Kingdom Plantae

8.1An overview of Kingdom Plantae

The kingdom Plantae, also known as the plant kingdom, is very diverse and includes hundreds of thousands of different species. Plants live in a wide range of habitats, from the freezing Arctic tundra to lush tropical rainforests and arid deserts. Because plants are so varied and so important in ecosystems, this chapter concentrates on terrestrial (land) plants and their evolutionary history.

The chapter follows the evolution of land plants, then the seedless plants, then the evolution of seed plants. The presence of plants on land has shaped terrestrial ecosystems and provides a foundation for other forms of life to thrive in them.

Origin of land plants

• In the beginning plants were restricted to aquatic (water) conditions.
• The migration of plants towards land started nearly 400 million years ago.
• Biologists have identified the freshwater green algae Charophyceae (a group of algae, for example Chara, a pond organism) as the closest relatives of land plants.

8.1.1Diagnostic features of plantsOften examined

Plants are multicellular eukaryotes with well-developed tissues and autotrophic nutrition (they make their own food). Their diagnostic (identifying) features are listed below.

• Cuticle: plants are protected from drying up in air by a cuticle formed from a waxy substance called cutin.
• Body plan: the plant body has roots, stems and leaves with the vascular tissues xylem and phloem.
• Cell walls: plant cells have walls rich in cellulose.
• Alternation of generations: plants show an alternation of a diploid sporophyte and a haploid gametophyte.
• Oogamous gametes: the gametophyte has multicellular sex organs that make morphologically and physiologically different gametes (a large egg and a small sperm).

Sporophyte

The diploid ($2n$) generation of a plant. It produces haploid spores by meiosis.

Gametophyte

The haploid ($n$) generation that develops from a spore. It has multicellular sex organs that produce gametes, and the gametes unite to form a diploid zygote.

Oogamous

Producing two different kinds of gametes, a large non-motile egg and a small motile sperm, that unite to form a diploid zygote.

Alternation of generations

The regular alternation of the sporophyte generation and the gametophyte generation in the life cycle of a plant.

Heteromorphic alternation of generations

Alternation in which the sporophyte and the gametophyte look different from each other (they show distinct morphological differences).

The life cycle in Figure 8.1 has five stages, which are the same in every plant group; only the size and importance of each generation change from group to group.

8.1.2Classification of Kingdom Plantae

In the phylogenetic classification system the kingdom Plantae is divided into two main groups: Bryophytes and Tracheophytes.

• Bryophytes are non-vascular plants. They include liverworts, mosses and hornworts and lack specialized tissues for efficient water and nutrient transport.
• Tracheophytes are vascular plants. They consist of two primary sister groups: Lycophytes (clubmosses and relatives) and Euphyllophytes (ferns, horsetails and seed plants).

Lycophytes, ferns and horsetails together are known as pteridophytes. They are free-sporing vascular plants with shared ancestral characteristics. When the sporophyte is discussed, pteridophytes (a paraphyletic group) are often compared with spermatophytes (seed plants, a monophyletic group), which helps in understanding evolutionary relationships. Seed plants divide further into Gymnosperms (non-flowering) and Angiosperms (flowering), and the angiosperms have unique structural traits that set them apart.

Phylogenetic classification

Classification of organisms according to their evolutionary relationships.

Monophyletic group

A group that contains a common ancestor and all of its descendants (for example the seed plants).

Paraphyletic group

A group that contains a common ancestor but not all of its descendants (for example the pteridophytes).

Pteridophytes

The free-sporing (seedless) vascular plants: lycophytes, ferns and horsetails.

Spermatophytes

The seed-producing vascular plants: gymnosperms and angiosperms.

8.2Bryophyta (non-vascular plants)

Plants are currently divided into two main groups: the nonvascular plants or bryophytes, and the vascular plants or tracheophytes. The division Bryophyta consists of plants that lack vascular tissues (xylem and phloem).

Bryophytes do not have true roots, stems and leaves. Their bodies have only root-like, stem-like and leaf-like structures, so they are said to be non-vascular.

8.2.1General characteristics of bryophytesOften examined

Bryophyta is a group of plants comprising liverworts, hornworts and mosses, the only nonvascular plants. Bryophytes are typically quite small, and a few exceed 2 centimetres in length. They mostly live in wet areas, but some can also thrive in dry environments. Water is crucial for their reproduction because their male gametes swim. For this reason they are sometimes called amphibious plants, but they are generally classified as land plants.

