---
title: "Life Cycles and Reproduction of Land Plants"
book: "University Biology — Year 2"
subject: biology
language: en
chapter: 5
exercises: 12
source: https://one-course.com/books/biology/4/en/chapter/5-life-cycles-and-reproduction-of-land-plants
---

# Chapter 5 — Life Cycles and Reproduction of Land Plants

The green cushion of [moss](#def-b2-plant-life-cycles-moss) on a wall is one plant; the brown stalks that rise from it in spring, each with a capsule of spores, are another — a different individual, with twice the chromosomes, growing out of the first and living on it. A [fern](#def-b2-plant-life-cycles-fern) frond carries spores on its underside, but the spores do not grow into [ferns](#def-b2-plant-life-cycles-fern): they grow into a heart-shaped green scale a few millimetres wide, which is where the sperm and eggs are made, and it is from that scale that a new [fern](#def-b2-plant-life-cycles-fern) springs. Every land plant alternates in this way between two bodies, one [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) and one [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis). This chapter follows the alternation from the algae to the seed, and shows how a sequence of changes to it — the shrinking of the [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) body, the invention of [pollen](#def-b2-plant-life-cycles-heterospory), the enclosure of the [ovule](#def-b2-plant-life-cycles-heterospory), the seed — freed reproduction from water and let plants cover the continents.

## 5.1 Three kinds of life cycle

**Definition 5.1 (Haplontic, diplontic, haplo-diplontic).**

Every sexual life cycle contains one *fertilisation*, which doubles the chromosome number, and one *[meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis)*, which halves it; the cycles differ in what happens between them. In a *haplontic* cycle the zygote is the only [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) cell and undergoes [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) at once; the organism is [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) and its gametes are made by mitosis (*Chlamydomonas*, many algae and fungi). In a *diplontic* cycle [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) makes gametes directly, the gametes are the only [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) cells, and the organism is [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) (animals, the brown alga *Fucus*). In a *haplo-diplontic* cycle [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) makes *spores*, which grow by mitosis into a [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) organism, the *gametophyte*, that makes gametes by mitosis; the zygote grows by mitosis into a [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) organism, the *sporophyte*, that makes spores by [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis). Two [multicellular](https://one-course.com/books/biology/4/en/chapter/1-diversity-of-unicellular-organisms#def-b2-unicellular-diversity-unicellular) bodies alternate — the *alternation of generations* — and this is the cycle of every land plant and of many algae.

![Three life cycles. Blue: the haploid phase; red: the diploid phase; the thick arcs are the multicellular bodies. Every cycle passes once through meiosis and once through fertilisation; they differ in which phase — or both — grows into an organism.](https://one-course.com/images/onecourse/chapters/biology-4/b2-plant-life-cycles/fig-8a2ccc31cea0.svg)

*Three [life cycles](#def-b2-plant-life-cycles-cycles). Blue: the [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) phase; red: the [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) phase; the thick arcs are the [multicellular](https://one-course.com/books/biology/4/en/chapter/1-diversity-of-unicellular-organisms#def-b2-unicellular-diversity-unicellular) bodies. Every cycle passes once through [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) and once through fertilisation; they differ in which phase — or both — grows into an organism.*

**Proposition 5.2 (What alternation of generations means).**

In a land plant the two generations are two organisms, not two stages of one: the [gametophyte](#def-b2-plant-life-cycles-cycles) and the [sporophyte](#def-b2-plant-life-cycles-cycles) have different genomes ([haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) and [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis)), different bodies, often different sizes by orders of magnitude, and each develops by mitosis from a single cell — a spore or a zygote. The [gametophyte](#def-b2-plant-life-cycles-cycles) makes gametes in [multicellular](https://one-course.com/books/biology/4/en/chapter/1-diversity-of-unicellular-organisms#def-b2-unicellular-diversity-unicellular) organs, *antheridia* (sperm) and *archegonia* (one egg each, in a flask whose neck the sperm swims down); the [sporophyte](#def-b2-plant-life-cycles-cycles) makes spores in *sporangia*. Across the land plants the balance shifts: in mosses the [gametophyte](#def-b2-plant-life-cycles-cycles) is the plant and the [sporophyte](#def-b2-plant-life-cycles-cycles) a dependent stalk; in [ferns](#def-b2-plant-life-cycles-fern) the [sporophyte](#def-b2-plant-life-cycles-cycles) is the plant and the [gametophyte](#def-b2-plant-life-cycles-cycles) a free-living scale; in seed plants the [gametophyte](#def-b2-plant-life-cycles-cycles) is reduced to a few cells hidden inside the [sporophyte](#def-b2-plant-life-cycles-cycles)’s tissues — the [pollen](#def-b2-plant-life-cycles-heterospory) grain and the contents of the [ovule](#def-b2-plant-life-cycles-heterospory).

