Development, Sequence of Development, Plasticity, Heterophylly, Factors Affecting Plant Development, Practice Problems and FAQs

Have you ever wondered how all the living organisms in the world change from their tiny new-born forms to the mature forms? The changes occur not just in terms of the physical body, but the mental ability of certain organisms also excel with time. 

You must be aware that all living organisms start their life as a single cell. Then how do multicellular organisms like us, like the tall oak trees, etc., turn into these highly organised biological systems composed of millions of specialised cells, tissues, organs and organ systems? In fact, our entire life cycle starting from birth to death involves several changes in our forms, size, skills, cognition, physiological and metabolic functions, etc.

The sequence of changes that occur in the structure and functioning of an organism, organ, tissue or cell, involving its formation, growth, differentiation, maturation, reproduction, senescence (ageing) death is termed as development. In fact, the entire lifecycle of living organisms is an attribute of this process.

In plants, the germination of seeds to seedlings and their gradual conversion to adult plants also involves several changes. To know more about it, let's dive into the topic of plant development.

Table of contents

• Plant development
• Sequence of plant development 
• Factors affecting plant development
• Practice problems
• FAQs

Plant development

Changes that occur in a plant from germination of seed to plant senescence is known as plant development. A plant goes through developmental stages of seed germination, seedling formation, juvenile phase, maturation, flowering, seed formation and senescence or ageing. Conversion of one phase to another is also considered to be development, e.g, leaf initiation to lef expansion, appearance of chloroplasts when a plant is exposed to sunlight, conversion from vegetative stage to flowering stage, etc.

Development and growth are sometimes used interchangeably; however, these two are completely different processes botanically. Sum of growth and differentiation is considered to be development.

Sequence of plant development 

Cell division, cell elongation, differentiation, maturation, and other phases are all included in the process of development. The promeristems undergo plasmatic growth in which the size of the plant cells increases. Fully grown cells undergo differentiation to form primary meristems. Cell division takes place in the primary meristem which results in cell elongation and expansion. Primary meristem differentiates to produce primary tissue. 

The cells of primary tissue are mature and have lost the ability to divide. These cells undergo dedifferentiation (regain the ability to divide) and produce secondary meristems. The secondary meristems again lose their ability to divide (dedifferentiation) and mature to perform specific functions. The tissue formed is known as secondary tissue. The secondary tissue ultimately dies due to ageing (senescence).

Fig: Sequence of plant development

Plasticity

Plants have the ability to alter their development in response to environmental factors (light quality, light intensity, temperature, light photoperiod, water depth, water flow etc.) and different phases of life to form different structures. This ability of the plant is termed as plasticity. An example of plasticity is heterophylly.

Heterophylly

During different phases of life or in response to different environmental stimuli, some plants have the ability to produce different leaf forms. This is known as heterophylly. 

In many plants, the early vegetative or juvenile phase has a different morphology, greater apical growth, rapid growth rate and delayed senescence. This juvenile phase is followed by an adult vegetative stage and then an adult reproductive stage. The juvenile stages can vary from the mature stages in the forms of their leaves. This phenomenon is known as developmental heterophylly and is shown by cotton, coriander and larkspur.

Fig: Developmental heterophylly in coriander

Environmental heterophylly is shown by buttercup (Ranunculus flabellaris) as the leaves of aquatic and aerial habitats vary in shape.

Fig: Environmental heterophylly in buttercup

In Limnophila heterophylla the submerged leaves look different compared to the floating terrestrial ones. Leaf shapes are related to their function, as submerged leaves are thin, narrow, and lack cuticles and stomata, whereas terrestrial leaves are thicker, expanded, and cutinized with stomata.

Fig: Environmental heterophylly in Limnophila heterophylla

In the American white water lily whose scientific name is Nymphaea odorata, 2 kinds of leaves are produced by the plant - leaves that either float on the surface of the water or aerial leaves that are are held above the water's surface. Over the course of a growing season, the predominant leaf form switches from surface leaves in the early season to aerial leaves in the midseason and then back to surface leaves at season's end. 

