Plantae are multicellular organisms which produce food by the process of photosynthesis. This kingdom includes organisms that range from a tiny moss to a giant tree. All plants are multicellular and eukaryotic. One of the characteristic features of plants is the presence of pigments like chlorophyll a and b and carotenoids which aid to convert sunlight into chemical energy by the process of photosynthesis.
Plants are said to have evolved from algae as they formed colonies on empty landmass. Some plant fossils suggest that plants would have appeared about 400 million years ago. They are said to have evolved from green algae and invaded the vacant land. The earliest fossils of vascular plants are about 370 million years old. Flowering plants are said to have evolved about 130 million years ago.
- Colonization of algae on land would have occurred after there was an intact ozone layer to protect the plants from harmful rays of the sun.
- Plants had to evolve and develop systems to prevent desiccation in the air, to absorb nutrients from the soil and to grow erect without support and also to reproduce on land.
- These adaptation during the course of evolution are acquired by the modification of existing structures and habits. The first plants to evolve on land were small mosses, liverworts.
- Plants like the ferns, pteridophytes are said to be the common ancestors of today's vascular plants.
- The next evolution was the development of seed plants. The first seed plants were the gymnosperms like conifers, cycads, etc. These forests dominated the late Pleaozoic era.
- Here after, evolution presented trees with flowers and seeds that were protected in fruits.
- Adaptations to different habitats have resulted in different types of flowering plants.
- Also, there is evolution and development of intricate symbiotic relationships between fungi, bacteria and insects, which relationships if disturbed may be a threat to the survival of the plants.
The most familiar plants to us are multicellular land plants, are known as embryophytes. Embryophytes include plants like ferns, mosses, liverworts, conifers and flowering plants. All these plants are eukaryotic and the cell walls are made of cellulose. They obtain their energy through photosynthesis.
The embryophyta is also known as metaphyta which includes liverworts, mosses, ferns, gymnosperms and angiosperms. They are known as land plants and they live in terrestrial habitats. All the metaphytes are multicellular, eukaryotes and have specialized reproductive organs.
Paleobotany is derived from Greek where 'paleo' means 'old' and 'botany' is the 'study of plants'. The branch paleobotany deals with the recovery and identification of plant remains from the geological framework. This includes the study of terrestrial plant fossils and also study of prehistoric marine autotrophs. This study is important as it permits the reconstruction of ancient ecological systems and climate and it is fundamental for the study of green plant development and evolution.
Plant systematics in practice involves relationships between plants and their evolutionary process at higher levels. Plant systematics considers the origin and diversification of land plants. It also explains the taxonomy, evolution, and the phylogenetics of plants. Taxonomy deals with the identification, nomenclature, classification of plants, evolution explains the speciation, reproductive biology, adaptation and biogeography; and phylogenetics deals with molecules, phenetics, and morphology. Plant taxonomy deals with the science that identifies, describes, classifies and names plants which makes it one of the main concept of taxonomy.
Diversity of plants is important for the environment in general and is also useful as an economical social resource. Plant diversity is considered at different levels using different criteria. Phenotypic variation, genotypic variation, and variation at the DNA levels is considered for the investigation of plant diversity. More than about 300,000 species of plants have been described.
Continuous analysis and identification of new species is being done using DNA based analysis. Evolutionary relationships are explained by these approaches. This also allows identification of genetic relationships between different groups of plants. Deducing evolutionary relationships is important in plant conversation and plant improvement. Diversity in the population of plant species can be considered as a primary level of variation. Reproductive processes determine plant diversity.
The life cycle of plants has an alteration between haploid and diploid generations. Plants are divided into two groups:
- Non-vascular plants - Non-vascular plants that are living include bryophytes like liverworts, mosses and hornworts.
- Vascular plants - Vascular plants are common plants like pines, corns, ferns, and oaks.
The process of photosynthesis carried out by plants and algae are the primary source of energy and organic material to all ecosystems. Photosynthesis has resulted in changing the early Earth's atmosphere, which is now with 21% oxygen. All animals and most of other organisms are aerobic and depend on oxygen. Plants are the primary producers in a food web and are the basis of all ecosystems. Many animals depend on plants for shelter, oxygen and food.
Land plants play a key role in water cycle and many other biogeochemical cycles. Some plants can also fix atmospheric nitrogen by means of nitrogen fixing bacteria making plants an important part of the nitrogen cycle. Roots of plants are important to soil development and prevention of soil erosion.
Many animals have coevolved with plants. Many animals like insects pollinate flower in turn receiving food in the form of pollen or nectar or some animals aid in seed dispersal by eating fruits. Plants also provide home, food to many animals.
Plants have mutualistic relationships with many fungal species. They also have parasites like the mycorrhizal fungi and epiparasites. Epiphytes are plants that grow on other plants without being parasites. There are about 600 plants that are carnivorous which trap small animals for nutrition.
Plant morphology deals with the study of the external plant structures and their physical form. Plant morphology is applied in visual identification of plants. It is the study of differnt plant parts, its forms and features like that of roots, stems, leaves flowers, fruits, seeds, etc.
A typical angiosperm plant is differentiated into:
- An underground roost system - The roots, stems and leave form the vegetative parts while the flowers are the reproductive plant parts.
- An aerial shoot system - The shoot system consists of stem, branches, leaves, flowers and fruits.
Plant physiology deals with the functioning and the physiology of plants. Plant processes like photosynthesis, respiration, plant nutrition, function of plant hormones, tropism, nastic movements, photoperiodism, circadian rhythms, seed germination, dormancy, transpiration, plant water relations, etc. are studied by plant physiologists.
Photosynthesis - Plants are photosynthetic which means that they can prepare their own food by the energy obtained from sun. The chlorophyll in the chloroplasts of the plant can capture light energy.
Vascular System - The vascular system of plants help in transport of nutrients from their different parts through structures called xylem, phloem. Longs root system of these plant are take up water and minerals. The xylem tissue moves water and mineral salts from roots to the other parts of plants. the phloem tissue transport sugars produced by the leaves to other plant parts and roots.
Plant cells are eukaryotic cells and are distinguished from other eukaryotic cells having cell wall which is rigid and made of cellulose, hemicellulose and pectin; and also they have a large water-filled central vacuole and chloroplasts. During the process of cell division there is development of a phragmoplast for the construction of cell plate during the later stage of cytokinesis. Plant cells also develop and differentiate into numerous cell types, the cells can differentiate into protective, storage, vascular, reproductive tissues.
The study of plant diseases is also known as plant pathology or phytology which deals with the scientific study of diseases caused by pathogens or infectious organisms and also due to environmental conditions. Organisms that cause infections are fungi, bacteria, viruses etc. Plant is said to be diseased when it is disturbed continuously by some casual agent that results in abnormal physiological process that disrupts the plant normal structure, growth, function or other activities. This might interfere with more than one plant's biochemical systems that give rise to pathological symptoms.
Plants are essential to maintain the nature's balance and also human life. Plants are the backbone of all life and are essential to human beings.
- Food - Food we consume comes directly or indirectly from plants.
- Air - Oxygen is the byproduct of photosynthesis, which is essential for our survival.
- Water - Plants play an important role in water cycle, they help water from the soil to atmosphere through transpiration.
- Habitat - Almost all organisms depend on plants for food and shelter.
- Climate - Plants utilize the carbon and help to keep carbon gases of the atmosphere in check.
- Medicine - Many of the drugs used come from plant derivatives. Most of the people rely on plant derivatives for personal and healthcare.