**Evidence.** Hofmeister (1851) germinated the spores of mosses, [ferns](#def-b2-plant-life-cycles-fern), horsetails and clubmosses, followed the development of the small green bodies they produced, found on them the antheridia and archegonia, and watched the embryo grow from the fertilised egg inside the [archegonium](#def-b2-plant-life-cycles-moss) into the spore-bearing plant. He then showed that the [ovule](#def-b2-plant-life-cycles-heterospory) of a conifer contains the same structures, reduced — archegonia within a tissue that is a retained [gametophyte](#def-b2-plant-life-cycles-cycles) — and so that mosses, [ferns](#def-b2-plant-life-cycles-fern) and seed plants are one series with one cycle. The chromosome counts that explained the two generations (Strasburger, 1894) came forty years later. ∎

## 5.2 Mosses: the gametophyte is the plant

**Definition 5.3 (The moss life cycle).**

A moss *spore* germinates into a branching green filament, the *protonema*, from which buds grow into the leafy shoots — the *[gametophyte](#def-b2-plant-life-cycles-cycles)*, a few centimetres tall, without true roots, xylem or phloem, anchored by rhizoids and drawing water over its whole surface. At the shoot tips it bears *antheridia*, which release biflagellate sperm into a film of rain or dew, and *archegonia*, each with one egg at the base of a neck; the sperm swim a few centimetres at most, guided by chemical attractants, and fertilise the egg in place. The zygote grows into the *[sporophyte](#def-b2-plant-life-cycles-cycles)*: a foot embedded in the [gametophyte](#def-b2-plant-life-cycles-cycles), a stalk (*seta*), and a *capsule* in which [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) makes tens of thousands of spores, shed through a ring of hygroscopic teeth that open in dry air. The [sporophyte](#def-b2-plant-life-cycles-cycles) photosynthesises a little but is fed by the [gametophyte](#def-b2-plant-life-cycles-cycles) through its foot and never lives alone.

![The moss cycle. The leafy plant is the haploid gametophyte; the sporophyte is a stalk and capsule that grows on it, fed by it, and makes spores by meiosis.](https://one-course.com/images/onecourse/chapters/biology-4/b2-plant-life-cycles/fig-a3a38fb4b566.svg)

*The [moss](#def-b2-plant-life-cycles-moss) cycle. The leafy plant is the [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) [gametophyte](#def-b2-plant-life-cycles-cycles); the [sporophyte](#def-b2-plant-life-cycles-cycles) is a stalk and capsule that grows on it, fed by it, and makes spores by [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis).*

![A moss cushion in spring: the green shoots are the gametophytes, and each reddish stalk with its capsule is a sporophyte grown from a fertilised egg, still attached to the shoot that made the egg.](https://one-course.com/images/onecourse/chapters/biology-4/b2-plant-life-cycles/img-b12214f3d89d.jpg)

*A [moss](#def-b2-plant-life-cycles-moss) cushion in spring: the green shoots are the [gametophytes](#def-b2-plant-life-cycles-cycles), and each reddish stalk with its capsule is a [sporophyte](#def-b2-plant-life-cycles-cycles) grown from a fertilised egg, still attached to the shoot that made the egg.*

**Example 5.4 (The cost of swimming sperm).**

A [moss](#def-b2-plant-life-cycles-moss) sperm swims at about $100\,\text{µ}\mathrm{m}/\mathrm{s}$; to reach an [archegonium](#def-b2-plant-life-cycles-moss) $2\,\mathrm{cm}$ away it needs $200\,\mathrm{s}$ of continuous water film — a splash of rain, a heavy dew. Fertilisation is therefore confined to wet weather and to short distances, and most mosses, [ferns](#def-b2-plant-life-cycles-fern) and their relatives live in damp places or reproduce in the wet season; the antheridia of some mosses sit in a splash cup whose shape throws raindrops, and the sperm they carry, half a metre. It is this dependence that the seed plants escaped.

## 5.3 Ferns: the sporophyte is the plant

**Definition 5.5 (The fern life cycle).**

The fern is the *[sporophyte](#def-b2-plant-life-cycles-cycles)*: a rhizome with roots, xylem and phloem, and fronds. On the underside of fertile fronds, clusters of *sporangia* (*sori*, often under a protective flap) each make 64 spores by [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) and fling them by the snap of a drying ring of thick-walled cells. A spore that lands on damp soil grows into a *prothallus*: a heart-shaped green [gametophyte](#def-b2-plant-life-cycles-cycles) a few millimetres across, one cell thick, with rhizoids, living free for a few weeks. It bears antheridia and archegonia on its underside; sperm swim in the soil film to the egg; the zygote grows into a young [sporophyte](#def-b2-plant-life-cycles-cycles) that draws its first food from the prothallus and then roots, and the prothallus dies. The two generations are both independent, but unequal: the [sporophyte](#def-b2-plant-life-cycles-cycles) lives for decades and can be metres tall, the [gametophyte](#def-b2-plant-life-cycles-cycles) for weeks and millimetres.

![The fern cycle. The plant we call a fern is the diploid sporophyte; the haploid gametophyte is a short-lived prothallus on which the sperm swim to the eggs.](https://one-course.com/images/onecourse/chapters/biology-4/b2-plant-life-cycles/fig-711f532e1cd3.svg)

*The [fern](#def-b2-plant-life-cycles-fern) cycle. The plant we call a [fern](#def-b2-plant-life-cycles-fern) is the [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) [sporophyte](#def-b2-plant-life-cycles-cycles); the [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) [gametophyte](#def-b2-plant-life-cycles-cycles) is a short-lived [prothallus](#def-b2-plant-life-cycles-fern) on which the sperm swim to the eggs.*

![The underside of a fertile fern frond: each brown dot is a sorus, a cluster of sporangia, some still covered by the flap that protects them until they ripen.](https://one-course.com/images/onecourse/chapters/biology-4/b2-plant-life-cycles/img-f15f90921e44.jpg)

*The underside of a fertile [fern](#def-b2-plant-life-cycles-fern) frond: each brown dot is a [sorus](#def-b2-plant-life-cycles-fern), a cluster of sporangia, some still covered by the flap that protects them until they ripen.*

