Growth and Development notes

Meaning and Features of Growth

Growth is an irreversible increase in the size or dry mass of an organism or one of its parts. It results from cell division, cell enlargement and cell differentiation. Development is a wider process that includes growth, differentiation and maturation.

In plants, growth usually continues throughout life because meristems remain active. In animals, most growth stops after maturity, although some tissues can continue to repair and regenerate.

• Growth may be measured by increase in length, area, volume, fresh mass, dry mass or number of cells.
• Cell division increases the number of cells, while cell enlargement increases their size.
• Differentiation is the development of cells into specialised types with specific structures and functions.
• Development includes the complete sequence from the beginning of life to maturity and ageing.
• Plant growth is generally indeterminate because root and shoot apical meristems remain active.
• Animal growth is usually determinate because it normally stops when the adult size is reached.
• The cells of the body are generally genetically identical, except for reproductive cells, which are formed through meiosis and have different genetic combinations.

Plant Growth Regions and Tissues

Plant growth occurs mainly in meristems. Meristematic cells are small, actively dividing cells with dense cytoplasm, a prominent nucleus and thin primary cell walls. They produce new cells that later enlarge and differentiate.

The root tip has different regions arranged in a definite order. The root cap protects the delicate tip. Behind it is the region of cell division, followed by the region of elongation and then the region of maturation.

The region of maturation lies behind the region of elongation. Root hairs develop from epidermal cells in the region of maturation. They increase the surface area for absorption of water and mineral salts.

• Apical meristems occur at the tips of roots and shoots and increase the length of the plant.
• Lateral meristems, such as vascular cambium and cork cambium, increase the thickness or girth of stems and roots.
• The root cap protects the root apical meristem as the root pushes through soil.
• The region of cell division contains actively dividing meristematic cells.
• The region of elongation increases the length of the root because cells enlarge rapidly.
• The region of maturation contains differentiated tissues and lies behind the region of elongation.
• Root hairs develop from the region of maturation.
• The pericycle of roots produces lateral roots.

Plant Growth Regulators

Phytohormones are chemical regulators synthesised by plants. They are produced in small quantities and influence physiological processes such as growth, dormancy, flowering, fruit development and leaf fall. Their effects depend on concentration, tissue type and environmental conditions.

Auxins promote cell elongation, especially in shoots, and are involved in apical dominance, phototropism, root initiation and fruit development. Auxin movement is largely basipetal, meaning movement from the shoot apex towards the base, although movement in roots can also occur in the opposite direction.

Gibberellins promote cell elongation and seed germination. Cytokinins stimulate cell division and delay ageing of plant parts. Abscisic acid generally inhibits growth and promotes dormancy and stomatal closure.

• Auxins are involved in cell elongation, apical dominance, phototropism and root formation.
• The movement of auxin is largely basipetal in shoots.
• Gibberellins promote seed germination and stem elongation.
• Cytokinins promote cell division and delay senescence of leaves and other plant parts.
• Coconut milk contains a factor with cytokinin activity.
• Abscisic acid causes stomatal closure and promotes seed and bud dormancy.
• Abscisic acid is a derivative of carotenoids.
• Phytohormones are regulators synthesised by plants and influencing physiological processes.

Plant Growth Responses and Movements

Plant growth is affected by light, temperature, water, minerals and gravity. Light intensity can change the rate of photosynthesis and growth. The quality or wavelength of light also affects growth because different pigments absorb different wavelengths.

Phytochrome is a pigment-protein system that absorbs red and far-red light. It helps plants respond to day length and influences seed germination, flowering and other developmental processes. Vernalization is the promotion of flowering by exposure to low temperature.

Plant movements may be growth movements or changes in turgor. Tropic movements are directional responses to a stimulus, whereas nastic movements do not depend on the direction of the stimulus. Paratonic movements are responses caused by external stimuli.

• Apical dominance is the common correlation in which the apical bud suppresses the growth of lateral buds, mainly through auxin.
• Light intensity affects the growth rate of plants.
• Light quality also influences growth rate, so the statement that quality of light has no influence is incorrect.
• Phytochrome absorbs red and far-red light.
• Vernalization stimulates flowering in plants such as carrot.
• The closure of the lid of a pitcher in a pitcher plant is a paratonic movement.
• Tropic movements are directional, while nastic movements are not dependent on stimulus direction.
• Phototropism is growth in response to light, and geotropism is growth in response to gravity.

Differentiation, Grafting and Permanent Tissues

Differentiation is the process by which unspecialised cells become specialised in structure and function. In plants, meristematic cells differentiate into tissues such as epidermis, parenchyma, collenchyma, sclerenchyma and vascular tissues.

Sclereids are specialised sclerenchymatous cells. They have thick, lignified walls and are usually dead at maturity. Their hard cells occur in seed coats, nut shells, pear pulp and other protective tissues.

