# The Colonisation of Land

Biology I · Life, Its Origin and Its Diversity · https://tryals.app/learn/biology-i/the-colonisation-of-land

## Leaving the Water

Land offered abundant light, plentiful carbon dioxide and, at first, no competitors or predators. It also posed problems that water had solved automatically.

| Problem | Solved in water by | Solution on land |
|---|---|---|
| Desiccation | Being surrounded by water | Waterproof cuticle, skin, exoskeleton |
| Support | Buoyancy | Lignin, skeletons, exoskeletons |
| Gas exchange | Dissolved gases | Stomata, lungs, tracheae |
| Reproduction | Water carries gametes | Pollen, seeds, internal fertilisation |
| Transport | Diffusion in a small body | Xylem and phloem, circulatory systems |

**Plants** solved these in a sequence still visible in the tree. Early bryophytes had no vascular tissue and stayed small and damp. The **cormus** — true roots, stem and leaves — arrived with vascular tissue: **xylem** carrying water upward and **phloem** carrying sugars, both stiffened by **lignin**, which is what allowed height. The **seed** freed reproduction from standing water, and the **flower** recruited animals as pollinators, which is far more targeted than releasing pollen to the wind.

Water rises in xylem without a pump. Evaporation from the leaves creates tension, and cohesion between water molecules transmits that tension down an unbroken column, the **cohesion-tension** mechanism, pulling from the top rather than pushing from below.

**Fungi** took a different route: a **mycelium** of fine hyphae with an enormous absorptive surface, feeding by external digestion, secreting enzymes and absorbing the products. Their partnership with plant roots, the **mycorrhiza**, is ancient and may have been what made plant colonisation possible at all.

**Animals** needed waterproofing, internal fertilisation or shelled eggs, and internal gas exchange. Arthropods managed it first with an exoskeleton that solved support and desiccation together. Vertebrates needed the **amniotic egg** before they could breed away from water entirely.

> **Common pitfall:** describing water as being "pushed" up a tree by the roots. Root pressure exists but is far too weak for a tall tree. The column is *pulled* from the top by evaporation, held together by the cohesion of water, a tension so strong that a severed trunk draws air in rather than leaking water out.

## Practice questions

7 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. A tree moves water from its roots to leaves 30 m above ground. Atmospheric pressure can support a water column of about 10 m. How many times taller than that limit is the tree?

**Answer:** 3 (within ±0.1)

**Why:** The tree is **3 times** taller than a pressure-driven column could reach. That is precisely why the cohesion-tension mechanism is needed: water is pulled from above under tension, not pushed from below.

Page: https://tryals.app/practice/biology-i/the-colonisation-of-land/a-tree-moves-water-from-its-roots-to-leaves-30-m-above-ground

### 2. Which problems did organisms have to solve in order to live on land?

A. Preventing water loss from the body surface
B. Obtaining light, which is scarcer on land than in water
C. Reproducing without standing water to carry gametes
D. Supporting the body without buoyancy

**Answer:** A. Preventing water loss from the body surface; C. Reproducing without standing water to carry gametes; D. Supporting the body without buoyancy

**Why:** Desiccation, support and reproduction were all real obstacles. Light was an *advantage* of land, not a problem, water absorbs light rapidly with depth, which is why the photic zone is so shallow.

Page: https://tryals.app/practice/biology-i/the-colonisation-of-land/which-problems-did-organisms-have-to-solve-in-order-to-live-on-land

### 3. Arrange these plant innovations in the order they evolved.

**Answer:**

1. Simple non-vascular plants confined to damp places
2. Vascular tissue with lignin, allowing height
3. Seeds, freeing reproduction from standing water
4. Flowers, recruiting animal pollinators

**Why:** Each step lifted a specific limit: lignified vascular tissue allowed height, seeds removed the dependence on water for fertilisation, and flowers replaced wasteful wind pollination with targeted animal delivery.

Page: https://tryals.app/practice/biology-i/the-colonisation-of-land/arrange-these-plant-innovations-in-the-order-they-evolved

### 4. Why is the mycorrhizal partnership between fungi and plant roots thought to have been important for the colonisation of land?

A. The fungus secretes a waterproof cuticle that prevents desiccation in the shoot
B. The fungal hyphae deposit lignin inside xylem vessels to provide structural support
C. The fungus greatly extends the root’s absorptive reach for water and nutrients
D. The fungus carries out photosynthesis in damp soil to provide the plant with sugars

**Answer:** C. The fungus greatly extends the root’s absorptive reach for water and nutrients

**Why:** A mycelium explores a far greater volume of soil than roots alone, supplying water and minerals in exchange for sugars. Fungi cannot photosynthesise, that is exactly why the trade works, and mycorrhizae appear in the earliest land-plant fossils.

Page: https://tryals.app/practice/biology-i/the-colonisation-of-land/why-is-the-mycorrhizal-partnership-between-fungi-and-plant-roots

### 5. Lignin is what allowed plants to grow tall on land.

**Answer:** True

**Why:** True, lignin stiffens cell walls and provides the compressive strength to stand against gravity. Without buoyancy, height required a structural polymer, and lignin is also why wood resists decay so well.

Page: https://tryals.app/practice/biology-i/the-colonisation-of-land/lignin-is-what-allowed-plants-to-grow-tall-on-land

### 6. Match each kingdom to the strategy it used on land.

**Answer:**

- Plants → Vascular tissue, lignin and seeds
- Fungi → A mycelium feeding by external digestion
- Arthropods → An exoskeleton solving support and water loss together
- Amniote vertebrates → A shelled egg that carries its own water supply

**Why:** The same set of problems produced four quite different solutions. The amniotic egg is the most elegant: it encloses an aquatic environment, letting vertebrates reproduce entirely away from water.

Page: https://tryals.app/practice/biology-i/the-colonisation-of-land/match-each-kingdom-to-the-strategy-it-used-on-land

### 7. A waterproof cuticle prevents desiccation on land but creates a barrier to atmospheric gases. How did plants resolve this trade-off without forfeiting water conservation?

A. They evolved regulated pores to balance gas diffusion and moisture loss
B. They replaced passive gas uptake with specialized internal transport rings
C. They allowed cuticles to become selectively permeable to ambient gases
D. They restricted all cellular gas exchange entirely to subterranean roots

**Answer:** A. They evolved regulated pores to balance gas diffusion and moisture loss

**Why:** Cuticles are impermeable barriers that solve desiccation but inevitably block direct gas diffusion; pores (stomata) provide an adjustable route for uptake. Confusing this with root respiration or non-existent permeable cuticles misses how structural trade-offs force active regulation.

Page: https://tryals.app/practice/biology-i/the-colonisation-of-land/a-waterproof-cuticle-prevents-desiccation-on-land-but-creates-a