Most plants are multicellular, autotrophic organisms, that is, able to produce their own food from inorganic elements by converting water and carbon dioxide to sugar. Plants are sessile, stationed in one spot throughout their lives.
Most plants have a complex life cycle called alternation of generations between diploid and haploid forms. The diploid generation, in which the plant body is made up of diploid cells, is called the sporophyte. The sporophyte produces haploid spores by meiosis.
These haploid spores then grow mitotically to produce the haploid generation, called the gametophyte, which produces haploid gametes. Gametes fuse to forma diploid zygote, a fertilized reproductive cell that marks the beginning of a new sporophyte generation.
Various methods may be used to group members of Plantae, which comprises about 300,000 species. Based upon where plants live, they can be divided into terrestrial (land) and aquatic plants. Members of these two groups vary widely in their size, body structure, and level of complexity as a result of their interactions with their environments.
Plants that live in water usually lack true roots, stems, and leaves as well as complex reproductive structures, such as flowers. Because they are surrounded by water, aquatic plants also lack the rigid supporting substances required by terrestrial plants.
Terrestrial plants are more complex and may be broadly grouped into vascular (plants with vessels) and nonvascular (plants without vessels). Vascular plants are divided into various phyla, based upon their adaptive features and complexity of structure.
Algae vs. Plantae
Those plants known as algae are not members of kingdom Plantae but instead are primarily members of another group of eukaryotic life, Protista, which also produce their own food by photosynthesis.
Algal pigments are red or brown, absorbing the green, violet, and blue lights that most readily penetrate the water. The combination of various pigments with chlorophyll resulted in the distinctive coloration of algae of the three phyla: red algae, brown algae, and green algae.
Within the kingdom Protista, three main algal phyla exist. Red algae (phylum Rhodophyta) are multicellular with red pigment (phycobilins) masking their green chlorophyll. Brown algae (phylum Phaeophyta) are multicellular and include the largest and most complex of the marine algae.
Green algae (phylum Chlorophyta) are mostly multicellular inhabitants of freshwater environments, with about seven thousand species. The algae are thought to be primitive evolutionary precursors to the species which now make up the kingdom Plantae.
Transition from Water to Land
The move from water to land requires adaptations and some newfeatures. The advantages to life on land seem obvious, with plentiful access to carbon dioxide and sunlight, reduced competition, fewer predators, and increased nutrient concentrations. The challenges are also plentiful: The supportive buoyancy of water is missing, and the air tends to dry things out.
Conditions on land favored the evolution of structures that support the plant body, vessels that transport water and nutrients throughout the body, and structures that conserve water.
Adaptations to dry land called conducting vessels (collectively known as the vascular system) emerged to transport water and minerals as well as products of photosynthesis.
Roots or rootlike structures evolved to help anchor the plant and absorb nutrients and water from the soil. A stiffening substance called lignin, made up of rigid polymer, enables plants to stand in wind, hence exposing maximal surface area to sunlight.
Numerous small pores called stomata in the leaves and stems open to allow gas exchange and close to conserve water when necessary. A waxy cuticle covering of the surfaces of leaves and stems also reduces the loss of water.
Based upon their structure, complexity, and distribution over the globe, land plants can be classified into three phyla of bryophytes and nine phyla of vascular plants, which include seedless plants, gymnosperms, and angiosperms.
Three phyla of plants—the liverworts, hornworts, and mosses—have been commonly known as bryophytes. The sixteen thousand species of bryophytes are among the least complex terrestrial plants.
They are the plant equivalent of amphibians. Although they have root like anchoring structures (rhizoids), they lack true roots, leaves, and stems. They are also non vascular, lacking well developed structures for conducting water and nutrients.
Because they must rely upon slow diffusion or poorly developed tissues for distribution of water and nutrients, their body size is limited. Most bryophytes are less than 1 inch (2.5 centimeters) tall.