**Theorem 5.6 (Dispersal of a spore).**

A spore of radius $r$ and density $\rho_s$ released at height $h$ into a horizontal wind of speed $u$ falls at the Stokes terminal speed $v_s = 2r^2(\rho_s - \rho_{\text{air}})g/9\eta_{\text{air}}$ and lands, in still-layered air, at a distance

$$
x = \frac{u\,h}{v_s}
$$

from the plant. A [fern](#def-b2-plant-life-cycles-fern) spore of radius $15\,\text{µ}\mathrm{m}$ and density $1100\,\mathrm{kg}/\mathrm{m}^{3}$ ($\eta_{\text{air}} = 1.8 \times 10^{-5}\,\mathrm{Pa}\,\mathrm{s}$) falls at $3\,\mathrm{cm}/\mathrm{s}$; from a frond at $0.5\,\mathrm{m}$ in a wind of $2\,\mathrm{m}/\mathrm{s}$ it travels about $30\,\mathrm{m}$, and in the turbulent air of a windy day, which lifts a fraction of the spores far above the release height, a few travel hundreds of kilometres — which is why the [fern](#def-b2-plant-life-cycles-fern) floras of oceanic islands are rich and their seed-plant floras poor.

**Proof.** The fall time is $h/v_s$ (the spore reaches its terminal speed in microseconds), during which the wind carries it $u\,h/v_s$. The terminal speed follows from balancing the Stokes drag $6\pi\eta r
v_s$ against the weight minus buoyancy $\tfrac43\pi r^3(\rho_s -
\rho_{\text{air}})g$, as in [Chapter 1](https://one-course.com/books/biology/4/en/chapter/1-diversity-of-unicellular-organisms#ch-b2-unicellular-diversity). ∎

## 5.4 Seed plants: the gametophyte is hidden

**Definition 5.7 (Heterospory, pollen, ovule).**

Mosses and most [ferns](#def-b2-plant-life-cycles-fern) are *homosporous*: one kind of spore, one kind of [gametophyte](#def-b2-plant-life-cycles-cycles) bearing both sexes. Seed plants (and a few [ferns](#def-b2-plant-life-cycles-fern) and clubmosses) are *heterosporous*: *microspores*, small and many, grow into male [gametophytes](#def-b2-plant-life-cycles-cycles); *megaspores*, large and few, into female ones. In seed plants both are reduced to almost nothing and neither leaves the [sporophyte](#def-b2-plant-life-cycles-cycles)’s tissues on its own. The male [gametophyte](#def-b2-plant-life-cycles-cycles) is the *pollen grain*: a microspore that has divided two or three times inside its wall, carried by wind or animals to the female organ, where it grows a *pollen tube* and delivers its sperm — no water needed. The female [gametophyte](#def-b2-plant-life-cycles-cycles) develops inside the megasporangium, which stays on the parent enclosed in one or two *integuments* with a pore, the *micropyle*: the whole structure is the *ovule*. After fertilisation the ovule becomes the *seed*: an embryo [sporophyte](#def-b2-plant-life-cycles-cycles), a store of food, and a coat made from the integuments, dormant until conditions are right.

![A gymnosperm ovule in section. The female gametophyte, grown from a single megaspore inside the megasporangium, is wrapped in an integument with a pore; the pollen grain, caught at the micropyle, grows a tube to the egg. After fertilisation the whole becomes the seed.](https://one-course.com/images/onecourse/chapters/biology-4/b2-plant-life-cycles/fig-a906e425d263.svg)

*A gymnosperm [ovule](#def-b2-plant-life-cycles-heterospory) in section. The female [gametophyte](#def-b2-plant-life-cycles-cycles), grown from a single [megaspore](#def-b2-plant-life-cycles-heterospory) inside the megasporangium, is wrapped in an integument with a pore; the [pollen](#def-b2-plant-life-cycles-heterospory) grain, caught at the micropyle, grows a tube to the egg. After fertilisation the whole becomes the seed.*

**Proposition 5.8 (The conifer cycle).**

A pine bears two kinds of cone. Small *male cones*, in clusters in spring, hold microsporangia in which [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) makes [microspores](#def-b2-plant-life-cycles-heterospory); each divides into a four-celled [pollen](#def-b2-plant-life-cycles-heterospory) grain with two air bladders and is shed by the million into the wind. *Female cones* carry two [ovules](#def-b2-plant-life-cycles-heterospory) on the upper face of each scale. [Pollen](#def-b2-plant-life-cycles-heterospory) blown between the scales is drawn to the micropyle by a drop of fluid; the female [gametophyte](#def-b2-plant-life-cycles-cycles), which has not yet developed, then grows over the following year from the single surviving [megaspore](#def-b2-plant-life-cycles-heterospory) into a tissue of a few thousand cells with two or three archegonia; a [pollen](#def-b2-plant-life-cycles-heterospory) tube grows slowly through the nucellus and, fifteen months after pollination, delivers a sperm to an egg. The embryo develops in the [gametophyte](#def-b2-plant-life-cycles-cycles), which becomes the food store of the seed; the seed, winged, is shed in the second autumn when the cone opens. Seeds take years, and cost a great deal; in return they are dispersed dry, survive winter and drought, and carry the seedling’s first weeks of food.

