Angiosperms: Characteristics, Evolution, and Classification
Angiosperms is the common name of the division or phylum that contains flowering plants, which are the dominant form of plant life. The members of this division are the source of most of the food on which humans and other mammals base their livelihood, as well as many raw materials and natural products. This includes almost all shrubs and herbaceous plants, most trees except pines and other conifers, and more specialized plants, like succulents, epiphytes, and aquatic plants. Although there are about 230,000 species, many are still unknown. Flowering plants have occupied almost all ecological niches and dominate most of the natural landscape. Approximately two-thirds of all species are unique to the tropics, but human activities are rapidly causing their extinction. Only a thousand species have economic importance worthy of consideration, and the bulk of the world’s food comes from only fifteen species. If properly investigated, several hundred more species could be used.
Two Major Groups of Flowering Plants
- Monocotyledons
- Dicotyledons
Description of Angiosperms
The most characteristic feature of angiosperms is the flower, whose function is to ensure the reproduction of the plant by seed formation. The flowers are highly modified leaves, composed of four main parts arranged in separate series, or whorls, which grow on the apex of specialized stalks.
- The outer whorl is the calyx, or all the sepals, which are modified leaves or bracts, usually green.
- Then comes the corolla, formed by petals; they are also modified leaves, but usually finer in texture and more vividly colored.
- The third whorl corresponds to the stamens, pollen-forming parts, which together constitute the androecium or male portion of the flower.
- The innermost whorl, the gynoecium, is formed by the carpels, the structures responsible for forming female seeds; the carpels are merged into a structure called the style, ending in a free end called the stigma.
The seeds of angiosperms develop within an ovary, the carpel that surrounds and protects the ovules. The seed is formed from the ovule after pollination and fertilization. Neither ovules nor seeds are elements unique to the angiosperms; naked seed plants, as mentioned above (Gymnosperms), have ovules that remain exposed on the surface of the leaves of specialized cones. What is unique to angiosperms is seed development from an ovule enclosed in an ovary; as the fertilized seed grows, it expands into a fruit.
Evolution of Angiosperms
The unique angiosperm flower probably evolved from a now-defunct gymnosperm cone pollinated by insects, combining male and female reproductive parts. Almost all living gymnosperms are wind-pollinated, but some fossil groups and survivors provide evidence of insect pollination. Although insects favored pollination, they also ate the ovules, and it is thought that the development of the carpel that encloses the ovule is an adaptation to protect it and the developing seed from predation by insects. The carpel also protects against other harmful environmental influences, such as drying, and has reduced the size of the ovule, perfecting the process of pollination and developing other parts of the flower, all of which increase the chances of successful reproduction.
Features of Kingdom Plantae
The kingdom Plantae includes mosses, ferns, conifers, and flowering plants in a range that exceeds 250,000 species, the second-largest group after arthropods in the animal kingdom. They are considered to be derived from algae. The main feature of the kingdom is the presence of chlorophyll, which captures light, producing carbon compounds; for this feature, they are autotrophs. Other features of this kingdom are that all are multicellular eukaryotes and possess cell walls composed mainly of cellulose.
Nutrition
Nutrition occurs through photosynthesis performed by the chlorophyll of the chloroplasts. There are some examples of partial or total heterotrophic plants.
Sexual Reproduction with Alternation of Generations
The diploid sporophyte and haploid gametophyte. Plants have alternating generations: diploid (2n) sporophyte alternating with individuals (or groups of cells produced by successive mitosis) haploid (n) called gametophytes. All plants have alternation of generations, in which a diploid phase (sporophyte) includes the embryo, and a haploid phase (gametophyte) that produces gametes by mitosis.
Photosynthetic Organisms
Photosynthetic organisms are those organisms such as plants, algae, and some bacteria that capture and utilize light energy to convert inorganic matter from their external environment into organic matter used for growth and development.
Differences Between Vascular and Nonvascular Plants
- Cellular or Thallophyte plants (algae, lichens, bryophytes) have their reproductive organs hidden and not visible, hence the name cryptogams (crypto = hidden, gams = sex), whereas vascular plants (flowering plants) have their reproductive organs visible as a flower for sexual reproduction.
- Cellular plants belong to the level of cellular organization because their cells do not differentiate into tissues, whereas vascular plants belong to the organization level of organ systems.
- Cellular plants reproduce through a life cycle of alternation of generations with an asexual or sporophytic and sexual gametophytic phase (ferns) or by a sexual or gametophytic phase and an asexual or sporophytic phase (Bryophytes and Mosses), whereas vascular plants reproduce sexually by double fertilization in angiosperms and simple fertilization in gymnosperms, resulting in seeds for the perpetuation of the species over time.
- Cellular plants do not have roots, stems, or leaves because their cells are not differentiated tissues; however, vascular plants have their corms or vegetative bodies differentiated into roots, stems, and leaves.
- Cellular plants do not have supporting or conducting tissues because their cells do not differentiate into tissues, whereas vascular plants have supporting tissues (collenchyma and sclerenchyma) for support and mechanical support of the aerial parts (stem, leaves, flowers, and fruits), and conducting tissues for transporting raw sap (xylem) formed by the woody vessels or upward, and elaborated sap (phloem) formed by sieve vessels or downward.
Differences Between Monocotyledons and Dicotyledons
The differences between flowers of a monocot and a dicot are:
- In monocotyledons, the cycles of the complete flower protectors cannot be differentiated well, as they are formed by pieces of the same color. When the calyx is not different from the corolla, it is called a perigone, and each of the pieces that make up the perigone is called a tepal. Besides, the flowers have 3 or 6 cycles of floral pieces.
- In dicotyledons, the cycles of the complete flower protectors can be distinguished since they are formed by pieces of different colors: the calyx, formed by green sepals, and the corolla, formed by petals of various colors except green. When the calyx is different from the corolla, it is called a perianth. Flowers present cycles of 4 or 5 floral pieces.