The liverworts, phylum Hepatophyta, comprise six thousand species of small, inconspicuous plants forming large colonies in moist, shaded soil or rocks, tree trunks, or branches.
Due to the livershaped gametophyte in some genera and the fiction that these plants might be useful in treating liver-related diseases, they were named liver worts many centuries ago. Liverworts are the simplest of all living land plants, lacking cuticle, stomata, and vascular tissue.
The hornworts, phylum Anthocerophyta, area small phylum of plants consisting of about one hundred species. By appearance, many species of hornworts have a remarkable resemblance to green algae. Hornworts, however, have stomata, an important structure for land plants. Like liverworts, hornworts lack specialized conducting tissue.
The mosses, phylum Bryophyta, constitute a diverse group of some ninety-five hundred species of small plants. Many species have both stomata and specialized conducting tissue, resembling the remaining phyla of land plants. Mosses usually thrive in relatively moist areas, where a variety of species can be found.
Some mosses are used to monitor air pollution because of their acute sensitivity to air pollutants such as sulfur dioxide. There are three classes of mosses: Bryidae (the “true” mosses), Sphagnidae (the peat mosses), and Andreaeidae (the granite mosses), each with distinctive features.
Seedless Vascular Plants
The overall pattern of plant diversification may be explained in terms of the successive rise to dominance of each of four major plant groups.
Early vascular plants, including Rhyniophyta, Zosterophyllophyta, and Trimerophyta, were primitive in morphology yet dominant during a period from about 420 million to 370 million years ago. Ferns, lycophytes, sphenophytes, and progymnosperms were dominant from about 380 million to 290 million years ago.
Seed plants arose about 360million years ago, with gymnosperms dominating the globe until 100 million years ago. Finally, the angiosperms, or flowering plants (phylum Anthophyta) appeared about 127million years ago and have been the most dominant and diverse group for the past 100 million years.
The seedless vascular plants dominated the landscape during the Carboniferous period. They are the primary source of coal, which was formed through gradual transformation of plant bodies under high pressure and heat.
The three dominant phyla—the Rhyniophyta, Zosterophyllophyta, and Trimerophyta—had become extinct by about 360 million years ago. The modern representatives of seedless vascular plants have become reduced in size and importance.
The landscape once dominated by seedless plants has largely been replaced by the more versatile seed plants. Five phyla of seedless plants have living representatives: Psilotophyta, Lycophyta, Sphenophyta, Pterophyta, and Progymnospermophyta.
Commonly called the whiskferns, the phylum Psilotophyta includes two living genera, Psilotum and Tmesipteris. Both are very simple plants. Psilotum is widely known as a greenhouse weed that prefers tropical and subtropical habitats.
In the United States, it is found in Arizona, Florida, Hawaii, Louisiana, Puerto Rico, and Texas. Tmesipteris is restricted to South Pacific regions such as Australia, New Caledonia, and New Zealand.
Psilotum is unique among vascular plants in that it lacks both true roots and leaves. The underground portion of Psilotum forms a system of rhizomes with many rhizoids that are a result of a symbiotic relationship between fungi and Psilotum.
Psilotumis homosporous, meaning the male and female gametophytes are produced through the germination of the spores of same origin. Psilotum sperm require water to swim to and fertilize the egg.
Tmesipteris grows as an epiphyte on tree ferns and other plants and in rock crevices. The leaf like appendages of Tmesipteris are larger than those of Psilotum. Otherwise, Tmesipteris is very similar to Psilotum.
There are about one thousand living species of phylum Lycophyta that belong to three orders of ten to fifteen genera. At least three orders of Lycophyta have become extinct; these include small and large trees, the dominant plants of the coal-forming forest of the Carboniferous period.
The three orders of living Lycophyta consist of herbs, each including a single family: Lycopodiaceae, Selaginellaceae, and Isoetaceae.
