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.