 |
| Angiosperms - Chinese Bladdernut Buds |
The name "angiosperms" has long been used by botanists to refer to the flowering plants, a group of approximately 235,000 species. All angiosperms are members of the phylum Anthophyta.
The name "angiosperm" is actually derived from two Greekwords, angeion,meaning "vessel" or "container", and sperma, meaning "seed". The name was given in reference to the fact that the seeds of all flowering plants develop from ovules that are enclosed in a structure called a carpel.
This characteristic sets the angiosperms apart from all other plants, which either do not have seeds or have seeds that are not developed in structures resembling a carpel. Although the name angiosperm is used widely, plant taxonomists and many botanists typically refer to them by the more formal name Anthophyta, the phylum that contains the flowering plants.
    |
Unique Features of Angiosperms
In addition to possessing enclosed seeds, Anthophyta differs from other plant phyla in a number of ways. The most obvious distinguishing feature is the flower, a complex structure containing the reproductive parts of the plant. The reproductive structures in other plants are much less complex and showy. The angiosperm life cycle differs from that of almost all other plants.
The sporophyte is the dominant, diploid stage and is the more visible form of the plant, with the leaves, stems, roots, and flowers. The haploid gametophyte is confined to life inside the ovary or anther of the flower, unlike the typically free-living gametophytes of most other plants.
Fertilization is also unique in angiosperms. Many angiosperms rely on insects or other animals to transfer pollen from one flower to another. Pollen grains produce two haploid sperm that travel through a pollen tube from the stigma into the ovary of the flower and into one of the embryo sacs.
Within the embryo sac one of the sperm fertilizes the egg, which will lead to formation of the diploid embryo, and the other sperm fuses with two or more polar nuclei to form the endosperm, which will nourish the embryo and young seedling. This process is often referred to as double fertilization. Other, less obvious features set Anthophyta apart as well, including a unique vascular anatomy, pollen structure, and various biochemical characteristics.
Size and Geographic Diversity
There are approximately 235,000 species of flowering plants, and they are found in almost all terrestrial habitats, with the exception of extremely high elevations and some polar regions. As mall proportion of flowering plants are aquatic (that is, found in freshwater habitats), and an even smaller number aremarine (found in saltwater habitats).
The greatest species richness is in tropical regions, especially tropical rain forests, and species richness steadily decreases at increasing latitudes north and south of the equator.
Angiosperms have been so successful in terres- trial ecosystems that they represent the majority of the herbs and shrubs and many of the trees as well. The diversity of growth forms is tremendous, represented by such diverse families as Poaceae (grasses and bamboos), which have greatly reduced and modified flowers; Cactaceae (cactuses), which have spines instead of leaves and very showy flowers; and Lemnaceae (duckweed), which has a highly reduced plant body sometimes comprising a single leaf with no true roots or stem and the smallest flowers of any angiosperm.
Other families include Asteraceae (sunflower or aster family), with reduced disc and ray flowers crowded together into inflorescences called heads; Salicaceae (willow family), a widespread, water-loving family of trees and shrubs with reduced flowers arranged in catkins; and Orchidaceae (orchid family), with some of the showiest and most intricate flowers of all, which have extremely numerous and minute seeds.
Economic Importance
Economically, angiosperms have made a profound impact. Essentially all of the world’s food crops, from rice, wheat, and corn to other fruits and vegetables, are derived from flowering plants. In fact, it is almost impossible to think of more than a handful of foods or food ingredients from plants that are not flowering plants.
 |
| Angiosperms - Stachyurus |
The same is true of ornamental plants. Although a few gymnosperms (such as conifers) and ferns are common as ornamentals, most of the remaining plants, even many valued for their foliage rather than their blooms, are flowering plants.
The only area where angiosperms do not dominate economically is in forest products, where conifers account for a significantly larger proportion of the harvest, but even there, hardwoods predominate for certain applications.
Medicine has also reaped many benefits from angiosperms. In fact, it was primarily the herbalists, from the Middle Ages to the Scientific Revolution, who expanded humankind’s understanding of flowering plants. Knowledge of flowering plants for food and medicine among many indigenous peoples has always been wide spread.
Modern medicine has capitalized onmuch of this knowledge and has even expanded the search for new medicines. Flowering plants have been the original source of many precursors to modern medicines, including aspirin (willows, Salix), quinine (Cinchona species), and digitalin and digoxin (Digitalis species).
Lifestyle Diversity
Along with the diversity in structure comes a diversity in lifestyles. Most angiosperms are free-living, that is, receiving their primary energy and carbon from photosynthesis and their nutrients from the soil.
