Patterns in Life: Diversity and Classification · Lesson 6 of 13
Kingdom Plantae — Multicellular, Autotrophic Eukaryotes with a Cell Wall
“Plants started with simple green bodies and gradually upgraded the entire transport department.”
• Describe the common cellular and nutritional features of plants. • Explain the organisation and habitat of thallophytes. • Describe lichens as a mutual association between an alga and a fungus. • Explain how bryophytes represent an early adaptation to land. • Compare thallophytes and bryophytes using body differentiation and dependence on water.
A tree, a moss growing on a damp wall and a thread-like alga in pond water look very different, yet all carry out photosynthesis and belong to the plant kingdom. Their differences reveal how plant bodies solve the problems of obtaining water, transporting materials, supporting themselves and reproducing in different surroundings.
Plants are multicellular eukaryotes. Their cells have a rigid wall made primarily of cellulose, and they prepare food through photosynthesis. Plants release oxygen and form the base of most food chains. Kingdom Plantae is divided into five major groups according to body differentiation, vascular tissues, seeds, flowers and dependence on water.
An autotroph is an organism that prepares its own food from simple substances using an energy source, as green plants do during photosynthesis.
Thallophyta (Algae) — Primitive Plants
A thallus is a simple plant body that is not differentiated into true roots, stems and leaves.
Thallophytes are among the simplest plant forms and are commonly found in water or moist environments. Their thallus directly exchanges gases, water and nutrients with the surroundings. This is effective in water because the entire body remains close to the materials it needs. Spirogyra, with its long green filaments, is an example.
The same simplicity that suits water becomes a challenge on dry land. A thallus lacks specialised roots for absorption, stems for support and vascular tissues for long-distance transport. It therefore cannot move water efficiently through a tall body or prevent rapid drying in the way more complex land plants can.
Problem
How can an undifferentiated body survive without roots or vascular tissue?
- 1.Water surrounds much of the plant body.
- 2.Water, dissolved substances and gases can exchange directly across the surface.
- 3.A small, simple body keeps most cells near the surroundings.
- 4.The arrangement is effective in water but poorly suited to a tall body on dry land.
Bridging Science and Society
White-green or grey-green patches seen on bark, stones and damp walls may be lichens. A lichen is not one organism living alone. It is a mutual association between an autotrophic alga and a heterotrophic fungus. The alga performs photosynthesis and supplies food, while the fungus provides shelter, protection and access to moisture and minerals.
Symbiosis is a close association between different organisms; in a mutual association, both partners benefit.
Lichens respond to air pollution and can be used as natural indicators of air quality. Some are traditionally collected for spices, medicines and dyes that colour wool or silk. However, some lichens are poisonous, making reliable classification and identification essential for safe use.
Bryophyta — First Steps on Land, Still Need Water
Mosses and liverworts represent an important movement from water to land. They grow as green mats on damp rocks, old walls and moist soil, especially during wet seasons. Their bodies are more differentiated than a thallus. They possess root-like rhizoids and may have simple stem-like and leaf-like parts.
Rhizoids are simple root-like structures that help anchor a bryophyte and assist in taking up water from its immediate surroundings.
Bryophytes do not possess true roots, stems, leaves or vascular tissues. Materials therefore move only over short distances, which limits their height. Their reproductive cells also require water, so they remain associated with moist and shady places. This combination of land growth and reproductive dependence on water gives them the description “amphibians of the plant kingdom”.
Activity: Let us Explore
A bryophyte sample can be observed in a watch glass using a hand lens and then under a dissecting microscope after adding a drop of water. Compare its small structures with the true leaves and roots of familiar garden plants. The goal is to distinguish simple leaf-like and root-like structures from fully developed organs.
| Characteristic | Thallophyta | Bryophyta |
|---|---|---|
| Typical habitat | Water or very moist surroundings | Moist and shady land |
| Body | Undifferentiated thallus | Slight differentiation into simple parts |
| Anchorage | No true roots | Rhizoids present |
| Vascular tissue | Absent | Absent |
| Dependence on water | Strong environmental dependence | Water required for reproduction |
| Example | Spirogyra | Marchantia or moss |
Problem
The organism has rhizoids and simple leaf-like parts but no vascular tissue. Which group fits best?
- 1.The body is more differentiated than a thallus.
- 2.Rhizoids are present instead of true roots.
- 3.Vascular tissues are absent.
- 4.The moist habitat supports water-dependent reproduction.
- 5.These combined features identify a bryophyte.
Thallophytes and bryophytes possess body plans suited to their habitats. Classification describes organisation and dependence; it does not arrange living groups as better or worse.
Quiz
Which description best matches Autotroph?
Which description best matches Thallus?
Which term matches this description: An autotroph is an organism that prepares its own food from simple substances using an energy source, as green plants do during photosynthesis.
Which term matches this description: A thallus is a simple plant body that is not differentiated into true roots, stems and leaves.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain why direct exchange with water is suitable for a thallus.
- Describe the contribution of each partner in a lichen.
- Why are bryophytes limited to short growth in moist surroundings?
- Compare thallophytes and bryophytes using three characteristics.
Key Takeaways
• Plants are multicellular autotrophic eukaryotes with cellulose walls. • A thallophyte has an undifferentiated body suited mainly to water or moist places. • Lichens combine an algal food producer with a protective fungal partner. • Bryophytes possess rhizoids and simple body parts but lack vascular tissue. • Bryophytes grow on land yet still require water for reproduction.