Grafting joins a scion, which is the upper shoot portion, to a stock, which is the rooted lower portion. Successful union requires the cambial regions to be placed together.

• Sclereids are usually dead cells at maturity, not groups of living cells.
• Sclereids have thick, lignified secondary walls and provide mechanical protection.
• Callus is a mass of undifferentiated or loosely organised cells formed at a wound.
• In grafting, formation of callus is the first step in union between stock and scion.
• After callus formation, new vascular connections develop between the stock and scion.
• The stock supplies the root system, while the scion supplies the desired shoot system.
• Successful grafting depends on proper contact between the cambium of the stock and the cambium of the scion.

Embryonic Development in Animals

Embryonic development begins after fertilisation. The zygote undergoes repeated mitotic divisions without a major increase in total size. These divisions are called cleavage and produce smaller cells called blastomeres.

The main sequence is zygote, cleavage, morula, blastula, gastrula and organogenesis. The morula is a solid ball of cells. The blastula is a hollow or partly hollow structure with a cavity. During gastrulation, cells move and rearrange to form the germ layers.

• The sequence of animal embryonic development is zygote, cleavage, morula, blastula, gastrula and organogenesis.
• Cleavage produces many blastomeres and causes cytoplasmic localization.
• Cytoplasmic localization means that different regions of the cytoplasm become distributed among different daughter cells during cleavage.
• The morula is a solid mass of cells formed after cleavage.
• The blastula contains a cavity called the blastocoel.
• Gastrulation is the stage in which the germ layers are formed.
• Organogenesis is the formation of organs from the germ layers.

Germ Layers, Chick Embryo and Extra-Embryonic Membranes

Gastrulation forms the primary germ layers: ectoderm, mesoderm and endoderm. These layers later differentiate into all tissues and organs of the body. The ectoderm generally forms the outer covering and nervous system, while the endoderm forms the lining of the digestive tract and associated organs.

The mesoderm forms connective tissues, muscles, blood, skeleton and many internal organs. In a chick embryo, the mesoderm becomes organised into structures including somites. Somites arise from dorsal mesoderm and contribute to the vertebral column, skeletal muscles and dermis.

Extra-embryonic membranes protect and support the embryo. Amnion surrounds the embryo with protective fluid, while chorion participates in exchange and protection. These membranes first appeared in reptiles, allowing reproduction away from water.

• Ectoderm, mesoderm and endoderm are the three primary germ layers.
• Germ layers are formed during gastrulation.
• Somites in a chick embryo are formed from dorsal mesoderm.
• Somites contribute to vertebrae, skeletal muscles and part of the dermis.
• The amnion surrounds the embryo and contains amniotic fluid.
• The chorion is an outer extra-embryonic membrane involved in protection and exchange.
• Extra-embryonic membranes such as amnion and chorion first appeared in reptiles.
• Reptile embryos can develop on land because these membranes reduce dependence on an aquatic environment.

Experimental Development and Regeneration

Regeneration is the replacement of lost or damaged body parts. It may involve simple repair of tissues or the formation of a complete structure. Its extent differs among organisms and depends on the type of tissue and the developmental ability of its cells.

Acetabularia is a single-celled alga used to study the role of the nucleus in development. Its nucleus is located in the rhizoid, while the stalk and cap are above it. Different species form different cap shapes, showing that hereditary information in the nucleus controls development.

In Acetabularia crenulata, the cap is irregularly shaped. Regeneration of a removed cap demonstrates that the cell can reform structures under the control of its nucleus.

• Regeneration is the restoration of lost or damaged cells, tissues or organs.
• Repair replaces damaged tissue, while true regeneration may restore a complete body part.
• The nucleus contains genetic information that controls development and regeneration.
• Acetabularia is a unicellular alga used in developmental experiments.
• The nucleus of Acetabularia lies in the rhizoid.
• Acetabularia crenulata has an irregularly shaped cap.
• Different cap forms in Acetabularia species show the role of nuclear hereditary information.

Ageing and Developmental Abnormalities

Ageing, or senescence, is the gradual decline in the structure and function of cells and tissues with advancing age. It may involve reduced cell division, accumulation of cellular damage, reduced repair and changes in metabolism.

In plants, cytokinin delays senescence and keeps leaves green for a longer time. In animals, ageing affects many systems, including the nervous, muscular, reproductive and immune systems. Genetic defects may disturb normal development.

Microcephaly is a condition in which the skull and brain are smaller than normal. It can result from genetic defects and may be associated with impaired brain development.

• Senescence is the natural deterioration of cells, tissues or organs with age.
• Cytokinin is the plant regulator that retards ageing or senescence of plant parts.
• Ageing may involve reduced regenerative ability and gradual loss of physiological efficiency.
• Microcephaly means an abnormally small skull, usually associated with reduced brain growth.
• Microcephaly may be caused by a genetic defect.
• Growth, differentiation and development are controlled by both genetic information and environmental conditions.