Plant Tropisms, Nasties, and Reproductive Strategies
Types of Tropisms
Tropisms are directional growth responses of a plant to environmental stimuli.
Phototropism
Response of a plant to light variation. It is manifested by a curvature of the plant. Stems exhibit positive phototropism (growing towards light), and roots exhibit negative phototropism (growing away from light).
Hydrotropism
Response of a plant to a stimulus based on water. Roots show positive hydrotropism, growing towards water sources.
Thigmotropism
Response to stimuli from touch or physical contact.
Chemotropism
Response of plants to the presence of chemicals.
Gravitropism or Geotropism
Response to stimuli of gravitational origin. Stems have negative geotropism (growing upwards against gravity), and roots have positive geotropism (growing downwards with gravity).
Nastic Movements
Nastic movements are non-directional responses of plants to stimuli. These are rapid and reversible changes that may be due to different types of stimuli, such as light, temperature, or contact.
Photonasties and Thermonasties
Responses of floral structures (petals) to stimuli of light and ambient heat.
Thigmonasties
Stimulation by friction in the leaves of some plants.
Plant Reproduction
Plants have two basic forms of reproduction: sexual and asexual reproduction.
Asexual Reproduction
Involves only one parent. Plants use various ways to reproduce asexually, including rhizomes, tubers, bulbs, and stolons.
Sexual Reproduction
Involves two individuals, each contributing a gamete.
Reproduction in Plants with Seeds
The reproductive function is located in a special organ called the flower.
The floral parts may be fertile (sexual) or sterile (non-sexual). A flower consists of a peduncle (stalk) whose end is widened to form the thalamus or floral receptacle, where a series of floral parts are inserted.
Sterile Parts
Sepals
Collectively known as the calyx. They are usually green and protect the developing flower bud.
Petals
Collectively known as the corolla. They often have bright colors to attract pollinators.
The calyx and the corolla protect the inner, fertile whorls of the flower.
Fertile Parts (Sexual Organs)
Androecium
The male sexual organ of the flower. It consists of stamens. Each stamen has a filament and a widened terminal called the anther. The anther is generally divided into two lobes, each containing pollen sacs filled with microspores that give rise to pollen grains.
Gynoecium
The female sexual organ. It consists of one or more carpels. Each carpel typically has three parts:
Ovary
The most widened part at the base. Inside, there are ovules which contain the female gametophyte.
Style
A tubular-shaped piece that connects the stigma to the ovary.
Stigma
The receptive surface where pollen grains are deposited.
Pollination and Fertilization
Pollination is the process by which pollen grains are transferred from the anther to the stigma.
Cross-pollination, the transport of pollen grains, can be achieved by different means, including:
- Anemophilous (wind pollination)
- Zoophilous (animal pollination)
- Hydrophilous (water pollination)
Fertilization occurs when the pollen tube reaches the ovule. The pollen grain divides into two male gametes that travel down the pollen tube to reach the egg cell (oosphere), where fertilization occurs, leading to the formation of a zygote.
Double Fertilization: One sperm nucleus fuses with the egg cell, and the other fuses with two polar nuclei of the embryo sac, causing a triploid nucleus that gives rise to a nutritive tissue called the endosperm.
The fertilized egg develops into an embryo within the seed. Simultaneously, the ovary undergoes changes that result in the formation of the fruit.
Seed and Fruit Dispersal
Methods of fruit dispersal include:
- Autochory (self-dispersal)
- Hydrochory (dispersal by water)
- Anemochory (dispersal by wind)
- Zoochory (dispersal by animals)