Stimuli, Responses, and Reproduction in Plants and Animals

Stimuli and Responses in Living Things

How Living Things Respond to Stimuli

Behavior is the sequence of actions triggered by a stimulus that causes a response. The organs producing a response are called effectors.

Movement: Predominantly in animals, involving muscle contractions.

Production of Substances: Glands produce secretions like sweat, saliva, and tears.

Responses in Plants

Tropisms: Changes in the direction of plant growth.

  • Positive Tropism: Growth towards the stimulus.
  • Negative Tropism: Growth away from the stimulus.

Types of Tropisms:

  • Phototropism (Light): Stems and leaves exhibit positive phototropism, while roots exhibit negative phototropism.
  • Geotropism (Gravity): Roots exhibit positive geotropism, while stems exhibit negative geotropism.
  • Hydrotropism (Moisture): Roots exhibit positive hydrotropism.
  • Haptotropism (Touch): Tendrils of plants coil around objects they contact.

Nasties: Small movements of a plant part.

  • Photonasty (Light): Flowers open and close in response to light changes.
  • Haptonasty (Touch): Triggered by contact, such as in insectivorous plants.

Stimuli Detection in Animals

Animals use receptors, often grouped into organs, to detect stimuli.

Chemical Stimuli: Detected by chemoreceptors, either by direct contact or through air or water.

Light Stimuli: Detected by photoreceptors, often located in eyes. Eye types include:

  • Camera Eye: Similar to a camera.
  • Compound Eye: Composed of many small facets.

Thermal Stimuli: Detect temperature variations, often scattered throughout the body. Some snakes have specialized pit organs for detecting heat.

Mechanical Stimuli: Sensitive to touch, pressure, and vibrations transmitted through air or water. Examples include:

  • Hearing in Vertebrates: Detects sound waves.
  • Lateral Line in Fish: Detects vibrations and water currents.

Communication Between Receptor and Effector

Nervous System: Sorts and interprets stimuli, directing responses. Components include:

  • Nerve Centers: Receive messages from receptors and formulate responses.
  • Nerves: Connect nerve centers with sensory and motor organs.

Hormonal System: Regulates internal body activities through hormones.

Reproduction

Asexual Reproduction

A simple and efficient process in stable environments, but offers limited adaptability to change.

In Unicellular Organisms:

  • Binary Fission: Cell nucleus and cytoplasm divide, creating two equal daughter cells.
  • Budding: A smaller bud forms and separates, growing into a new organism.
  • Sporulation: Multiple spores are formed and released, each capable of developing into a new organism.

In Animals:

  • Fragmentation: The animal splits into fragments, each regenerating a complete individual.
  • Budding: A multicellular bud forms and develops into a new individual.

In Plants and Fungi:

  • Vegetative Reproduction: A part of the parent plant generates a new plant. This can occur through tubers, bulbs, or runners.
  • Spore Formation: Spores are formed in sporangia, released, and germinate into new individuals.

Sexual Reproduction

More complex but generates genetic diversity, enhancing adaptability.

Fertilization Types:

  • External Fertilization: Eggs and sperm are released into the external environment.
  • Internal Fertilization: Sperm is deposited inside the female’s body.

The fusion of sperm and egg creates a zygote, which develops into an embryo.

Embryo Development:

  • Oviparous: Embryo develops inside an egg outside the mother’s body.
  • Viviparous: Embryo develops inside the mother’s body, receiving nourishment from her.
  • Ovoviviparous: Embryo develops inside an egg within the mother’s body, relying on egg yolk for nourishment.

Offspring Development:

  • Direct Development: Young resemble adults at birth.
  • Indirect Development: Young undergo metamorphosis, with larval stages distinct from the adult form.
    • Simple Metamorphosis: Nymphs gradually develop into adults through molting.
    • Complex Metamorphosis: Larvae transform into pupae before becoming adults.

Sexual Reproduction in Seed Plants

Pollination: Transfer of pollen from anther to pistil.

  • Anemophily: Wind-mediated pollination.
  • Entomophily: Insect-mediated pollination.

Fertilization: Pollen grain reaches the ovary, leading to double fertilization.

  • One male gamete fuses with the egg cell to form the embryo.
  • The other male gamete fuses with other cells to form the endosperm, providing nutrients.

The pistil develops into the fruit, containing the seed.

Germination: Seed sprouts under suitable conditions.

Advantages of Reproduction:

  • Colonization of New Territories: Dispersal of offspring.
  • Survival in Adverse Conditions: Spores and zygotes can remain dormant.