Plants are photosynthetic eukaryotes and they are also called embryophytes since they produce an embryo that is protected by tissues of the parent plant. Plants are derived from a single branch of the evolutionary tree and hence said to be monophyletic. As per the fossil evidence, plants were derived from green algae, 500 million years ago. Invading the land was difficult for plants to adapt for several reasons. Hence, plants underwent a number of adaptations like development of roots, stems, leaves and seeds.
Plant Cell Structure
A plant cell contains a polysaccharide cell wall, a central vacuole and chloroplasts, which distinguish them from animal cells. Plant cells also contain mitochondria, a nucleus, Golgi apparatus, endoreticulum membranes, plasma membranes. On plasma membrane and cell wall, there are channels called plasmodesmata connecting neighboring cells for cell-cell communication. Plants are also made of tissues like animals do. There are three basic types of plant tissues, vascular tissues, ground tissue and dermal tissues. Vascular tissues include xylems and phloems, which are responsible for water, solute and organic chemical transportation. Ground tissues include tissue surrounding the vascular tissue. Its main function is for photosynthesis and storage. Dermal tissues cover the surface of a plant. Different tissue types include different types of cells.
Plants absorb nutrients from air and soil. According to the amount the plant requires, the plant nutrients can be classified as macronutrients and micronutrients. Primary macronutrients include nitrogen (N); phosphorus (P), and potassium (K), secondary macronutrients include calcium (Ca), magnesium (Mg), and sulfur (S). There are total 9 micronutrients: boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). Macronutrients are usually not proficient in cultivation soil and need to be supplied in fertilizer. Other ways to obtain nutrients include Nitrogen fixation by bacteria, carnivorous feeding and symbiotic associations with fungi. Natural community recycles nutrients. Root absorbs nutrients from the soil. These nutrients are transported upwards via xylem. The driving force is the evaporation from leaves via guard cells. This process (evaporation of water via guard cells pulls up water from root) is called transpiration. On the contrary to transpiration, leaves make carbohydrates and these need to be supplied to roots and stems. This transportation occurs in phloem in vascular tissue and it is both directions, downwards and upwards.
Root and Stem
A plant body is made up of three major parts: root, leave and stem. Leaves and stems form a shoot. Each part contains the three major tissue types. Root is often under the ground and anchors the plant. From cross section, a typical root contains epidermis, cortex, endodermis and vascular bundles. From longitudinal view, a root contains a root tip which functions in gravity sensing, a zone of cell division, zone of elongation and zone of maturation. The function of a root is for anchorage of the plant, and water and mineral uptake. A stem is the part of the plant from which shoots and buds arise. The function of stem is for support of the plant, for transportation of water, mineral and food. Some stems have storage and reproduction function. Stems have specialized structure celled xylem and phloem for transportation.
Leaves and Flowers
A bud is an undeveloped shoot and normally occurs in the axial of a leaf or at the tip of the stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. A bud can be differentiated into a leaf or a flower. Leaves are the major sites for photosynthesis which provide food for almost all life forms. Upper surface of leaves are covered by cuticular wax, trichomes may be seen on some cells. The lower dermis cells have a specialized type called guard cells which regulate the opening of stomata. Flowers are the reproduction organs for plants.
Plants developed both physical and chemical defense strategy. The physical barriers include cuticle wax on surface of leaves, trichomes, spines and bark. The chemical barrier is secondary metabolites or chemical toxins which could cause problems for the predators, for example, isoprene, phenolic compounds and alkaloids. Plants also developed a systemic response when they are wounded, in such a condition as insects attacking. Upon attacking, plants first synthesize a small peptide called systemin which then triggers a series of biochemical reaction to release proteinase inhibitors which are toxic to insects. Upon infection by plant virus, plants can undergo a hypersensitive response (HR) which leads to programmed cell death on the infected sites. There is a gene-for-gene theory for plant defense against virus. Basically, to every pathogen avirulence (avr) gene, there is a corresponding R gene (resistance gene) in plant to trigger HR. HR is commonly followed by a slower response that leads to systemic acquired resistance (SAR). SAR occurs when a hormone, which may be salicylic acid, travels from the infection site to nearby tissues and triggers the expression of a specific set of genes.
Plants adopt two pathways to reproduce themselves: sexual and asexual reproduction approaches. Sexual Reproduction involves male gametes (sperms) and female gametes (eggs), they combine together to form zygotes. After that, female structures house the embryo during development. Asexual reproduction refers to offspring produced by mitosis and therefore they are genetically identical to parent. Flowers are the reproductive organs for plants. The reproduction process typically includes flower blossom, pollination, seed development and seed maturation.