---
title: "How Plants Spread"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 39
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/39-how-plants-spread
---

# Chapter 39 — How Plants Spread

A new road cutting is bulldozed in March: raw, bare earth, not a green speck. Come back in September and it is stitched all over with [seedlings](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle) — poppies, dandelions, willow shoots, grasses. Nobody sowed it. Yet [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), which cannot walk a single step, colonized the bare ground within months. This chapter is about their travel department.

## 39.1 Plants travel as seeds and spores

**Proposition 39.1 (Sedentary plants, traveling beginnings).**

An adult [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) is fixed for life ([Definition 3.1](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant)) — so a [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) spreads by sending out its *beginnings*: [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) and [fruits](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) ([Chapter 31](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#ch-g5-flower-to-fruit)), [spores](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-spore) ([Definition 24.1](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-spore)), or [runners](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ex-g4-plants-without-seeds-runner) and other vegetative advances ([Definition 24.5](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-vegetative)). Occupying new ground this way is called *colonization*.

**Proof.** *Admitted at this level.* ∎

**Definition 39.2 (Dispersal).**

*Dispersal* is the carrying of [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform), [fruits](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) or [spores](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-spore) away from the parent [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant). The carriers are few and ancient: the *wind*, *[animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal)*, *water* — and some [fruits](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) that fling their [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) themselves.

**Example 39.3 (Why leave home at all?).**

A [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) dropped at the parent’s foot [germinates](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-germination) in the parent’s shade, among the parent’s [roots](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) — competing for the very [light](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) and water of [Proposition 22.3](https://one-course.com/books/biology/1/en/chapter/22-what-plants-need#prop-g4-what-plants-need-needs) with a rival a hundred times its size. [Dispersal](#def-g6-how-plants-spread-dispersal) is the escape from that losing match: distance is the first gift a [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) gives its young.

## 39.2 The carriers, and the fruits built for them

**Proposition 39.4 (Built to match the carrier).**

[Fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) and [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) shapes match their carriers — [Definition 28.1](https://one-course.com/books/biology/1/en/chapter/28-adapted-to-their-world#def-g4-adapted-to-their-world-adaptation)’s fit, applied to luggage:

- *wind* : parachutes (the dandelion’s plumed [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) ), wings (maple and ash keys that spin down slantwise), or dust-fine [spores](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-spore) that simply float (the fern’s millions);
- *[animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal), outside* : burrs — burdock, cleavers — whose hooks grip [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) and socks and ride until groomed off;
- *[animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal), inside* : sweet fleshy [fruits](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) — cherries, blackberries, rose hips — eaten whole; the tough-coated [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) pass through unharmed and are deposited elsewhere, with fertilizer ( [Exercise 16.10](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#exo-g3-flowers-fruits-seeds-10) ’s answer, completed);
- *water* : floating [fruits](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) — the alder’s tiny rafts, the coconut’s ocean-going [shell](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) ;
- *self-flinging* : pods that dry, tense and snap — gorse and violets shoot [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) metres from the parent.

**Proof.** *Admitted at this level.* ∎

![Four pieces of travel equipment. Read each shape as an address label: it names the carrier.](https://one-course.com/images/onecourse/chapters/biology-1/g6-how-plants-spread/fig-4034766269ba.svg)

*Four pieces of travel equipment. Read each shape as an address label: it names the carrier.*

![The wind service, mid-departure: each plumed seed a one-way long-range traveler.](https://one-course.com/images/onecourse/chapters/biology-1/g6-how-plants-spread/img-db9d4a0953b3.jpg)

*The wind service, mid-departure: each plumed [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) a one-way long-range traveler.*

**Example 39.5 (Reading the roadside).**

September’s road cutting, decoded: dandelions and willows — plumed [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform), flown in on the wind; poppies — their tiny [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) lay already in the moved earth, waiting as only [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) can ([Exercise 9.9](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#exo-g2-from-seed-to-plant-9)); blackberry [seedlings](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle) under the fence posts — deposited by perching [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), fertilizer included; burdock by the path — carried in on a dog. Every colonist’s arrival route is written in its equipment.