A few groups of plants receive their energy or nutrients in other ways. Some are saprophytes, which receive their energy and carbon from decaying organic material in the soil and their nutrients from other soil components, much like other plants.
Some of the best-known saprophytes are in Ericaceae (heath family). Their most distinctive feature is that they are either white or some shade of pink or red and are never green. Monotropa uniflora (Indian pipes), for example, is a ghostly white and has no chlorophyll.
Parasitism is an alternative for some angiosperms. One well-known parasite is the mistletoe (Loranthaceae), popular as a Christmas decoration, which is a branch parasite on trees. Many types of mistletoe have green foliage and therefore receive some of their energy from photosynthesis, but their primary nourishment comes from the host tree.
Some species have foliage that is brown or yellow and do not photosynthesize much at all. The seeds of mistletoe are spread from tree to tree when birds eat their berries and defecate the seeds on the branch of another tree. Probably the most unusual parasite is Rafflesia, from Malaysia and Sumatra.
It parasitizes species of Tetrastigma, a vine that grows on the forest floor and has no stems or leaves of its own. When it blooms it has the largest flowers in the world, and it is often called the corpse flower because it has a very strong odor, like that of rotting flesh.
Other parasites receive varying proportions of their energy and nutrients from their host and conventional means, and when the contributions are nearly equal they are referred to as hemiparasites. Hemiparasites are common in Castilleja (paint-brushes), and many species invade the roots of other plants to obtain part of their nutritional needs.
Aunique approach to obtaining nutrients is represented by insectivorous plants, commonly known as carnivorous plants These plants use a variety of adaptations for trapping and absorbing nutrients from insects.
Sundews (Droseraceae) have special glands on their leaves that excrete a sticky fluid that traps insects like flypaper. Pitcher plants (Nepenthaceae and Sarraceniaceae) have special tubular leaves that resemble cups or pitchers.
The inside of the leaves fill with water near the base, and the lip and inside surface of the pitcher are slippery. Once an insect gets inside, it slips into the water at the bottom. Venus’s flytrap (Dionaea, also in Droseraceae) is even more intricate, with leaves specially modified with traps that spring shut when an insect lands or walks on them.
There is even an aquatic carnivore, the bladderwort (Utricularia), which has saclike leaves with small openings that can close after a small aquatic insect or crustacean is sucked in.Although insectivorous plants do obtain some of their nutrients from insects, they also obtain nutrients from the soil or, in the case of bladderworts, surrounding water.
Angiosperm Classification
Traditionally Anthophyta has either been considered as a single class Angiospermae or Magnoliopsida, with two subclasses, or has been divided into two classes, Eudicotyledones, or Magnoliopsida, and Monocotyledones, or Liliopsida. The second of these two options is more commonly accepted by contemporary plant taxonomists, and the two classes are often referred to by the common names dicotyledons or dicots and monocotyledons or monocots, respectively.
The monocot/dicot dichotomy has long been considered a major evolutionary split in the angiosperms. The two classes a differ fromeach other in a number of ways. Monocots generally have bladelike leaves with parallel venation, whereas dicots more typically have pinnate or palmate venation. Monocots have fibrous root systems without taproots; dicots typically have taproots.
The flower parts in monocots occur typically in threes, whereas they occur most often in fours and fives in dicots. Monocots lack cambial secondary growth,which is common in dicots. Monocots have scattered vascular bundles in their stems, as opposed to the more orderly arrangement seen in dicot stems.
It has long been proposed that the monocots branched off fromthe dicots very early in the evolution of the angiosperms, but until recently it was difficult to sort out the probable events and the resulting classification system that would be needed to reflect them.
With the advent of molecular tools, such as deoxyribonucleic acid (DNA) sequencing, the study of early angiosperm evolution is getting much more attention. It has now become clear that, if the classification system is to reflect evolutionary history, Anthophyta must be divided intomore than just two classes.
Currently there is no agreement on how many other classes there should be, but Monocotyledones and Eudicotyledones will retain most of the taxa. This new approach to the classification of Anthophyta has also resulted in changing the common name of the "dicots" to "eudicots", meaning "true dicots".
Many of the remaining taxa not included in the monocots or eudicots are now often referred to as magnoliids and are considered to represent taxonomic groups that have branched off fromthe early angiosperms before the monocot/eudicot split. Some of these groups include the orders Magnoliales (which includes Magnoliaceae, long considered as having many primitive characteristics), Winterales, and Laurales.
The placement of a few taxa, such as Ceratophyllaceae and Chloranthaceae, is particularly controversial. With continued analyses of DNA sequences it is hoped that a clearer picture of the relationships among the magnoliids and related taxa will be obtained and a more phylogenetically based classification system can be devised.