The main plant body of a bryophyte is the gametophyte. The gametophytes are green and make their own food, and they are relatively large and evident compared with the sporophytes. Some sporophytes are completely enclosed within gametophyte tissue; others are not enclosed and turn brownish or straw-coloured at maturity.

The four main features of bryophytes

1. They lack specialized vascular tissues.
2. Multicellular sex organs produce the embryo.
3. Sporophytes are always smaller and obtain their food from the gametophyte.
4. The sporophyte is homosporous (it makes one kind of spore).

Land adaptations in bryophytes

Land adaptations in bryophytes

Those characteristics of bryophytes that help them to live in a land habitat.

During evolution bryophytes developed several adaptations to colonize land:

1. They form compact multicellular plant bodies that reduce water loss by minimizing the surface area exposed to dry conditions. They also have cuticles to reduce water evaporation further.
2. Photosynthetic tissues are organized into special chambers that allow efficient carbon dioxide absorption while minimizing water loss and exposure to excessive light.
3. They develop rhizoids, which help in water absorption and provide anchorage.
4. They show heterogamy or oogamy: two types of gametes, non-motile eggs with stored food and motile sperms.
5. Special multicellular organs, the antheridia and archegonia, protect and help in the production of gametes.
6. They form multicellular embryos that are retained and protected within the female reproductive structures during development.
7. They show alternation of generations, between spore-producing sporophytes and gamete-producing gametophytes. This allows them to explore various genetic combinations for adapting to diverse terrestrial conditions.

Subdivisions of bryophytes

Bryophytes are divided into three subdivisions (Figure 8.4 shows a liverwort, a moss and a hornwort).

8.2.2Life cycle of Polytrichum (a moss plant)Often examined

The genus Polytrichum belongs to the group Bryopsida (mosses) of bryophytes. Like other bryophytes it shows two generations, the sporophyte and the gametophyte, which regularly alternate with each other.

Gametophytic generation

The main plant body is the gametophyte. It consists of an erect, growing stem-like part bearing several green leaf-like appendages. This leafy shoot is anchored in the ground by root-like structures called rhizoids. It produces gametes and so reproduces by the sexual method; the sex organs are at the apex of the shoot.

The sex organs are developed in clusters in which they are mixed with multicellular hair-like structures, the paraphyses. In Polytrichum the antheridia and archegonia are found on separate plants: this state is called dioecious. In other bryophytes such as Funaria the sex organs may occur on the same plant: this is monoecious.

Fertilization

Sporophytic generation

The zygote is the first cell of the sporophytic generation. It divides repeatedly by mitosis to make a multicellular embryo, which develops into the mature sporophyte. The sporophyte consists of:

• a foot, embedded in the tissue of the gametophyte;
• a meristematic region, the seta, which grows into a long stalk;
• a spore-producing sac-like structure, the capsule (or sporangium), on the tip of the stalk.

Germination of spores

Spores are formed in the sporophyte by meiosis, so the spores are haploid. The spore is the first cell of the gametophytic generation. It germinates into an alga-like structure, the protonema, which has buds and branches. The bud gives rise to the mature gametophyte.

8.3Tracheophyta (vascular plants)

Tracheophytes are vascular plants: they have xylem and phloem for the conduction of water, minerals and organic solutes. They earn their name from the universal presence of tracheids in their xylem.

8.3.1General characteristics of Tracheophytes (vascular plants)Often examined

This group of land plants is highly successful, with flowering plants being particularly dominant in land habitats today. The success is due to the evolution of several complex vegetative and reproductive characteristics.

1. The sporophyte is the main plant body. It consists of roots, stems and leaves, and has vascular systems in these structures.
2. The evolution of protected sporangia, which ultimately led to the development of seeds, played a crucial role.
3. The adaptation of a pollen tube allowed safe, water-independent transmission of male gametes to female gametes.
4. The presence of flowers and fruits further enhanced reproductive success.
5. The life cycle involves heteromorphic alternation of generations, which gives flexibility in adapting to various terrestrial conditions.

Tracheophytes are further classified into four subdivisions: Psilopsida, Lycopsida, Sphenopsida and Pteropsida.