**Method 39.6 (Naming the carrier from the fruit).**

Given a [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) or [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed):

1. look for flight gear — plumes, wings, dust fineness: the wind;
2. look for hooks: [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) , outside;
3. fleshy, colored, sweet? [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) gut, inside — check the [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) for tough coats;
4. buoyant, near water? water;
5. a dried, twisted, sprung pod? self-flinging;
6. test your verdict: drop it, float it, press it to a woolly sleeve.

## 39.3 Colonizing without seeds

**Example 39.7 (The ground advance).**

Alongside the airborne arrivals, some colonists walk on their own vegetative feet ([Chapter 24](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ch-g4-plants-without-seeds)): couch grass advances underground by creeping [stems](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts), metre by metre; brambles arch over and [root](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) where their tips touch; the strawberry’s [runners](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ex-g4-plants-without-seeds-runner) hop across open ground. Slow, short-range — and unstoppable across a neglected garden, as any gardener knows.

**Remark 39.8 (Two-speed colonization).**

The two systems divide the work. [Seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) and [spores](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-spore) are the *long-range* service — rare lucky landings far away, the dandelion crossing the town. Vegetative advance is the *short-range* service — dense, sure occupation of the ground next door. The bramble patch spreading from the hedge uses both at once: arching [stems](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) for the metre, bird-carried [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) for the mile.

## 39.4 Exercises

**Exercise 39.1 ★.**

What is [colonization](#prop-g6-how-plants-spread-travel)? What is [dispersal](#def-g6-how-plants-spread-dispersal), and what are the main carriers?

**Solution of Exercise 39.1.**

[Colonization](#prop-g6-how-plants-spread-travel) is a [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species)’ occupation of new ground. [Dispersal](#def-g6-how-plants-spread-dispersal) is the carrying of [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform), [fruits](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) or [spores](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-spore) away from the parent; the carriers are the wind, [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) (outside and inside), water — and some self-flinging [fruits](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform).

**Exercise 39.2 ★.**

Why is a [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) better off germinating far from its parent?

**Solution of Exercise 39.2.**

At the parent’s foot it would [germinate](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-germination) in the parent’s shade and among its [roots](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts), competing for [light](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) and water against a rival a hundred times its size. Distance escapes the losing match.

**Exercise 39.3 ★.**

Name the carrier for each: the dandelion’s plume, the burdock burr, the cherry, the maple key, the coconut.

**Solution of Exercise 39.3.**

Dandelion plume: the wind. Burdock burr: [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), outside. Cherry: [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) gut, inside. Maple key: the wind (spinning wing). Coconut: water.

**Exercise 39.4 ★.**

How do a bird-eaten blackberry’s [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) survive the journey, and what do they arrive with?

**Solution of Exercise 39.4.**

Their tough coats let them pass through the bird’s [digestion](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) unharmed; they arrive deposited far from home, with a dose of fertilizer.

**Exercise 39.5 ★.**

Which travel service do gorse and violets use? Describe the mechanism.

**Solution of Exercise 39.5.**

Self-flinging: their pods dry, tense like springs, and snap open, shooting the [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) metres from the parent.

**Exercise 39.6 ★.**

Name two vegetative colonists and how each advances.

**Solution of Exercise 39.6.**

Couch grass, advancing underground by creeping [stems](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts); brambles, arching over and rooting at the tips (or the strawberry, hopping by [runners](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ex-g4-plants-without-seeds-runner)).

**Exercise 39.7 ★★.**

Decode the road cutting: for dandelion, poppy, blackberry and burdock, give the arrival route the chapter names.

**Solution of Exercise 39.7.**

Dandelion: flown in on the wind by its plumes. Poppy: its [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) already lay in the soil, woken when the earth was moved. Blackberry: deposited by perching [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) inside droppings. Burdock: carried in hooked to a dog’s [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering).

**Exercise 39.8 ★★.**

Run [Method 39.6](#met-g6-how-plants-spread-name) on: (a) a plumed thistle [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed); (b) a rose hip; (c) a cleavers ball that sticks to your pullover.

**Solution of Exercise 39.8.**

(a) Flight gear — plumes: the wind. (b) Fleshy, colored, tough-coated [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) inside: eaten by [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal), [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) ride the gut. (c) Hooks that grip wool: [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) [fur](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering) — and the pullover test just confirmed it.

