monocot and dicot root systems are tiny root hairs, which reside near the root tip. The purpose of these root hairs is to increase the surface area of the root for optimal absorption of water and minerals.
Plant Stems
Plant stems are a system of nodes, internodes, axillary buds and terminal buds.
- Nodes: the point where leaves attach to stem
- Internodes: stem segments between nodes
- Axillary buds: structures that can form vegetative branches, but are usually dormant
- Terminal buds: where growth of young shoots occurs. Terminal buds have developing leaves and a complete series of nodes and internodes. Terminal buds suppress the growth of axillary buds. This is referred to as apical dominance.
Apical dominance is an evolutionary adaptation that makes the plant grow taller and this exposes the plant to more sunlight. In cases where the top of the plant is damaged (ie: eaten by an animal), or light intensity is strongest at the sides of the plant then the top, axillary buds break dormancy and start to grow, complete with their own terminal buds, axillary buds and leaves.
Plant Leaves
Most photosynthesis occurs in the leaves although green stems can also perform photosynthesis. Leaves generally consist of:
- A flattened blade
- A stalk
- The petiole: which joins the leaf to the node of the stem
Leaves can vary in structure, however. Grasses for example (and many other monocots) lack petioles. Instead the leaf base forms a sheath around the stem. Plant taxonomists use plant leaves to determine plant identity and classification.
Differences in plant leaves aside from shape, spatial arrangement and vein pattern, are:
- Simple leaf: leaf that has a single, undivided blade
- Compound leaf: which are divided into several leaflets
- Double compound leaf: leaf that is further divided into several leaflets
Most large leaves are compound leaves or doubly compound, which allows for strength against strong wind (less tearing) and protection against pathogen spread (ie: able to confine some pathogens to a single leaf rather then whole leaf).
Resource and further reading:
Campbell and Reece, 2002, 'Plant structure and growth', in Biology 6th edition, ed. Benjamin Cummings, San Francisco, chap 35.
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