8.4Pteridophyta (seedless vascular plants)

Pteridophytes are seedless vascular plants. They consist of the earlier three complete subdivisions of tracheophytes (Psilopsida, Lycopsida, Sphenopsida) and the Fillicinae (ferns) class of Pteropsida.

8.4.1Psilopsida: whisk ferns

The Psilopsida are the simplest of the vascular plants. They are generally called whisk ferns because they resemble whiskbrooms.

• No true roots: instead they have underground stems called rhizomes, which bear rhizoids for anchorage.
• No true leaves: the aerial stems have very small green scales and the stems themselves are the sites of photosynthesis.
• Sporangia (for reproduction) are usually at the apical tips or along the lateral sides of the aerial branches.
• The gametophyte is a flat, undifferentiated structure. It lacks colour and stays beneath the soil. Inside its cells lives a fungus that supplies nutrients to the gametophyte and, in exchange, the fungus gets protection from it.
• Many members of the group have become extinct; a notable example is Rhynia. Among the surviving genera, Psilotum is the most prevalent and best known.

8.4.2Lycopsida: club mosses

Lycopsida comprises a variety of plants including club mosses, spike mosses and quillworts. The plant body has a branching rhizome that produces aerial stems, typically not more than 30 cm in height.

• The stems and branches are densely covered with scale-like leaves called microphylls. A microphyll is unique in having only a single strand of vascular tissue.
• In club mosses the sporangia are borne on terminal clusters of leaves called strobili (singular: strobilus), which look club-shaped.
• Club mosses are the only living plants with microphylls.
• The gametophyte of Lycopsida is mainly underground.
• Prominent genera are Lycopodium and Selaginella.

Microphyll

A small, scale-like leaf with only a single strand of vascular tissue, found in club mosses.

Strobilus

A terminal cluster of sporangium-bearing leaves with a club-shaped appearance (plural: strobili).

8.4.3Sphenopsida: horsetails

Sphenopsida, commonly known as horsetails, grow all over the world in waste and moist places. The group has more fossil species than living species: only a single surviving genus, Equisetum, is found today.

• The plants develop from rhizomes, which give rise to two kinds of aerial stems: fertile and sterile.
• The aerial stems are slender, green, hollow and jointed, so horsetails are also called arthrophytes (jointed plants).
• The sterile stems bear whorls of leaves and branches at the joints (the nodes). The internode (the region between two successive nodes) has no leaves.
• Spores are produced in rings of sporangia arranged in cones (strobili), usually at the tips of the fertile shoots.
• The gametophytes are thalloid and grow on clayey soil and on mud.

8.4.4Pteropsida (ferns, Fillicinae)Often examined

Pteropsida can be split into three classes: (i) Fillicinae, (ii) Gymnospermae and (iii) Angiospermae. The members of Fillicinae are also known as ferns. They are seedless vascular plants, so they are included in the group Pteridophyta. Gymnospermae and Angiospermae consist of seed-producing plants, which together make the group Spermatophyta.

Fillicinae (ferns)

Ferns belong to the group Pteropsida, in the subgroup or class Fillicinae, which is the most abundant group of seedless vascular plants in warm and moist tropical regions. Ferns range in size from reduced aquatic forms less than a centimetre to a tree fern that may have trunks more than 24 metres tall with leaves up to 5 metres or more in length.

• Except for a few, all ferns are homosporous (one kind of spore).
• The sporophyte generation is much larger, more conspicuous and more complex than the gametophyte, and the sporophyte is completely independent.
• Sporangia are foliar: they are attached to special leaves or fronds. The fronds are adapted for both reproduction and photosynthesis.
• Young fronds are coiled and later become flattened. This kind of growth is called circinate vernation, an important characteristic of this group.

Circinate vernation

The coiled arrangement of a young fern frond, which uncoils and flattens as it grows (the young coiled leaf is called a fiddlehead).

Frond

A fern leaf, adapted for both photosynthesis and spore production (the sporangia are foliar).

8.4.4Life cycle of fern (Adiantum)Often examined

Adiantum (maiden hair fern) is a herbaceous fern that grows near moist walls and watercourses. Its life cycle has two generations, the sporophyte and the gametophyte, and both are independent.

Sporophyte generation

The main plant body is the sporophyte, which consists of a stem, roots and leaves.