**Exercise 39.9 ★★.**

Explain [Remark 39.8](#rem-g6-how-plants-spread-twospeeds)’s “two-speed” idea with the bramble patch: which system does the metre, which the mile?

**Solution of Exercise 39.9.**

The arching, tip-rooting [stems](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) do the metre — dense, sure occupation of the ground next door. The bird-eaten berries do the mile — rare long-range landings that found new patches far away.

**Exercise 39.10 ★★.**

The fern colonized the new cutting’s damp north bank but not its dry south bank. Combine [Proposition 24.3](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#prop-g4-plants-without-seeds-damp) and [Proposition 37.6](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#prop-g6-exploring-our-environment-decide) into the explanation.

**Solution of Exercise 39.10.**

[Spores](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-spore) reach both banks — they float everywhere — but a [spore-plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant)’s reproduction needs dampness, so only the north bank’s [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) accept the arrival. [Dispersal](#def-g6-how-plants-spread-dispersal) delivers; [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) decide who may stay: the inhabitants match the [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions), arrivals notwithstanding.

**Exercise 39.11 ★★.**

An island rises bare from the sea, kilometres offshore. Which [dispersal](#def-g6-how-plants-spread-dispersal) services can reach it at all? Predict the equipment of its first [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) colonists.

**Solution of Exercise 39.11.**

Only the long-range services: wind (plumed and dust-fine [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform), [spores](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-spore)), sea water (floating [fruits](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) like the coconut), and [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) (gut-carried [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) of fleshy [fruits](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform)). Predict plumed, dust-fine, floating or berry-borne equipment among the pioneers — no heavy, carrier-less [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform).

**Exercise 39.12 ★★★.**

Sweet flesh is expensive for a [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) to build. Using [Example 39.3](#ex-g6-how-plants-spread-whyleave) and the [inside-animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) service, explain what the cherry tree buys with all that sugar — and why the [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform)’ tough coats are the other half of the purchase.

**Solution of Exercise 39.12.**

The sugar buys transport: a bird paid in flesh carries the [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) far from the parent’s shade and deposits them with fertilizer — distance, the first gift. The tough coats are the other half because the courier is a [digestive tract](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract): the payment must be digestible and the passengers must not be. Sweetness recruits the carrier; the coats survive it.

## 39.5 Problem: The Abandoned Field

**Problem 39.1.**

Weekend problem — ten years of colonization, watched from one gate

A ploughed field is abandoned in autumn. A naturalist photographs it from the gate every June for ten years, and keeps notes.

**Part I — Year one.**

1. June of year one: the bare earth is dotted with poppies, dandelions and thistles. For each, name the likely source and route — poppies from the soil itself, the others airborne. What equipment made the airborne arrivals possible?
2. No blackberries appear in the field’s middle in year one, but three [seedlings](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle) stand under the old fence posts. Explain the pattern with the [inside-animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) service — why exactly under the posts?
3. The naturalist finds no [ferns](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#prop-g4-plants-without-seeds-damp) anywhere, though a wood full of them lies upwind. [Ferns](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#prop-g4-plants-without-seeds-damp) ’ [spores](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#def-g4-plants-without-seeds-spore) certainly arrived ( [Example 24.2](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ex-g4-plants-without-seeds-fern) counts their millions). What condition of the open field, by [Proposition 24.3](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#prop-g4-plants-without-seeds-damp) , refused them?
4. Couch grass invades from the field’s edge only, a metre or two a year. Which [colonization](#prop-g6-how-plants-spread-travel) system is this, and why can it not appear in the field’s middle the way dandelions did?

**Part II — The years after.**

5. By year three, grasses and tall herbs have closed the bare ground, and the poppies have vanished from the standing vegetation. Where are the poppies now, and in what form? (Recall the road cutting and [Proposition 38.6](https://one-course.com/books/biology/1/en/chapter/38-life-through-the-seasons#prop-g6-life-through-seasons-herbs) .)
6. From year four, bramble arcs and young hawthorns dot the field, thickest near the hedges and under the fence line. Name the two services that placed them, and the evidence in the pattern.
7. By year eight, willow and birch saplings stand taller than the brambles — both trees with tiny plumed or [dust-light](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) . Why are the heavy-seeded oaks and chestnuts still almost absent, and what [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) courier eventually [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) even them?
8. The photographs show the field’s cast changing year by year although no gardener touched it. State the driving rule: what does each year’s vegetation change about the field’s own [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) — [light](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) at ground level above all — and who does that change favor next?