![The timetable of a pine seed: pollination in the first spring, fertilisation more than a year later, seed shed in the second autumn — about eighteen months from pollen to seed.](https://one-course.com/images/onecourse/chapters/biology-4/b2-plant-life-cycles/fig-d86afb16c129.svg)

*The timetable of a pine seed: pollination in the first spring, fertilisation more than a year later, seed shed in the second autumn — about eighteen months from [pollen](#def-b2-plant-life-cycles-heterospory) to seed.*

![A pine shoot in spring: a cluster of yellow male cones shedding pollen, and a small red female cone at the tip of another shoot, its scales open to catch it.](https://one-course.com/images/onecourse/chapters/biology-4/b2-plant-life-cycles/img-d1ced1b18a30.jpg)

*A pine shoot in spring: a cluster of yellow male cones shedding [pollen](#def-b2-plant-life-cycles-heterospory), and a small red female cone at the tip of another shoot, its scales open to catch it.*

**Example 5.9 (Pollen in the wind).**

A pine [pollen](#def-b2-plant-life-cycles-heterospory) grain, $50\,\text{µ}\mathrm{m}$ across with two bladders, falls at about $3\,\mathrm{cm}/\mathrm{s}$; a large tree releases some $10^{11}$ grains in a season, enough to coat ponds yellow. The chance that one grain lands on a given [ovule](#def-b2-plant-life-cycles-heterospory) is tiny, and the strategy works only in numbers: wind pollination is cheap per grain, costly in grains, and confined to plants that grow in stands of their own species — conifers, grasses, oaks. The flowering plants ([Chapter 6](https://one-course.com/books/biology/4/en/chapter/6-sexual-reproduction-of-flowering-plants#ch-b2-angiosperm-reproduction)) found a way to deliver [pollen](#def-b2-plant-life-cycles-heterospory) by animals, one grain at a time to the right address.

## 5.5 Trends across the land plants

**Proposition 5.10 (Four trends).**

From mosses to [ferns](#def-b2-plant-life-cycles-fern) to seed plants: (1) the [sporophyte](#def-b2-plant-life-cycles-cycles) grows from a dependent stalk to the whole plant, and the [gametophyte](#def-b2-plant-life-cycles-cycles) shrinks from the plant to a scale to a few cells; (2) homospory gives way to [heterospory](#def-b2-plant-life-cycles-heterospory), and the female [gametophyte](#def-b2-plant-life-cycles-cycles) stays on the parent; (3) fertilisation passes from swimming sperm in surface water to a [pollen](#def-b2-plant-life-cycles-heterospory) tube, so that reproduction no longer needs rain; (4) the dispersal unit passes from the spore — a single cell with a day’s reserves — to the seed, an embryo with food and a coat, which can wait. The [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) generation is favoured on land because a [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) body masks [recessive](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-vocabulary) [mutations](https://one-course.com/books/biology/4/en/chapter/3-mutations-and-genome-diversification#def-b2-genome-diversification-mutation), can grow larger and longer-lived with a vascular system, and because a spore made by [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) on a tall [sporophyte](#def-b2-plant-life-cycles-cycles) travels further than a gamete swimming from a low [gametophyte](#def-b2-plant-life-cycles-cycles). The trend continues into the flowering plants, where the female [gametophyte](#def-b2-plant-life-cycles-cycles) is seven cells and the seed is wrapped in a fruit.

**Example 5.11 (The groups compared).**

|  | mosses | [ferns](#def-b2-plant-life-cycles-fern) | conifers |
| --- | --- | --- | --- |
| dominant generation | [gametophyte](#def-b2-plant-life-cycles-cycles) | [sporophyte](#def-b2-plant-life-cycles-cycles) | [sporophyte](#def-b2-plant-life-cycles-cycles) |
| [gametophyte](#def-b2-plant-life-cycles-cycles) | leafy shoot, cm | [prothallus](#def-b2-plant-life-cycles-fern), mm, free | [pollen](#def-b2-plant-life-cycles-heterospory) grain (4 cells); [ovule](#def-b2-plant-life-cycles-heterospory) contents (thousands of cells) |
| spores | one kind | one kind | [microspores](#def-b2-plant-life-cycles-heterospory), [megaspores](#def-b2-plant-life-cycles-heterospory) |
| fertilisation | sperm swim in water | sperm swim in water | [pollen](#def-b2-plant-life-cycles-heterospory) tube |
| dispersal unit | spore | spore | seed |
| vascular tissue | none | xylem, phloem | xylem, phloem, wood |

## 5.6 Exercises

**Exercise 5.1 ★.**

Define [gametophyte](#def-b2-plant-life-cycles-cycles), [sporophyte](#def-b2-plant-life-cycles-cycles), spore and gamete, and say by which kind of division (mitosis or [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis)) each of the four is produced.

**Solution of Exercise 5.1.**

[Gametophyte](#def-b2-plant-life-cycles-cycles): the [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) [multicellular](https://one-course.com/books/biology/4/en/chapter/1-diversity-of-unicellular-organisms#def-b2-unicellular-diversity-unicellular) generation, grown by mitosis from a spore, that makes gametes by mitosis. [Sporophyte](#def-b2-plant-life-cycles-cycles): the [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) generation, grown by mitosis from a zygote, that makes spores by [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis). Spore: a [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) cell made by [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) that grows without fusing. Gamete: a [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) cell made by mitosis (in plants) that must fuse with another.

**Exercise 5.2 ★.**

The [fern](#def-b2-plant-life-cycles-fern) *Ophioglossum reticulatum* has $1260$ chromosomes in its leaf cells. How many in a spore, a [prothallus](#def-b2-plant-life-cycles-fern) cell, a sperm, an egg, a zygote?

**Solution of Exercise 5.2.**

Leaf cells are $2n = 1260$: spore 630, [prothallus](#def-b2-plant-life-cycles-fern) cell 630, sperm 630, egg 630, zygote 1260.

**Exercise 5.3 ★.**

Which generation is the green cushion of a [moss](#def-b2-plant-life-cycles-moss)? The brown stalk with a capsule? A [fern](#def-b2-plant-life-cycles-fern) frond? A [prothallus](#def-b2-plant-life-cycles-fern)? A [pollen](#def-b2-plant-life-cycles-heterospory) grain? A pine seed’s food store? A pine seed’s embryo?