• Stem: a short, stout, horizontally growing underground rhizome. It is protected by brownish scales called ramenta and stays shielded by the lasting bases of the leaves.
• Roots: the lower side of the rhizome gives rise to fibrous adventitious roots.
• Leaves: on the upper side of the rhizome emerge large pinnately compound fronds. Young leaves (fiddleheads) show circinate vernation.
• Rachis: the main stalk of the leaf is dark, smooth and shiny, which is why the plant is called the 'maiden hair fern'.
• Leaflets: pinnae (first-order leaflets) and pinnules (second-order leaflets) show dichotomous venation.
• Sori: clusters of sporangia on the underside of the spreading lobes along the margin of the leaflet, protected by the bent margin of the leaflet, which forms a false indusium.

A mature sporangium is flattened, spherical or ellipsoidal. It consists of a stalk and an upper swollen capsule covered by a single-layered wall. The wall has two portions:

Inside the sporangium, spores are produced by meiosis of spore mother cells; many spores form in each sporangium. The wall of a spore has two layers, the exine (outer) and the intine (inner).

Gametophyte generation

The gametophyte (prothallus) of Adiantum is heart-shaped, with a central notch that acts as the growing point. It is multicellular and thick at its centre and narrows to only one cell at the margins.

• Rhizoids form on the underside of the gametophyte and serve for anchorage and the absorption of water and nutrients.
• The gametophyte contains chloroplasts, so it photosynthesizes and functions independently.
• Two types of reproductive organs, antheridia and archegonia, develop on it.
• Archegonium: a flask-shaped structure with two parts, the venter (contains the egg or oosphere) and the neck (contains the neck canal cells).
• Antheridium: a globular structure that produces many antherozoids (sperms). They are mature, spiral-coiled and have many cilia, so they need water to move.

Fertilization

Chemotactic movement

Movement of a cell in response to a chemical stimulus.

Prothallus

The heart-shaped haploid gametophyte of a fern.

Sorus (plural: sori)

A cluster of sporangia on the underside of a fern leaflet.

Heteromorphic alternation of generations

A life cycle in which the sporophyte and gametophyte generations are morphologically different, as in the fern: a large leafy sporophyte and a small heart-shaped gametophyte.

8.4Importance of seedless vascular plants

The book lists the following uses of the seedless vascular plants (Science Tidbits).

8.5Spermatophyta (seed producing vascular plants)Often examined

Spermatophytes are seed-producing vascular plants. They include the two classes of Pteropsida: Gymnosperms (plants with exposed seeds) and Angiosperms (plants with seeds enclosed within fruits). Gymnosperm seeds are borne openly on sporophylls within cones, while angiosperm seeds are encased within fruits produced from flowers. Both classes show reproductive adaptations that are crucial for the successful colonization of land.

Common adaptations of all spermatophytes

1. Heterospory: two distinct kinds of spores. Microspores are small male spores that develop into male gametophytes; megaspores are large female spores that give rise to female gametophytes (embryo sacs) on germination.
2. The indehiscent (non-rupturing) nature of the megasporangium (ovule) keeps the megaspore inside and lets it germinate there.
3. Formation of integuments, protective layers around the megasporangium, which contribute to its indehiscence.
4. Reduction to one functional megaspore: of the four megaspores made by meiosis of the megaspore mother cell in each sporangium, only one survives. This reduces competition among developing female gametophytes.
5. A micropyle (a small opening) at the distal end of the megasporangium, which makes pollen tube penetration easy.
6. A narrow pollen tube inside the male gametophyte, which carries sperm safely to the female gametophyte and allows water-independent reproduction on land.
7. After fertilization the zygote matures into an embryo (a miniature plant) and the entire megasporangium becomes the seed (a fertilized or ripened ovule); the integuments form the seed coat.

Heterospory

The production of two kinds of spores, microspores and megaspores.

Indehiscent

Not rupturing (not splitting open) when ripe, as the megasporangium.

Integuments

Protective layers that surround the megasporangium and later form the seed coat.

Micropyle

The small opening at the tip of the ovule through which the pollen tube enters.

Seed

A fertilized (ripened) ovule containing an embryo, usually with stored food, and covered by a seed coat.

8.5.1GymnospermsOften examined

Gymnosperms are a highly successful group of seed plants with a global distribution; they make up about one-third of the world's forests. They may be tall, woody, perennial trees or shrubs. The name gymnospermae means 'naked seeded' (gymno = naked, spermae = seed): the ovules are exposed on the surface of fertile leaves (megasporophylls), unlike angiosperms in which the ovules are enclosed. These heterosporous plants produce seeds but do not produce fruits.