Lycopodiaceae are commonly known as club mosses. All except two genera of Lycophyta belong to this family. Most of the estimated four hundred species of Lycopodiaceae are tropical.
They rarely form conspicuous elements in any plant community, except in some temperate forests where several species may form distinct mats on the forest floor. Because they are evergreen, they are most noticeable during winter months. Lycopodiaceae are homosporous and require water for fertilization.
Among the genera that grow in the United States and Canada are Huperzia (the fir mosses, seven species), Lycopodium (tree club mosses, five species), Diphasiastrum (club mosses and ground pines, eleven species), and Lycopodiella (six species).
Selaginella is the only living genus of the family Selaginellaceae. Among its seven hundred species, most are tropical, growing in moist habitats.
A few species occur in deserts, becoming dormant during the driest season. The well-known resurrection plant, S. lepidophylla, grows in Mexico, New Mexico, and Texas. Selaginella are heterosporous, having separate male and female gametophytes.
The only member of the family Isoetaceae is Isoetes, commonly known as quillwort. Isoetes may be aquatic or grow in pools that have alternate dry and wet seasons.
Isoetes is also heterosporous. The unique feature of some species of Isoetes is their ability to acquire carbon dioxide for photosynthesis from soil where they grow rather than from the atmosphere. Their leaves have thick cuticles and lack stomata.
Only one genus, Equisetum, consisting of fifteen living species, makes up the phylum Sphenophyta. Members of this phylum are known as the horsetails, believed to be the oldest surviving plants on earth. They are widely distributed in moist or damp places, by streams, and along the edges of woods and roadsides.
The leaves are reduced to tiny scales on the branches. The ribs are tough and strengthened with silicon deposits. Due to its abrasive texture, Equisetum was used in past times to scour pots, pans, and floors. Hence, they were also called “scouring rushes.”
The roots of Equisetum are adventitious, emerging at the nodes of the rhizomes. The aerial stems of Equisetum arise from branching underground rhizomes. Although the plants may die back during the dry season, the rhizomes are perennial. Equisetum is homosporous. Its gametophytes are green and free-living, most being about the size of a pinhead.
They become established mainly in mud that has recently been flooded and is rich in nutrients. The gametophytes, which reach sexual maturity in three to five weeks, are either bisexual or male. The sperm require water to swim to the eggs. A fertilized egg develops into an embryo or young sporophyte.
Members of the phylum Pterophyta are commonly called ferns, the largest group of plants other than the flowering plants. About eleven thousand living species of ferns are widely distributed on the earth, among which three-fourths of the species are found in the tropics.
There the greatest diversity of fern species exists, and they are abundant in many plant communities. In the small tropical country of Costa Rica, 1,000 species of ferns have been identified, whereas only 380 species of fern occur in the United States and Canada combined.
In both form and habitat, ferns exhibit amazing diversity. Some are small and have undivided leaves, while others can reach up to 30 meters in height, with a trunk of more than 30 centimeters in diameter. Most living ferns are homosporous, except for two orders of water ferns.
Two genera of the orderOphioglossales, the grape ferns (Botrychium), and the adder’s-tongues (Ophioglossum), are wide spread in the north temperate region.
A single leaf is usually produced each year from the rhizome. Each leaf consists of two parts: the blade (the vegetative portion) and a fertile segment that typically bears two rows of eusporangia, hence the name eusporangiate ferns.
The order Filicales consists of 35 families and 320 genera, with more than 10,500 species. Most the familiar ferns are members of this order, such as the garden and woodland ferns of temperate regions. They have rhizomes that produce new sets of leaves each year.
The root system is primarily adventitious, arising from the rhizomes near the bases of the leaves. They are homosporous.With a high surface-to-volume ratio, their bodies capture sunlight much more effectively than those of the lycophytes.
The water ferns, Marsileales and Salviniales, are the only living heterosporous ferns. Members of Marsileales grow in mud, on damp soil, or often with their four-leaf-clover-like leaves floating on the surface of water.
