Growth and Development of an organism

Introduction

One of the fundamental characteristics of living organisms is their ability to grow, i.e., to increase in size and/or cell number. Even monerans and protists undergo growth by multiplication. In higher organisms including plants and animals, growth is accompanied by development, which usually entails the evolution of certain special abilities and functions, and changes in morphological and physiological forms.

Growth

Any sort of irreversible increase in the size of an organism is referred to as growth. Growth is a result of an increment in the number of cells and enlargement of an individual cell.

Types of growth in plants

• Primary growth − The division and elongation of the meristematic cells at the root and shoot apices, leading to increased root and shoot lengths.

• Secondary growth − The division of the lateral meristems (i.e., the vascular cambium and cork cambium), leading to an increase in the width or thickness of some plants.

Characteristics of Growth

• Growth is irreversible, contributing to the increasing complexity of an organism’s form and function

• It includes an increase in the number of cells and the size of the cells

• At the cellular level, growth involves an increase in the amount of protoplasm in cells

• Growth may be determinate (i.e., may stop after some time), or indeterminate (i.e., growth continues throughout life)

• Growth is measurable

Development

Development refers to the formation of a complex multicellular organism from a single cell. It entails differentiation of the zygote into different cell types, morphogenesis, development of functional, metabolic, and physiological capabilities, and environmental integration.

Stages of development in humans

• Infancy − (from birth to 1 year old): The infant undergoes important developments and must be provided with ample nourishment by the mother

• Childhood − (1 to 10 years): characterised by cognitive, physical, motor, and emotional developments.

• Adolescence − (11 -17 years): marked by various transformations, including rapid physical, psychosocial and bio-behavioural developments. The most important development is that of puberty, i.e., attainment of sexual maturity

• Adulthood − begins from 18 years of age, characterised by the completion of sexual maturity and cognitive development.

• Middle age − (40-60 years). Generally characterised by declining body functioning and decreased physical strength, and an overall decline in the functioning of the CNS, eventually leading to old age.

• Old age − (60 and above): aka senescence, the final stage in the lifetime of a human, characterised by the overall diminishing of an individual's cognitive, emotional, sensory, perceptual, and physical abilities

Stages of development in plants

• The vegetative stage − This stage involves the development of the plant from the embryo. The radicle and the plumule develop into the roots and the shoot respectively. Leaves grow and perform photosynthesis.

• The reproductive stage − during this stage the plant develops its inflorescence, to receive the pollen during pollination. The reproductive stage ends with fertilization. and the eventual formation of a fruit with seeds inside it

• Senescence − This stage is the final step in the development of a plant, marked by extreme changes in the leaves including degradation of chlorophyll and redistribution of nutrients.

Areas of Development

Quantitative and Qualitative development

Quantitative development refers to the changes in size and the number of cells in an organism. Qualitative development refers to the changes in the constitution and nature of an organism. For example, the growth of a neonate into an adult is accompanied by a plethora of developments in functional abilities and morphological characteristics.

Structural and Functional Development

The beginning of a single-celled zygote and the gradual but complex development into a fully functioning multicellular organism is a result of various kinds of structural and functional developments. Structural development involves the transformation of the cells of the body via the processes of cell commitment and differentiation.

Progressive and Regressive Development

Development is usually associated with increasing complexities in the body organisation and higher functional capabilities, known as progressive development. However, development can also be regressive, entailing the loss of certain functional body parts, or a size reduction. Progressive and regressive development often occur simultaneously during the growth of an organism.

Factors affecting Growth and Development

• Heredity − An individual organism inherits the majority of its form and function from its parents. It not only influences the morphological characteristics but also the metabolic and functional capacities and health of an organism.

• Hormones − These chemicals influence different areas of development in both plants, and animals. For example, the growth hormone in humans stimulates growth, the follicle-stimulating hormone and testosterone influence reproductive abilities, etc. In plants, hormones such as auxins, gibberellins, cytokinins and abscisic acid are essential for ensuring optimal growth and development.

• Environment − An organism's physical environment greatly influences its growth and functioning. Nutrients, water, temperature and climate conditions, atmospheric pressure, presence of pollutants and teratogens, etc affect the growth rate and course of development.

Laws of Growth and Development

• Cephalocaudal principle − Growth and development occur along the top-bottom axis, i.e., from the head and downwards. The brain develops first, being the control centre of the body. As observed distinctly in foetuses of vertebrates, the head develops first and is very prominent, and eventually, the hand and leg buds are also formed. Moreover, an infant first learns to control its head, then gains control of its hands and eventually learns to crawl, stand and walk.

• Proximodistal principle − Development occurs from the centre of the body in an outward manner. Hence, the spinal cord develops first, followed by the development of the arms, hands, and finally the fingers. Moreover, a child learns to control his shoulders and hands first and eventually learns to hold things with his fingers.

• Development is a continuous phenomenon, from the birth to the death of an organism

• Growth and development always occur sequentially. For example, a human always develops from the fetus through infancy, childhood, adolescence adulthood, and eventually reaches old age.

Conclusion

• An increase in the size and the number of cells in an organism is termed growth. In plants, growth is of two types- primary and secondary

• Development refers to the transformation of a single-celled zygote into a complex and fully functional organism. It involves quantitative, qualitative, structural, functional, progressive and regressive changes.

• Growth and development are continuous and sequential and always occur from the top-bottom and from the proximal to distal axes of the body. They are affected by intrinsic factors, including heredity and hormones, and environmental factors

FAQs

Q1. What happens when the growth hormone is under or over-secreted?

Ans. Hyposecretion of growth hormone leads to pituitary dwarfism, while its hypersecretion causes gigantism. In adults, hypersecretion causes acromegaly.

Q2. In which group of plants is secondary growth absent?

Ans. Herbaceous plants lack lateral meristems. Hence, secondary growth is absent in such plants.

Q3. What is the main difference between angiosperms and animal development?

Ans. Gastrulation and the haplodiplontic life cycle are characteristic of angiosperms development which does not occur in animals

Q4. What is cell commitment?

Ans. Cell commitment refers to the dedication to a particular specialised fate, it includes cell specification, cell determination and cell differentiation.