Characteristics of gymnosperms

• Leaves show xerophytic features: a thick, tough cuticle, stomata sunken in pits and wax on the surface.
• Xylem consists of tracheids and xylem parenchyma. Vessels and wood fibres are generally absent (the exception is Gnetales).
• Companion cells are absent in the phloem.
• They show regular heteromorphic alternation of generations with a dominant, independent sporophyte and a less conspicuous, dependent gametophyte.
• Microspores and megaspores, which develop on microsporophylls and megasporophylls, give rise to the two gametophytes. The megasporophylls, bearing ovules, remain unfolded and unjoined at the margins, exposing the seeds on their surface.
• Microsporophylls and megasporophylls are arranged spirally on a central axis to make the male and female cones (strobili).
• Polyembryony (several embryos in one seed) is common, but finally a single embryo matures.

Classification and genera

Prominent genera within the group are given below (Figure 8.12 shows seven of them).

8.5.1Life cycle of PinusOften examined

The pine, Pinus, belongs to the order Coniferales of the class Gymnospermae. Many pine species are suited to mountainous and high-altitude habitats, in areas with cold temperatures and large seasonal changes. In the life cycle of Pinus the dominant diploid sporophyte alternates with an inconspicuous haploid gametophyte.

Sporophytic generation

The main plant body is the sporophyte.

• A woody trunk, typically straight and tall, with branches radiating from it in a spiral fashion.
• Evergreen needle-like or scale-like leaves, adapted to conserve water.
• A taproot system with lateral roots for anchorage and for absorbing water and nutrients from the soil.
• The spore-producing parts are called cones or strobili, which is why these plants are called conifers. There are two types: the male cone (staminate strobilus) and the female cone (ovulate strobilus).

Figure 8.14 shows the whole life cycle in eight numbered stages. The detailed steps of pollination, gametophyte development, fertilization and seed formation are given in the next section.

Strobilus (cone)

A spore-bearing structure made of sporophylls arranged spirally on a central axis.

Microsporophyll

A scale-like leaf of the male cone that bears microsporangia.

Megasporophyll

A scale of the female cone that bears ovules (megasporangia); in gymnosperms it is unfolded, so the ovules are exposed.

Functional megaspore

The one haploid megaspore, out of four, that survives to form the female gametophyte.

8.5.1Pollination, gametophytes and seed formation in PinusOften examined

Pollination

The microspores have wings by which they can fly to a large distance, and only a few of them reach their target, the surface of the megasporangium in the female cone. This movement of microspores from the male cone to the female cone is called pollination. During this event the microspores are called pollen grains. In gymnosperms the pollen grains are developing male gametophytes.

Development of the male gametophyte

The microspores begin to germinate into male gametophytes within the microsporangium (this is endosporic development).

Development of the female gametophyte

The female gametophyte develops within the megasporangium (ovule). The functional megaspore (n) divides repeatedly by mitosis to form a multicellular female gametophyte or embryo sac, which contains one to several archegonia. The structure of an archegonium is quite similar to that of bryophytes and pteridophytes; each archegonium contains the egg in its basal swollen part.

Fertilization and development of the seed

8.5.2AngiospermsOften examined

Angiosperms, or flowering plants, are a diverse group of plants known for having their seeds enclosed within fruits. The name literally means 'enclosed seed': 'angio' means closed and 'sperm' means seed. The key reproductive structures are the flowers, which house specialized leaves containing ovules. These leaf structures come together and fold at their edges to form the ovaries within the flowers. Once fertilization occurs, the ovaries transform into fruits.

Angiosperms have a well-developed vascular system, with vessels and fibres in the xylem and companion cells in the phloem, unlike gymnosperms. In their life cycle they follow heteromorphic alternation of generations: a dominant, independent sporophyte and a less prominent, dependent gametophyte. This group of plants is both extensive and successful, with many familiar plant species belonging to it.

8.5.2Life cycle of a flowering plantOften examined

The life cycle of an angiospermic plant consists of a diploid sporophyte and a haploid gametophyte that regularly alternate with each other.

Sporophytic generation

The adult angiospermic plant is a diploid sporophyte with roots, stems and leaves. It may be a herb, shrub or tree. When it reaches maturity it begins to produce flowers.