Their unique, drought resistant, bean-shaped reproductive structures are able to germinate even after one hundred years of dry storage. Members of Salviniales, genera Azolla and Salvinia, are small plants that float on the surface of water. They are harvested and used as feed or fertilizer in some Asian countries.
Gymnosperms, literally “naked seeds,” are the earliest-evolved plants that produce seeds. Living gymnosperms comprise four phyla: Cycadophyta, Ginkgophyta, Coniferophyta, and Gnetophyta.
The life cycles of all bear a remarkable resemblance: an alteration of heteromorphic generations, with large, independent sporophytes and greatly reduced gametophytes. The ovules are exposed on the surfaces of the megasporophylls.
At maturity the female gametophyte of most gymnosperms is multi cellular in structure, with several archegonia. The male gametophytes develop as pollen grains. Except for the ginkgo and cycads, the sperm cells of seed plants are non motile.
In seed plants, water is not required for transfer of spermor fertilization of an egg. Pollen grains that usually contain two sperm nuclei may be transferred to the egg via various means, such as wind, insects, and animals.
A pollen grain then germinates and sends one nucleus to fuse with the egg, which in turn develops into embryo. After fertilization, each ovule develops into a seed.
Among the four phyla, the conifers (Coniferophyta) are the largest and most wide spread gymnosperms,with about 50 genera and 550 species.
They still dominate many of the earth’s plant communities, with pines, firs, spruces, and other familiar evergreen trees over wide stretches of the north. Living cycads (Cycadophyta) constitute 11 genera, and some 140 species grow primarily in tropical and warm regions.
Cycads are palm like plants, with trunks and sluggish secondary growth. Only one living species of ginkgo (Ginkgophyta) exists. The phylum Gnetophyta consists of three genera that are close relatives of the angiosperms, with which they share many characteristics.
As plants adapted to terrestrial environments, more effective means of reproduction and distribution emerged. Flowers, fruits, and seeds resulted in the dominance by angiosperms: flowering plants, the most diverse group of plants. Modern flowering plants, or angiosperms, constitute the phylum Anthophyta, which are incredibly diverse, with more than 230,000 species.
They range in size from a few millimeters in diameter, such as the duckweed that floats on ponds, to more than 320 feet (about 100 meters) tall, such as the eucalyptus. From desert cacti to tropical orchids to grasses to major food crops, angiosperms dominate the plant kingdom.
Anthophyta is divided into two large classes: Monocotyledons (65,000 species) and Eudicotyledones, or “true” dicots (170,000 species).
In addition to some features shared with gymnosperms, angiosperms have a few unique characteristics. Within seeds, nutrients and food are usually stored in a triploid tissue called an endosperm.
The presence of carpels makes flowers shiny and more attractive for pollinators, enhancing their reproductive success. The nutritious fruit-encasing seeds offer protection and ensure the wide distribution of angiosperms as various animals eat fruits and disperse the seeds throughout the ecosystem.
Angiosperms have broad leaves, giving them the advantage of collecting more sunlight for photosynthesis, especially in warm, moist climates. The extra energy gained in spring and summer allows trees to drop their leaves and enter a dormant period, which reduces water evaporation when water is in short supply.
Pollination in angiosperms takes place by the transfer of pollen from anther to stigma. Each pollen grain contains sperm, typically with two or three cells. One cell grows, sending pollen tubes to the ovule, where one sperm nucleus unites with the egg and produces a diploid zygote.
The other cell or cells fuse with two polar nuclei, giving rise to primary endosperm nucleus. This phenomenon, called double fertilization, is a unique characteristic for angiosperms. The zygote then develops into an embryo (sporophyte).
The primary endosperm grows and matures into a nutritive endosperm. Both self-pollination and cross-pollination occur in flowering plants. The angiosperm domination of earth began about 100 million years ago and has persisted to the present.