Fig: Environmental heterophylly in Nymphaea odorata

Factors affecting plant development

Plant development is affected by two types of factors:

• Intrinsic factors
• Extrinsic factors

Intrinsic factors

Intrinsic factors include the intercellular (Plant growth regulators) and intracellular (genetic) factors affecting the plant development.

Extrinsic factors

Extrinsic factors include all the environmental factors affecting plant development. Some of the extrinsic factors are 

1. Light 
2. Temperature
3. Oxygen
4. Water
5. Nutrients

Practice Problems

Q1. Assertion: Sum of growth and differentiation is considered to be development.

Reason: Plants only develop by cell division.

A. Both assertion and reason are true, the reason gives the correct explanation of the assertion.
B. Both assertion and reason are true, the reason does not provide the correct explanation of the assertion.
C. Both assertion and reason are incorrect.
D. Assertion is correct and reason is incorrect.

Solution: Plant development refers to the changes that occur in a plant from germination to senescence. Growth, differentiation, and development are all strongly linked in the life of a plant, hence development is defined as the total of growth and differentiation in plants. The development process in plants include cell division, cell elongation, differentiation, maturation, and other stages. Hence, option d is correct.

Q2. Which of the following shows the correct sequence of tissues formed in plant development
A. Primary meristem→Secondary growth→Secondary meristem
B.  Primary meristem→Secondary meristem→Secondary tissue
C. Primary meristem→Secondary meristem→Secondary tissue
D. Primary meristem→Secondary meristem→Primary tissue

Solution: The development in plants starts with promeristem which undergoes plasmatic growth which gives rise to primary meristem. The cells of the primary meristem undergo cell division causing cell elongation and expansion to produce primary tissue. The primary tissue regains the ability to divide again (dedifferentiation) and produces secondary meristem. Secondary growth and redifferentiation occurs in the secondary meristem which tends the cells to mature, losing their strength to divide. This forms secondary tissue. The secondary tissue ultimately dies through senescence. Hence, option c is correct.

Q3. Choose the correct statement from the following.

A. Plant development depends only on intrinsic factors.
B. Intrinsic factors include the environmental factors.
C. Plant growth regulators are intrinsic factors.
D. Light is an intrinsic factor.

a. I
b. I and II
c. III
d. II and III

Solution: The development in plants depends on the intrinsic and the extrinsic factors. All environmental elements that affect plant development are classified as extrinsic factors. Intercellular (Plant growth regulators) and intracellular (genetic) factors regulating plant development are examples of intrinsic factors. Hence, option c is correct.

Q4. Why are plant growth regulators intrinsic factors in plant development?
Solution: The two types of elements that serve to regulate plant development are intrinsic factors (internal) and extrinsic factors (external). Plant growth regulators are factors that can be created and controlled by the plant itself and operate as intrinsic intercellular regulators of plant growth and development.

FAQs

Question 1. What are plant growth regulators?
Answer: Plant growth regulators are organic substances, excluding nutrients, which in very low concentrations act as hormones that lead to promotion, inhibition or modification of growth by influencing various physiological activities. Natural plant growth regulators are broadly grouped under five classes - auxins, gibberellins, cytokinins, ethylene and abscisic acid.

Question 2. When do differentiation and redifferentiation occur in plants?
Answer: Differentiation occurs when cells emerging from primary meristematic cells lose their ability to divide and specialise into cells of specific tissues with specific structure and function. This occurs during the formation of the primary cortex, primary xylem and primary phloem.

When differentiated cells become meristematic again by dedifferentiation, they form secondary meristematic tissues such as vascular cambium and cork cambium which divide and form cells which permanently lose their ability to divide and differentiate again to form cells of the secondary tissues. This is known as redifferentiation. Redifferentiation occurs during the formation of the secondary xylem, secondary phloem and secondary cortex.

Question 3. Name two secondary meristematic tissues in plants.
Answer: The cork cambium that arises from the cells of the pericycle and the interfascicular vascular cambium that arise from the cells of the medullary rays by dedifferentiation, during secondary growth of a plant, are examples of secondary meristematic tissues in plants.

Question 4. Do humans also show plasticity?
Answer:  in humans, a given genotype can produce different phenotypes in response to different environments. This ability is termed as plasticity. It increases the adaptability of humans to different environmental conditions.

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