A flower is essentially a modified shoot. It generally has a stalk called the pedicel, a central part called the thalamus (torus), and floral leaves: sepals, petals, stamens and carpels. These are arranged in four concentric rings (whorls) on the thalamus: calyx, corolla, androecium and gynoecium.

The calyx and corolla, the outer two whorls, are like the protectors of the flower. They protect the stamens and carpels and attract insects for pollination, but they do not take part in reproduction directly, so they are called accessory whorls. The androecium and gynoecium, the inner two whorls, are the reproductive parts, so they are called necessary whorls.

Pedicel

The stalk of a flower.

Thalamus (torus)

The central part of the flower on which the floral whorls are arranged.

Stamen

The male part of a flower: a long stalk, the filament, with a bilobed swollen tip, the anther.

Pollen sacs (microsporangia)

Four sacs in the lobes of an anther that produce haploid pollen grains (microspores) by meiosis.

Carpel (pistil)

The female part of a flower: a sac-like ovary at the bottom, a long tubular style above it and a swollen stigma at the tip of the style.

Ovule

An integumented, indehiscent megasporangium in the ovary. Each ovule has one spore mother cell that divides by meiosis into four megaspores, of which only one is functional.

Parts that make the spores

Each stamen has a long stalk, the filament, and a bilobed swollen tip, the anther. In the anther lobes there are four pollen sacs (microsporangia), which produce haploid pollen grains (microspores) by meiosis.

Each carpel or pistil has a sac-like ovary at the bottom, a long tubular style above the ovary and a swollen tip, the stigma, on the style. The ovary contains one to numerous ovules, which are integumented indehiscent megasporangia. Each ovule has one spore mother cell that divides by meiosis to produce four megaspores; of these only one is functional.

8.5.2Pollination and gametophytes of angiospermsOften examined

Pollination

When the microspores are formed, the wall of the pollen sacs bursts and the microspores are liberated. They are transferred from the anther of the stamen to the stigma of the carpel by wind, insects or other animals. This event is known as pollination, and the microspores during pollination are called pollen grains.

Development of the male gametophyte

In angiosperms the pollen grains are not developing male gametophytes, because they begin to germinate on the stigma of the carpel after pollination. Therefore the male gametophyte in angiosperms is called exosporic (it develops outside the microsporangium).

Development of the female gametophyte

In the ovule the functional megaspore nucleus undergoes three rounds of mitosis, giving a total of 8 nuclei.

8.5.2Fertilization, seed formation and germinationOften examined

Fertilization

The pollen tube elongates through the style, enters the ovule and reaches the female gametophyte. There it releases the male gametes.

Seed formation and germination

After fertilization the zygote develops into an embryo, while the endosperm nucleus develops into a multicellular nutritive tissue, the endosperm. The integuments of the ovule become a protective seed coat known as the testa and tegmen, while the ovary wall develops into the fruit. Seeds typically enter a period of dormancy and then, under favourable conditions, germinate to produce a seedling. The seedling gradually grows into a sporophyte, giving a regular alternation between a dominant sporophyte generation (2n) and a less conspicuous gametophyte generation (n).

8.5.3Classification of angiospermsOften examined

Angiosperms can be further classified into two subclasses: Dicotyledonae (dicots) and Monocotyledonae (monocots). The two subgroups can be distinguished by several characteristics, which are outlined in Table 8.1.

8.5.4Common angiospermic plant families

The monocot and dicot subclasses are further divided into orders and families. Commonly occurring angiospermic plants belong to the following families: Brassicaceae, Solanaceae and Poaceae.

8.5.4Family Brassicaceae (mustard or cabbage family)

Brassicaceae, previously called Cruciferae, is a medium-sized and economically important family of dicot plants. They are commonly known as the mustards, the crucifers or the cabbage family.

General characteristics

Economic importance

8.5.4Family Solanaceae (night shade or potato family)

The Solanaceae, also known as the potato or deadly nightshade family (night shade family), is one of the most utilized and important plant families. It contains some of the world's most important food plants: the potato, tomato, all peppers, ground cherries (tomatillo) and eggplant.

General characteristics

Economic importance

Members of the family provide drugs and food; some are weedy, some are poisonous and others are handsome ornamentals.

8.5.4Family Poaceae (Gramineae, grass family)

Poaceae, formerly called Gramineae, is the most economically important monocot plant family. It provides staple foods from domesticated cereal crops such as maize, wheat, rice, barley and millet, as well as feed for meat-producing animals.