**Part III — The general lesson.**

9. Order the colonists by arrival: soil’s waiting [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) , wind-riders, animal-carried shrubs, heavy-seeded trees. Pair each wave with the [dispersal](#def-g6-how-plants-spread-dispersal) service that timed it.
10. A town square’s pavement cracks sprout the same year-one cast — poppies, dandelions — but never reach year four’s shrubs. Give two reasons the succession stalls there.
11. The naturalist claims: “left alone, this field will be woodland.” Defend the claim with the ten-year trend — and name the meadow exception of [Proposition 35.1](https://one-course.com/books/biology/1/en/chapter/35-people-and-nature#prop-g5-people-and-nature-managed) that proves “left alone” is the operative phrase.
12. Summarize the whole problem in two sentences using the words *[dispersal](#def-g6-how-plants-spread-dispersal)* , *[conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions)* and *[colonization](#prop-g6-how-plants-spread-travel)* .

**Solution of Problem 39.1.**

**1.** Poppies: waked from [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) already in the ploughed soil — the annual’s waiting stock. Dandelions and thistles: flown in on the wind, by their plumed [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform).

**2.** [Birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) eating blackberries elsewhere perch on the fence posts and deposit the [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) below them, fertilizer included — so the [seedlings](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#prop-g2-from-seed-to-plant-cycle) map the perches, not the wind.

**3.** Its dryness: an open, sunlit field offers none of the dampness a fern’s delicate spore-grown stages need. Arrival happened; establishment was refused.

**4.** Vegetative advance — creeping underground [stems](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts). It moves only ground-to-adjacent-ground from established [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), so it must walk in from the edge; it has no airborne stage to land in the middle.

**5.** In the soil, as [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform): [annuals](https://one-course.com/books/biology/1/en/chapter/38-life-through-the-seasons#prop-g6-life-through-seasons-herbs) beaten in the standing competition persist as the [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) bank, waiting for the next bare ground.

**6.** The [inside-animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) service (berries eaten, [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) deposited from perches — thickest under fence line and hedges) and, for hawthorn equally, the same bird-courier; the perch-shaped pattern is the evidence.

**7.** Oak acorns and chestnuts are heavy, carrier-less without help — they fall at the parent’s foot. Their courier is the jay (and the squirrel): burying acorns as winter stores and forgetting some, they [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) oaks far from the parent.

**8.** Each year’s vegetation shades the ground below it, cooling and darkening the surface: the sun-loving pioneers’ own success removes their [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions), and the changed [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) favor shade-bearers next — colonists alter the site, and the altered site chooses new colonists.

**9.** Wave 1, the soil’s waiting [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) (instant — they were already there). Wave 2, the wind-riders (weeks: the air delivers constantly). Wave 3, animal-carried shrubs (years: perch by perch). Wave 4, heavy-seeded trees (last: they wait for jays’ and squirrels’ buried forgettings).

**10.** The cracks offer no room: a shrub’s [roots](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) need soil volume a pavement seam lacks; and the succession’s later waves need their couriers and [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) — perching [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)’ deposits mostly miss the cracks, and the baked dry seam refuses what does arrive. Year one’s wind-riders are the only service the site can accept.

**11.** The trend runs bare earth, herbs, shrubs, pioneer trees — each wave taller and shadier: extended, it ends in woodland, as the willows and birches already show. The exception: a mowed or grazed meadow, where human touch removes the shrub wave every year — proof that only *left alone* does the field walk to forest.

**12.** For example: “[Dispersal](#def-g6-how-plants-spread-dispersal) delivered colonists to the bare field — from the soil, the wind, the [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) — and at each stage the field’s [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) decided which arrivals could stay. [Colonization](#prop-g6-how-plants-spread-travel) thus ran in waves, each wave changing the [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) and choosing the next.”