The Structure and Diversity of Angiosperms
Investigating the Forms and Types of Flowering Plants
Aug 17, 2009
Plants may be informally grouped into three categories based on structure and mode of reproduction: nonvascular spore plants which are simple in structure (no vascular system) and reproduce by spores, vascular spore plants which are advanced in structure (possess a vascular system) but reproduce by spores, and vascular seed plants which are advanced in structure and reproduce by seeds.
Fossils indicate that nonvascular spore plants and vascular spore plants were the dominant plant types during the age of dinosaurs. However, as the climate underwent a series of upheavals, new group of plants emerged to fill the void; plants that reproduced not by fragile spores but by hardy seeds. The age of seed plants had begun and it has continued unabated to present times.
The vascular seed plants consist of five taxonomic divisions: Cycadophyta, Ginkgophyta, Pinophyta, Gnetophyta, and Magnoliophyta. The first four of these divisions are often grouped into an informal nontaxonomic group known as the gymnosperms (meaning “naked seed”). Their name reflects the fact that their seeds, after maturation, are exposed to the environment (hence, “naked” seed). The Magnoliophyta ,or angiosperms as they are informally known, are the flowering plants.
With over 250,000 identified species and perhaps a million or more awaiting discovery, the angiosperms (or flowering plants) reign supreme in the green world. This lofty status has been attained not only by the sheer weight of their numbers but also by the complexity of their bodies (especially their reproductive structures) and the scope of their diversity and distribution. No other group of living things comes in a wider array of sizes, shapes, and colors or occupies a greater range of land, fresh water and salt water habitats than do the flowering plants.
The Secret is in the Seed
What accounts for the rise to supremacy of seed plants? Why have they become the most successful and dominant plant group? The answers lie in their seeds. A complex seed offers more survival value than does a simple spore.
The protection that a seed affords the enclosed embryo and the stored food that is available to that embryo at the critical stages of germination and establishment give seed plants a great advantage over the spore-forming plants. Couple a tough, durable seed with an adaptable body structure and you have the formula for the most successful group of plants that have ever lived.
Types of Angiosperms
The flowering plants comprise the division Magnoliophyta (newer designation) or Angiospermae (older designation). This division in turn can be broken down into two smaller groups or classes: Magnoliopsida (formerly dicotyledons) with about 170,000 species and Liliopsida (formerly monocotyledons) with about 65,000 species.
The similarities between these two groups are far greater than the differences. However, the two classes do possess distinguishing physical traits. Liliopsidaphytes which include such familiar plants as the grasses, lilies, irises, orchids, cattails, and palms have one cotyledon in the seed, parallel veins in the leaf, and flower parts in multiples of three. The Magnoliopsidaphytes which include almost all the familiar trees and shrubs (other than gymnosperms) and most other non-woody plants possess two cotyledons in the seed, net venation, and flower parts in multiples of four or five.
The Structure of Angiosperms
The angiosperms make up the major portion of what we humans see as the plant world. From lawn and gardens, fields of crops, trees, shrubs, fruits and vegetables in the market, to bright splashes of color from wildflowers in a meadow, and from duckweed and water lilies, eel grass, and turtle grass in the water, to saguaro and prickly pear cacti in blazing deserts.
In size flowering plants range from species of Eucalyptus trees well over 100 meters (330 feet) tall with trunks some 20 meters (66 feet) in girth to some aquatic floating duckweeds barely 1 millimeter (0.004 inches) across.
Regardless of their appearance and size, all angiosperms are vascular in nature and possess true roots, stems, and leaves.
The angiosperms are truly the monarchs of the plant realm. With their size and abundance they have come to dominate every type of habitat available to them. Wherever one goes, flowers are there is some form or another.
 |
 |