**Solution of Exercise 5.3.**

Green cushion: [gametophyte](#def-b2-plant-life-cycles-cycles). Stalk and capsule: [sporophyte](#def-b2-plant-life-cycles-cycles). [Fern](#def-b2-plant-life-cycles-fern) frond: [sporophyte](#def-b2-plant-life-cycles-cycles). [Prothallus](#def-b2-plant-life-cycles-fern): [gametophyte](#def-b2-plant-life-cycles-cycles). [Pollen](#def-b2-plant-life-cycles-heterospory) grain: male [gametophyte](#def-b2-plant-life-cycles-cycles). Seed’s food store (in a pine): female [gametophyte](#def-b2-plant-life-cycles-cycles). Seed’s embryo: the next [sporophyte](#def-b2-plant-life-cycles-cycles).

**Exercise 5.4 ★.**

Name the three kinds of [life cycle](#def-b2-plant-life-cycles-cycles) and give an organism for each. Where does [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) take place in each?

**Solution of Exercise 5.4.**

Haplontic (*Chlamydomonas*): [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) in the zygote, at once. Diplontic (animals, *Fucus*): [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) makes the gametes. Haplo-diplontic (mosses, [ferns](#def-b2-plant-life-cycles-fern), seed plants, many algae): [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) makes spores in the [sporophyte](#def-b2-plant-life-cycles-cycles)’s sporangia.

**Exercise 5.5 ★★.**

A [moss](#def-b2-plant-life-cycles-moss) sperm swims at $100\,\text{µ}\mathrm{m}/\mathrm{s}$. How long does it take to reach an [archegonium](#def-b2-plant-life-cycles-moss) $3\,\mathrm{cm}$ away? A dew film evaporates in $20\,\mathrm{min}$ after sunrise: what is the maximal range of fertilisation? Comment on the sizes of [moss](#def-b2-plant-life-cycles-moss) colonies.

**Solution of Exercise 5.5.**

$0.03/10^{-4} = 300\,\mathrm{s}$, five minutes. In $20\,\mathrm{min}$, $12\,\mathrm{cm}$. Fertilisation works only between plants a few centimetres apart, so mosses grow in dense cushions with both sexes (or both organs) within reach.

**Exercise 5.6 ★★.**

Compute the settling speed in air of a [fern](#def-b2-plant-life-cycles-fern) spore of radius $15\,\text{µ}\mathrm{m}$ and density $1100\,\mathrm{kg}/\mathrm{m}^{3}$ ($\eta =
1.8 \times 10^{-5}\,\mathrm{Pa}\,\mathrm{s}$, $\rho_{\text{air}} = 1.2\,\mathrm{kg}/\mathrm{m}^{3}$), and the distance it travels from $0.5\,\mathrm{m}$ in a $2\,\mathrm{m}/\mathrm{s}$ wind. Redo it for a pine seed of mass $5\,\mathrm{mg}$ whose wing gives it a descent speed of $0.8\,\mathrm{m}/\mathrm{s}$, shed from $20\,\mathrm{m}$.

**Solution of Exercise 5.6.**

$v_s = 2\times(1.5\times 10^{-5})^2\times 1099\times 9.81/(9\times
1.8\times 10^{-5}) = 0.030\,\mathrm{m}/\mathrm{s}$, $3\,\mathrm{cm}/\mathrm{s}$; distance $uh/v_s
= 2\times 0.5/0.03 = 33\,\mathrm{m}$. Seed: $2\times 20/0.8 =
50\,\mathrm{m}$ — a heavier unit shed from higher with a wing goes about as far.

**Exercise 5.7 ★★.**

A [fern](#def-b2-plant-life-cycles-fern) frond carries $200$ sori of $40$ sporangia, each [sporangium](#def-b2-plant-life-cycles-fern) making $64$ spores. How many spores per frond? If one spore in $10^{5}$ becomes a [prothallus](#def-b2-plant-life-cycles-fern) and one [prothallus](#def-b2-plant-life-cycles-fern) in $50$ produces a [sporophyte](#def-b2-plant-life-cycles-cycles), how many young [ferns](#def-b2-plant-life-cycles-fern) does a frond yield?

**Solution of Exercise 5.7.**

$200\times 40\times 64 = 512\,000$ spores per frond; $5.1$ prothalli; $0.1$ young [sporophyte](#def-b2-plant-life-cycles-cycles) per frond.

**Exercise 5.8 ★★.**

Explain why [heterospory](#def-b2-plant-life-cycles-heterospory) is a precondition for the seed, and why a homosporous plant could not enclose its female [gametophyte](#def-b2-plant-life-cycles-cycles) in an [ovule](#def-b2-plant-life-cycles-heterospory).

**Solution of Exercise 5.8.**

A seed is a retained [ovule](#def-b2-plant-life-cycles-heterospory): the female [gametophyte](#def-b2-plant-life-cycles-cycles) must be kept on the parent, enclosed, and fed, while the male [gametophyte](#def-b2-plant-life-cycles-cycles) must travel to it. That needs two kinds of spore with two fates. A homosporous plant’s one spore must be shed to grow into a free bisexual [gametophyte](#def-b2-plant-life-cycles-cycles); retaining it would retain the male function as well and force self-fertilisation on every plant, and there would be nothing to travel.

**Exercise 5.9 ★★.**

Compare a [fern](#def-b2-plant-life-cycles-fern) spore (a sphere of radius $15\,\text{µ}\mathrm{m}$, density $1100\,\mathrm{kg}/\mathrm{m}^{3}$) and a pine seed ($5\,\mathrm{mg}$) as dispersal units: mass, energy content (take $20\,\mathrm{kJ}/\mathrm{g}$ of dry matter, $50\,\%$ dry), and what each can and cannot do on landing.

