Virtually all other living creatures depend on plants to survive. Through photosynthesis, plants convert energy from sunlight into food stored as carbohydrates. Because animals cannot get energy directly from the sun, they must eat plants (or other animals that have had a vegetarian meal) to survive. Plants also provide oxygen to humans and animals for breath, because plants use carbon dioxide for photosynthesis and release oxygen into the atmosphere. Plants are found on land, in oceans and in fresh water. They have been on Earth for millions of years. Plants were on Earth before animals and currently number about 260,000 species. Vascular plants also known as tracheophytes that mean ―tracheid plant. It refers to the water-conducting cells called tracheary elements or tracheids that show spiral bands like those in the walls of the tracheae or air tubes, of insects.
The division comprises a tremendous diversity of plants among its four subgroups:
- psilopsids, leafless and rootless primitive forms commonly known as whisk ferns (though not true ferns);
- sphenopsids, feathery leaved plants commonly called horsetails;
- lycopsids, low-lying plants called club mosses and pteropsids, comprising the ferns, gymnosperms (pines, spruces, firs, etc.) and flowering plants. The vascular system consists of xylem (wood), concerned mainly with the conduction of water and dissolved minerals and phloem, which functions mainly in the conduction of foods, such as sugar.
- Tracheophytes are believed to have originated from the green algae (Chlorophyta).
Three features distinguish plants from animals
- Plants have chlorophyll, a green pigment necessary for photosynthesis.
- Their cell walls are made sturdy by a material called cellulose.
- They are fixed in one place (they do not move).
Classification of plants In order to study the billions of different organisms living on earth, biologists have sorted and classified the plants based on their similarities and differences. This system of classification is also called taxonomy and usually features both English and Latin names for the different divisions. All plants are included in one so-called kingdom (Kingdom Plantae), which is then broken down into smaller and smaller divisions based on several characteristics, including
Whether they can circulate fluids (like rainwater) through their bodies or need to absorb them from the moisture that surrounds them.
The majority of the 260,000 plant species are flowering herbs. To describe all plant species, the following divisions (or phyla) are most commonly used to sort them. The first grouping is made up of plants that are non-vascular; they cannot circulate rainwater through their stems and leaves but must absorb it from the environment that surrounds them. The remaining plant species are all vascular (they have a system for circulating fluids). This larger group is then split into two groups; one that reproduces from spores rather than seeds and the other that reproduces from seeds. The classification of plant kingdom can be done on the basis of the vascular tissue’s presence or absence, accordingly called vascular plants and non-vascular plants. The non-vascular plants are also called as bryophytes. There is no conductive tissue present in them for the transport of nutrients, water and sugars. The vascular plants include mosses, liverworts, hornworts. The vascular plant is further divided into three parts namely seedless (pteridophytes), gymnosperms and angiosperms. Together they can also be called as tracheophytes. All the tracheophytes have vascular tissue which helps in the transport of water and nutrients through the plant. The angiosperms are the flowering plant, they have one or two cotyledons. The monocotyledons are one seed leaf and dicotyledons are two seed leaves. Monocots have fibrous root system whereas eudicots have taproot.
Non-Vascular Plants Mosses and allies, or related species (Bryophyta and allies).
Mosses or bryophyte are non-vascular. They are an important foundation plant for the forest ecosystem and they help prevent erosion by carpeting the forest floor. All bryophyte species reproduce by spores not seeds, never have flowers, and are found growing on the ground, on rocks and on other plants. Stomata are present in mosses and hornworts but absent iin liverworts.
Originally grouped as a single division or phylum, the 24,000 bryophyte species are now grouped in three divisions:
- Mosses (Bryophyta),
- Liverworts (Hepatophyta)
- Hornworts (Anthocerotophyta).
Also included among the non-vascular plants is Chlorophyta, a kind of fresh-water algae.
Vascular Plants with Spores Ferns and allies (Pteridophyta and allies). Unlike mosses, ferns and related species have a vascular system, but like mosses, they reproduce from spores rather than seeds. The ferns are the most plentiful plant division in this group, with 12,000 species. Other divisions (the fern allies) include Club mosses or Lycopods (Lycopodiophyta) with 1,000 species, Horsetails (Equisetophyta) with 40 species and Whisk ferns (Psilophyta) with 3 species.
Vascular Plants with Seeds Conifers and allies (Coniferophyta and allies). Conifers reproduce from seeds, but unlike plants like blueberry bushes or flowers where the fruit or flower surrounds the seed, conifer seeds (usually cones) are ―naked‖. In addition to having cones, conifers are trees or shrubs that never have flowers and that have needle-like leaves. Included among conifers are about 600 species including pines, firs, spruces, cedars, junipers and yew. The conifer allies include three small divisions with fewer than 200 species all together: Ginko (Ginkophyta) made up of a single species, the maidenhair tree; the palm-like Cycads (Cycadophyta) and herb-like plants that bear cones (Genetophyta) such as Mormon tea.
Flowering Plants (Magnoliophyta)
The vast majority of plants (around 230,000) belong to this category, including most trees, shrubs, vines, flowers, fruits, vegetables and legumes. Plants in this category are also called angiosperms. They differ from conifers because they grow their seeds inside an ovary, which is embedded in a flower or fruit.
Thallophyta is a unicellular phylum of plants of very diverse habit and structure. They include algae, fungi and lichens. Thallophyta reproduce vegetatively or by means of asexual spores and exist almost exclusively as gametophytes. The simpler forms, as many blue-green algae, yeasts, etc., are unicellular and reproduce vegetatively or by means of asexual spores; in the higher forms the plant body is a thallus, which may be filamentous or may consist of plates of cells; it is commonly undifferentiated into stem, leaves and roots, and shows no distinct tissue systems; the fronds of many algae, however, are modified to serve many of the functions of the above named organs. Both asexual and sexual reproduction, often of a complex type, occur in these forms. The thallophyta exist almost exclusively as gametophytes, the sporophyte being absent or rudimentary. By those who do not separate the Myxophyta from the thallophyta as a distinct phylum the latter is treated as the lowermost group in the vegetable kingdom.
Green Algae (Chlorophyta)
The green algae is the most diverse group of algae, with more than 7000 species growing in a variety of habitats. Green color is from chlorophyll a and b in the same proportions as the higher plants
Red Algae are red because of the presence of the pigment phycoerythrin; this pigment reflects red light and absorbs blue light. Because blue light penetrates water to a greater depth than light of longer wavelengths, these pigments allow red algae to photosynthesize and live at somewhat greater depths than most other algae. Some rhodophytes have very little phycoerythrin and may appear green or bluish from the chlorophyll and other pigments present in them
Characteristics Red algae have a number of general characteristics that in combination distinguish them from other eukaryotic groups
- Absence of flagella and centrioles.
- Floridean starch as a storage product and the storage of starch in the cytoplasm.
- Phycoerythrin, phycocyanin and allophycocyanin as accessory pigments.
- Unstacked thylakoids in plastids.
- No chloroplast, endoplasmic reticulum.
Brown Algae are the largest type of algae. Brown algae are in the phylum Phaeophyta, which means dusky plants. Brown algae are brown or yellow-brown in color and found in temperate or arctic waters. Brown algae typically have a root-like structure called a “holdfast” to anchor the algae to a surface