General characteristics

Economic importance

STEAM activity: Botanical detective

The book closes the chapter with a STEAM activity (Science, Technology, Engineering, Arts and Mathematics) called 'Botanical Detective: Unveiling the Green Mysteries'.

Objective

To explore plant morphology and taxonomy and to develop skills in plant identification through an interactive, hands-on activity.

Materials

1. Potted plants or plant specimens with distinct morphological features.
2. Hand lenses or magnifying glasses.
3. Notebooks and pencils.
4. Digital devices for research purposes.
5. Plant identification field guides or online resources.

Procedure

Figure 8.23 in the book is a partly empty dichotomous key; students fill the empty boxes. The completed key is drawn below. A dichotomous key divides the plants into two groups at every step, using one clear feature, until each plant is named.

Key terms

Plantae

The plant kingdom: multicellular eukaryotes with well-developed tissues and autotrophic nutrition.

Charophyceae

A group of freshwater green algae (for example Chara) identified as the closest relatives of land plants.

Cutin

The waxy substance that forms the cuticle protecting plants from drying up in air.

Sporophyte

The diploid generation of a plant, which produces haploid spores by meiosis.

Gametophyte

The haploid generation of a plant, with multicellular sex organs that produce gametes.

Oogamous

Producing different (large egg and small sperm) gametes that unite to form a diploid zygote.

Alternation of generations

The regular alternation of a sporophyte and a gametophyte generation in the life cycle of a plant.

Heteromorphic alternation of generations

Alternation in which the sporophyte and gametophyte generations are morphologically different.

Bryophytes

Non-vascular plants (liverworts, hornworts and mosses) whose main body is the gametophyte.

Tracheophytes

Vascular plants with xylem and phloem; they are named after the tracheid of the xylem.

Pteridophytes

Free-sporing seedless vascular plants: lycophytes, whisk ferns, horsetails and ferns.

Monophyletic group

A group of organisms that includes a common ancestor and all its descendants, for example the seed plants.

Amphibious plants

A name sometimes given to bryophytes because their swimming male gametes need water for reproduction, although they are classified as land plants.

Land adaptations in bryophytes

Characteristics of bryophytes that help them to live on land, such as compact bodies, cuticles, rhizoids, protected embryos and multicellular sex organs.

Homosporous

Producing only one kind of spore (bryophytes and most ferns).

Rhizoids

Root-like structures that anchor bryophytes and the gametophytes of some plants and absorb water.

Antheridium

The male sex organ, which produces sperms (antherozoids).

Archegonium

The flask-shaped female sex organ with a venter containing the egg and a neck.

Dioecious

Having the antheridia and archegonia on separate plants, as in Polytrichum.

Monoecious

Having the antheridia and archegonia on the same plant, as in Funaria.

Protonema

The alga-like, branched structure into which a moss spore germinates; its bud gives the mature gametophyte.

Tracheid

The water-conducting cell present in the xylem of all vascular plants.

Rhizome

An underground horizontal stem, as in whisk ferns, club mosses, horsetails and ferns.

Microphylls

Small scale-like leaves with a single strand of vascular tissue, found in club mosses.

Strobilus

A cone: a cluster of sporangium-bearing leaves arranged on a central axis (plural: strobili).

Circinate vernation

The coiled arrangement of young fern fronds that later flatten.

Frond

The leaf of a fern, adapted for both photosynthesis and reproduction.

Sorus

A cluster of sporangia on the underside of a fern leaflet (plural: sori).

False indusium

The bent margin of a leaflet that protects the sori of Adiantum.

Annulus

The part of the sporangium wall with thick-walled cells that contracts to burst the sporangium.

Stomium

The thin-walled weak region of the sporangium wall where it bursts.

Prothallus

The heart-shaped haploid gametophyte of a fern.

Heterospory

The production of two kinds of spores: small male microspores and large female megaspores.

Microspore

A small male spore that develops into the male gametophyte.

Megaspore

A large female spore that develops into the female gametophyte (embryo sac).

Integuments

Protective layers around the megasporangium that form the seed coat.

Micropyle

The small opening at the tip of the ovule through which the pollen tube enters.

Pollen tube

The narrow tube of the male gametophyte that carries sperm to the female gametophyte without need of water.