**Solution of Exercise 5.9.**

Spore: $\tfrac43\pi(1.5\times 10^{-5})^3\times 1100 =
1.6 \times 10^{-11}\,\mathrm{kg}$, $16\,\mathrm{ng}$; energy $0.5\times 1.6\times
10^{-8}\times 2\times 10^{4} = 1.6 \times 10^{-4}\,\mathrm{J}$. Seed: $5\,\mathrm{mg}$, $0.5\times 5\times 10^{-3}\times 2\times 10^{4} = 50\,\mathrm{J}$ — $3\times 10^{5}$ times more. The spore can grow one small photosynthetic scale and nothing before light; the seed can grow a root and a shoot in the dark, wait through a season, and survive being eaten or dried.

**Exercise 5.10 ★★★.**

A pine releases $10^{11}$ [pollen](#def-b2-plant-life-cycles-heterospory) grains over a forest; at [ovule](#def-b2-plant-life-cycles-heterospory) height the grains are spread in a layer $20\,\mathrm{m}$ deep over $1\,\mathrm{km}^{2}$. Estimate the [pollen](#def-b2-plant-life-cycles-heterospory) concentration (grains per cubic metre), the number of grains per second passing through an [ovule](#def-b2-plant-life-cycles-heterospory)’s micropylar opening ($0.1\,\mathrm{mm}^{2}$) in a $2\,\mathrm{m}/\mathrm{s}$ wind, and the time for an [ovule](#def-b2-plant-life-cycles-heterospory) to receive one grain. What does this say about the timing of cone opening?

**Solution of Exercise 5.10.**

$10^{11}/(10^{6}\times 20) = 5000$ grains per cubic metre; flux $5000\times 10^{-7}\times 2 = 1 \times 10^{-3}\,$ grains per second; about $1000\,\mathrm{s}$, a quarter of an hour, per grain. The cone must be open and receptive for hours to days, and precisely when the male cones shed — pollination is timed to the week.

**Exercise 5.11 ★★★.**

Give three reasons why the [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) generation came to dominate the [life cycle](#def-b2-plant-life-cycles-cycles) of land plants, and one reason why the [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) generation did not disappear altogether.

**Solution of Exercise 5.11.**

Diploidy masks [recessive](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-vocabulary) deleterious [mutations](https://one-course.com/books/biology/4/en/chapter/3-mutations-and-genome-diversification#def-b2-genome-diversification-mutation); a [diploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) body with vascular tissue can be large and long-lived, and height helps both light capture and [spore dispersal](#thm-b2-plant-life-cycles-dispersal); spores made high on a [sporophyte](#def-b2-plant-life-cycles-cycles) disperse in air, whereas gametes must swim. The [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) generation persists because [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) and fertilisation require a [haploid](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) phase, and the [pollen](#def-b2-plant-life-cycles-heterospory) grain and embryo sac are the minimal bodies that carry gametes to each other and feed the embryo.

**Exercise 5.12 ★★★.**

Hofmeister worked before chromosomes were known. Explain what he could and could not establish about the [alternation of generations](#def-b2-plant-life-cycles-cycles) by microscopy and culture alone, and what the chromosome counts added.

**Solution of Exercise 5.12.**

By culturing spores and following them under the microscope he could establish that a spore grows into a small body bearing sex organs, that the embryo arises from the fertilised egg inside the [archegonium](#def-b2-plant-life-cycles-moss), that the spore-bearing plant grows from it, and that the same organs exist in reduced form in the conifer [ovule](#def-b2-plant-life-cycles-heterospory): the alternation as a sequence of bodies and organs. He could not know what distinguished the two generations at the level of the cell. Chromosome counts showed that the two bodies differ by a factor two in chromosome number, that [meiosis](https://one-course.com/books/biology/4/en/chapter/4-meiosis-genetic-mixing-and-heredity#def-b2-meiosis-heredity-meiosis) sits at spore formation and fertilisation at the egg, and so explained why the alternation is obligatory.

## 5.7 Problem: From Spore to Seed

**Problem 5.1.**

Weekend problem — the reproductive arithmetic of a fern and a pine compared, from the number of spores and pollen grains to their dispersal, the fate of gametophytes, and the cost of a seed, ending on the number of propagules each plant must make for one replacement

A [fern](#def-b2-plant-life-cycles-fern) plant bears $10$ fertile fronds, each with $200$ sori of $40$ sporangia making $64$ spores each. Spores: radius $15\,\text{µ}\mathrm{m}$, density $1100\,\mathrm{kg}/\mathrm{m}^{3}$. A pine produces $2\times 10^{4}$ female cones’ worth of [ovules](#def-b2-plant-life-cycles-heterospory) over its life — take $100$ [ovules](#def-b2-plant-life-cycles-heterospory) per cone — and $10^{11}$ [pollen](#def-b2-plant-life-cycles-heterospory) grains per year. Seeds: $5\,\mathrm{mg}$, $50\,\%$ dry matter at $20\,\mathrm{kJ}/\mathrm{g}$. Air: $\eta = 1.8 \times 10^{-5}\,\mathrm{Pa}\,\mathrm{s}$, $\rho =
1.2\,\mathrm{kg}/\mathrm{m}^{3}$; $g = 9.81\,\mathrm{m}/\mathrm{s}^{2}$.

