Plant Physiology: Relationships and Reproduction
Plant Relationships and Stimuli Responses
Plants can receive and transmit stimuli from the external environment and have the capacity for movement. The regulation of a plant’s physiological activities depends on many internal and external factors. Internal factors are mainly plant hormones or phytohormones, while external factors include the length of day and night.
Plant Hormones
A hormone is a chemical produced by specialized cells, which acts on other cells of the individual, away from the hormone’s production site. Plant hormones, or phytohormones, are substances that, in low concentrations, promote, inhibit, or modify plant development.
Tropism
Tropisms are continued growth movements that involve the curvature of plants in response to an external, unilateral stimulus. It is considered positive tropism when the response is a movement towards the stimulus, and negative tropism otherwise.
Types of Tropisms
- Phototropism: Plant response to light variation. It is manifested by a curvature of the plant. Stems exhibit positive phototropism, while roots exhibit negative phototropism.
- Hydrotropism: Plant response to a water-based stimulus. Roots exhibit positive hydrotropism.
- Thigmotropism: Response to stimuli from touch.
- Chemotropism: Plant response to the presence of chemicals.
- Geotropism or Gravitropism: Response to gravitational stimuli. Stems have negative geotropism, and roots have positive geotropism.
Nasties
Nastic movements in plants are responses where the direction of the stimulus determines the movement. Nasties are rapid and reversible changes that may be due to different types of stimuli: light, temperature, contact, etc.
- Photonasties and Thermonasties: Responses of floral structures (petals) to stimuli of light and ambient heat.
- Thigmonasties: Stimulus from rubbing the leaves of some plants.
Plant Reproduction: Asexual and Sexual
Plants have two basic forms of reproduction: asexual and sexual.
- Asexual Reproduction: Involves only one parent plant, using various methods to reproduce: rhizomes, tubers, bulbs, and stolons.
- Sexual Reproduction: This type of reproduction involves two individuals, each contributing a gamete.
Reproduction in Seed-Bearing Plants
The reproductive function is located in a specialized structure called the flower. Floral parts may be fertile or sterile. A flower consists of a peduncle, whose enlarged end forms the thalamus or floral receptacle, where a series of pieces are inserted.
Fertile (Sexual) Parts
- Androecium: This is the male sexual organ of the flower. It consists of the stamens, each comprising a filament and a widened terminal part, the anther. Generally, the anther is divided into two units called thecae. Each theca has a pair of pollen sacs filled with microspores that give rise to pollen grains.
- Gynoecium: This is the female sexual organ. It consists of one or more carpels. Shaped like small bottles, they have three parts:
- Ovary: This is the most widened part at the base. Inside, there are the ovules, which form the female gametophyte.
- Style: This is a tubular-shaped piece that connects the stigma to the ovary.
- Stigma: This is the opening where the pollen grains are deposited.