Seed

A fertilized (ripened) ovule containing an embryo and covered by a seed coat.

Gymnosperms

Non-flowering seed plants with naked seeds borne on sporophylls in cones.

Polyembryony

The development of several embryos in one ovule, common in gymnosperms.

Pollination

The transfer of microspores (pollen grains) from the male cone or anther to the female cone or stigma.

Single fertilization

Fertilization in which each male gametophyte contributes only one sperm, as in gymnosperms.

Angiosperms

Flowering plants whose seeds are enclosed in fruits formed from the ovaries.

Calyx

All the sepals of a flower; the outermost whorl, an accessory whorl.

Corolla

All the petals of a flower; an accessory whorl.

Androecium

The male whorl of a flower, made of stamens (filament and anther).

Gynoecium

The female whorl of a flower, made of carpels (stigma, style and ovary).

Accessory whorls

The calyx and corolla, which protect the flower and attract insects but are not directly part of reproduction.

Necessary whorls

The androecium and gynoecium, the reproductive whorls of a flower.

Ovule

An integumented, indehiscent megasporangium in the ovary that becomes the seed after fertilization.

Exosporic

Describes the angiosperm male gametophyte, which develops on the stigma after pollination, outside the microsporangium.

Embryo sac

The female gametophyte of an angiosperm: egg, synergids, polar nuclei or secondary nucleus, and antipodals.

Double fertilization

In angiosperms, the fusion of one sperm nucleus with the egg (zygote, 2n) and the other with the polar nuclei (endosperm nucleus, 3n).

Endosperm

The triploid nutritive tissue of the seed formed from the endosperm nucleus.

Dicots

Dicotyledonae: angiosperms with two cotyledons, reticulate venation, vascular bundles in a ring and a tap root.

Monocots

Monocotyledonae: angiosperms with one cotyledon, parallel venation, scattered vascular bundles and fibrous roots.

Siliques

Long, slender fruits that contain seeds, typical of the Brassicaceae.

Culm

The hollow or solid, often round stem of a grass (Poaceae).

Caryopsis

The one-seeded dry grain fruit of grasses, which does not split open.

FBISE 2025 and 2026 questions

Questions from the FBISE HSSC-I 2026 (1st Annual) and FBISE HSSC-I 2025 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

Exam focus

1. Differentiate monocots and dicots (leaf, root, stem, seed, flower) in a table. (short)
   Asked in 2026 Set D Q.2 (xii) OR, 2025 Set B Q.2 (xii) and 2025 Set D Q.2 (vi), and in the book's short Q14 and long Q12. The most repeated question of the unit.
   Go to answer
2. Write three main features of bryophytes, and why they are called amphibious plants. (short)
   2026 Set B Q.2 (vi), 2026 Set D Q.2 (vi) and 2025 Set B Q.2 (ii); also book short Q3 and Q6.
   Go to answer
3. Characteristics of vascular plants (tracheophytes) and why they are successful land plants. (short)
   2025 Set D Q.2 (iii) OR; book short Q5 and long Q5 and Q8.
   Go to answer
4. Characteristics of angiosperms: vascular tissues, alternation of generations, and what makes them successful on land. (short)
   2026 Set B Q.2 (xii) OR, 2025 Set B Q.2 (xiv) OR, and MCQ 11 of 2026 Set D on enclosed seeds.
   Go to answer
5. General characteristics of the seedless vascular groups: whisk ferns and club mosses, with examples. (short)
   2026 Set B Q.2 (i) OR (whisk ferns) and 2026 Set D Q.2 (i) (club mosses); book long Q6.
   Go to answer
6. Classify plants into groups and give a character and an example of each (complete the table). (short)
   2025 Set D Q.2 (v) asked bryophytes, seedless plants and the rose; 2026 Set B MCQ 11 asked an example of a vascular plant.
   Go to answer
7. Draw and label the life cycles of moss (Polytrichum), fern (Adiantum), Pinus and a flowering plant. (long)
   No past-paper question from this unit in 2025 or 2026 asked for a life cycle, but the book has long Q3, Q7 and Q13 and Section C often asks for a labelled life cycle, so prepare them.
   Go to answer
8. Land adaptations of bryophytes and the importance of seedless vascular plants. (short)
   Book short Q4, Q9 and long Q4 and Q9; these link to the paper questions on bryophytes and are likely follow-ups.
   Go to answer