**Part I — The [fern](#def-b2-plant-life-cycles-fern)’s spores.**

1. Compute the number of spores the plant sheds in a season.
2. Compute the mass of one spore and of the whole crop.
3. Compute the spore’s settling speed in air.
4. From fronds at $0.5\,\mathrm{m}$ in a $1.5\,\mathrm{m}/\mathrm{s}$ wind, how far do the spores travel? Over how large an area (a disc of that radius) are they spread, and how many land per square metre?
5. Explain why turbulence carries a few spores much further, and what this implies for the colonisation of a new island.
6. Compute the energy content of one spore ( $50\,\%$ dry matter at $20\,\mathrm{kJ}/\mathrm{g}$ ) and of the whole crop, and compare with the energy of one pine seed.

**Part II — The [gametophytes](#def-b2-plant-life-cycles-cycles).** One spore in $2\times 10^{4}$ lands on soil damp enough to grow into a [prothallus](#def-b2-plant-life-cycles-fern); a [prothallus](#def-b2-plant-life-cycles-fern) lives $6$ weeks and needs a wet night — probability $0.2$ per week — for fertilisation; a fertilised [prothallus](#def-b2-plant-life-cycles-fern) gives a young [sporophyte](#def-b2-plant-life-cycles-cycles) with probability $0.5$; a young [sporophyte](#def-b2-plant-life-cycles-cycles) survives to adulthood with probability $0.02$.

7. How many prothalli does the plant’s crop produce?
8. Compute the probability that a [prothallus](#def-b2-plant-life-cycles-fern) experiences at least one wet night in its six weeks.
9. Compute the expected number of adult [ferns](#def-b2-plant-life-cycles-fern) produced by the crop of one season.
10. If the parent lives $30$ years, how many adult offspring does it leave, and what does this say about the [fern](#def-b2-plant-life-cycles-fern) population?
11. A [fern](#def-b2-plant-life-cycles-fern) sperm swims at $100\,\text{µ}\mathrm{m}/\mathrm{s}$ and a [prothallus](#def-b2-plant-life-cycles-fern) ’s archegonia lie $1\,\mathrm{mm}$ from its own antheridia. How long does self-fertilisation take? Why do many [ferns](#def-b2-plant-life-cycles-fern) nevertheless avoid it (give one mechanism)?
12. In what sense is the [prothallus](#def-b2-plant-life-cycles-fern) the weak link of the [fern](#def-b2-plant-life-cycles-fern) cycle?

**Part III — The pine’s [pollen](#def-b2-plant-life-cycles-heterospory).**

13. A [pollen](#def-b2-plant-life-cycles-heterospory) grain is a sphere of radius $25\,\text{µ}\mathrm{m}$ of effective density $600\,\mathrm{kg}/\mathrm{m}^{3}$ (air bladders included). Compute its settling speed.
14. From a cone at $15\,\mathrm{m}$ in a $3\,\mathrm{m}/\mathrm{s}$ wind, how far does it travel?
15. The $10^{11}$ grains are spread through $1\,\mathrm{km}^{2}$ of forest to a depth of $20\,\mathrm{m}$ . Compute the concentration.
16. A micropyle presents an opening of $0.1\,\mathrm{mm}^{2}$ to a wind of $2\,\mathrm{m}/\mathrm{s}$ . Compute the number of grains entering it per hour, and the time to receive the first grain.
17. Explain why a lone pine $5\,\mathrm{km}$ from the forest sets almost no seed, and why wind-pollinated species grow in stands.
18. Compare the numbers: how many [pollen](#def-b2-plant-life-cycles-heterospory) grains does the forest make per [ovule](#def-b2-plant-life-cycles-heterospory) , if it holds $1000$ pines with $2000$ [ovules](#def-b2-plant-life-cycles-heterospory) each?

**Part IV — The pine’s seeds.** Of the tree’s [ovules](#def-b2-plant-life-cycles-heterospory), $60\,\%$ are pollinated and $80\,\%$ of those fill a seed; a seed becomes a seedling with probability $0.05$ and a seedling an adult with probability $0.01$.

19. Compute the number of seeds the tree makes in its life, and their total mass and energy.
20. A winged seed descends at $0.8\,\mathrm{m}/\mathrm{s}$ . From $20\,\mathrm{m}$ in a $3\,\mathrm{m}/\mathrm{s}$ wind, how far does it travel? Compare with the [pollen](#def-b2-plant-life-cycles-heterospory) .
21. Compute the expected number of adult offspring, and compare with the [fern](#def-b2-plant-life-cycles-fern) ’s.
22. Compute the energy the tree invests per adult offspring, and the [fern](#def-b2-plant-life-cycles-fern) per adult offspring (spores only), and compare.
23. The [fern](#def-b2-plant-life-cycles-fern) spore can wait a few weeks; the pine seed several years. Explain, with the energy figures, why the seed can afford dormancy and the spore cannot.
24. Give two advantages that make the seed worth its cost and two conditions under which the [fern](#def-b2-plant-life-cycles-fern) ’s strategy is the better one.
25. State the result: propagules per adult offspring for the [fern](#def-b2-plant-life-cycles-fern) and for the pine, and the energy each spends per adult offspring.

**Solution of Problem 5.1.**

**1.** $10\times 200\times 40\times 64 = 5.1\times 10^{6}$ spores. **2.** $\tfrac43\pi(1.5\times 10^{-5})^3\times 1100 =
1.6 \times 10^{-11}\,\mathrm{kg}$ per spore; crop $8\times 10^{-5}$ kg, $80\,\mathrm{mg}$. **3.** $v_s = 2\times 2.25\times 10^{-10}\times 1099\times
9.81/(1.62\times 10^{-4}) = 0.030\,\mathrm{m}/\mathrm{s}$. **4.** $x = 1.5\times 0.5/0.030 = 25\,\mathrm{m}$; disc of $\pi\times 25^2 = 2000\,\mathrm{m}^{2}$; about $2600$ spores per square metre. **5.** Updrafts of a few centimetres per second exceed $v_s$, so a spore caught in turbulence stays aloft for hours and travels hundreds of kilometres; a single spore founds a population if its [prothallus](#def-b2-plant-life-cycles-fern) can self-fertilise (it bears both organs), which is why remote islands have rich [fern](#def-b2-plant-life-cycles-fern) floras. **6.** Spore: $0.5\times 1.6\times 10^{-8}\times 2\times 10^{4} =
1.6 \times 10^{-4}\,\mathrm{J}$; crop: $820\,\mathrm{J}$; one pine seed: $50\,\mathrm{J}$ — the whole spore crop equals sixteen seeds. **7.** $5.1\times 10^{6}/2\times 10^{4} = 256$ prothalli. **8.** $1 - 0.8^{6} = 1 - 0.26 = 0.74$. **9.** $256\times 0.74\times 0.5\times 0.02 = 1.9$ adult [ferns](#def-b2-plant-life-cycles-fern) per season. **10.** $30\times 1.9 \approx 57$: far more than the one replacement of a stable population, so the survival figures are optimistic — in a full habitat most young [sporophytes](#def-b2-plant-life-cycles-cycles) die of crowding and shade. **11.** $10^{-3}/10^{-4} = 10\,\mathrm{s}$. Many [ferns](#def-b2-plant-life-cycles-fern) mature antheridia and archegonia at different times, or release a hormone (antheridiogen) that makes neighbouring young prothalli male, so that sperm come from another individual. **12.** It is one cell thick, rootless, unprotected, needs liquid water for fertilisation and lives a few weeks: nearly all the mortality of the cycle falls on it (one spore in $2\times 10^{4}$ becomes one; a quarter of those never see a wet night). **13.** $v_s = 2\times 6.25\times 10^{-10}\times 599\times
9.81/(1.62\times 10^{-4}) = 0.045\,\mathrm{m}/\mathrm{s}$. **14.** $3\times 15/0.045 = 1000\,\mathrm{m}$. **15.** $10^{11}/(2\times 10^{7}) = 5000$ per cubic metre. **16.** $5000\times 10^{-7}\times 2 = 1 \times 10^{-3}\,\mathrm{s}^{-1}$: 3.6 per hour; the first arrives after about $1000\,\mathrm{s}$, a quarter of an hour. **17.** Five kilometres downwind the cloud has spread sideways and upward and most grains have settled (a $1\,\mathrm{km}$ range per $15\,\mathrm{m}$ of height), so the concentration is orders of magnitude lower and an [ovule](#def-b2-plant-life-cycles-heterospory) may wait days for a grain; the tree’s own [pollen](#def-b2-plant-life-cycles-heterospory) mostly gives selfed seeds, which abort. Wind pollination needs a dense cloud, hence stands. **18.** $10^{14}$ grains for $2\times 10^{6}$ [ovules](#def-b2-plant-life-cycles-heterospory): $5\times
10^{7}$ grains per [ovule](#def-b2-plant-life-cycles-heterospory). **19.** $2\times 10^{6}$ [ovules](#def-b2-plant-life-cycles-heterospory) $\times 0.6\times 0.8 = 9.6\times
10^{5}$ seeds; $4.8\,\mathrm{kg}$; $48\,\mathrm{MJ}$. **20.** $3\times 20/0.8 = 75\,\mathrm{m}$: the [pollen](#def-b2-plant-life-cycles-heterospory) goes a kilometre, the seed a few tree-heights; the genes travel by [pollen](#def-b2-plant-life-cycles-heterospory), the plant by seed. **21.** $9.6\times 10^{5}\times 0.05\times 0.01 = 480$ adult offspring, against the [fern](#def-b2-plant-life-cycles-fern)’s 57. **22.** Pine: $4.8\times 10^{7}/480 = 100\,\mathrm{kJ}$ per adult offspring; [fern](#def-b2-plant-life-cycles-fern): $820/1.9 = 430\,\mathrm{J}$ per adult offspring, two hundred times less. **23.** Both are half dry matter at $20\,\mathrm{kJ}/\mathrm{g}$, so at a basal rate of, say, $0.5\,\mathrm{mW}$ per gram dry both last $10^{4}\,
\text{J/g}/(5\times 10^{-4}\,\text{W/g}) = 2\times 10^{7}$ s, about eight months: the duration of dormancy is not what the seed’s energy buys. It buys what happens after germination — a root and a shoot built in the dark from reserves, weeks before the seedling feeds itself — and, with the coat, protection meanwhile; the spore’s $1.6 \times 10^{-4}\,\mathrm{J}$ builds a few cells that must photosynthesise at once. **24.** Seed: fertilisation without water; a provisioned, protected, dormant embryo that establishes in dry or seasonal places; dispersal by wings, fruits and animals. [Fern](#def-b2-plant-life-cycles-fern) strategy wins in wet, shaded, stable habitats, and in colonising distant or newly opened ground, where cheap, light, numerous propagules matter more than provisioning. **25.** [Fern](#def-b2-plant-life-cycles-fern): $5.1\times 10^{6}$ spores per season for 1.9 adults, $2.7\times 10^{6}$ spores per adult offspring, $430\,\mathrm{J}$ each; pine: $9.6\times 10^{5}$ seeds for 480 adults, $2000$ seeds per adult offspring, $100\,\mathrm{kJ}$ each.