Chapter 30 - Plant Diversity II – The Evolution of Seed Plants
Concept 30.1 - The reduced gametophytes of seed plants are protected in ovules and pollen grains
 In addition to seeds all seed plants share the following: reduced gametophytes, heterospory, ovules and pollen.
 Advantages of a Reduced Gametophyte:
The tiny gametophytes develop from spores within the sporongia of the paternal sporophyte. Since the gametophyte is
now surrounded by sporophyte tissue it is protected from environmental stress. The sporophyte tissue also serves as the
gametophyte’s nutritional source.
 Heterospory: The Rule Among Seed Plants:
Most of the seed plants are heterosporous
o Megasporangia in megasporophylls produce megaspores that give rise to female gametophytes.
o Microsporangia in microsporophylls prodce microspores that give rise to male gametophytes.
 Ovules and Production of Eggs:
The megasporangium is protected by layers of sporophyte tissue called integuments.
An ovule is made of a megasprongium, megaspore and their integument(s). A female gametophyte develops from
megaspore and produce one or more eggs.
 Pollen and Production of Sperm:
Microspores develop into pollen grains: where male gametophytes are found.
Pollination occurs once the transferred pollen (by wind/animals) reaches the part of the plant seed that contains the
ovule.
If the pollen grain germinates it forms a tube that discharges two sperms into the female gametophyte within the
ovule.
 Evolutionary Advantage of Seeds:
An ovule develops into a seed. A seed is made of an embryo and food supply packaged within a protective coat
derived from the integument(s).
Before seeds, spores were the protective stage in any plant life cycle.
Seeds are much more complex and resistant than spores.
Seeds are dispersed by winds and animals
Concept 30.2 – Gymnosperms bear “naked” seeds, typically cones
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Gymnosperms are plants that have naked seeds, seeds that are not enclosed in ovaries.
Gymnosperm seeds are exposed on modified leaves that form cones.
Gymnosperms have cone-bearing plants called conifers which include trees like pines, firs and red woods.
The Four Plant Phyla that are Gymnosperms:
Phylum Cycadophyta: the second largest after conifers. They have large cones and palm like leaves.
Phylum Ginkgophyta: has only one species called Ginkgo biloba. This tree has fern like leaves that turn gold in
autumn.
Phylum Gnetphyta: plants in this phylum are called gnetophytes, and they consist of 3 genera: Gnetum, Ephedra and
Welwitschia. Some of these species live in deserts and some are tropical.
Phylum Coniferophyta: the largest of gymnosperms phyla. This phylum has many large trees like cypresses and red
woods. Most conifers are evergreen.
 Gymnosperm Evolution:
By the late Devonian period, some plants begun to adapt characteristics of seed plants.
Archaeopteris was a heterosporous tree that produced wood but did not produce seeds.
o These types of seedless vascular species are called progymnosperms.
Early gymnosperms lived in Carboniferous ecosystems with other seedless vascular plants. The Carboniferous period
was then replaced by the Permian period (drier climates) which favoured gymnosperms.
 Life Cycle of a Pine:
In conifers, the two types of cones of spores are produced by separate cones.
o Small pollen cones have microsporocytes which divide using meiosis and produce microspores. Each
microspore develops into a pollen grain containing male gametophyte.
o In ovulate cones megasporocytes undergo meiosis to produce megaspores which develop into female
gametophytes.
Pollen carried by wind enters through the microphyle and forms a tube through the megasporangium.
2 sperms are released through the tube and fertilization occurs.
Mature seeds travel on the wind once released from ovulate cones.
Concept 30.3 – The reproductive adaptations of angiosperms include flowers and fruits
 Angiosperms (flowering plants): are seed plants that produce the reproductive structures called flowers and fruits.
 Characteristics of Angiosperms:
 Flowers: an angiosperm structure specialized for sexual reproduction.
Flowers have up to four rings of modified leaves called floral organs:
o Sepals: found at the base of the flower. Sepals are usually green and enclose the flower before it opens.
o Petals: are brightly coloured in most flowers and they attract pollinators.
o Stamens: the microsporophylls, produce microspores that give rise to pollen grains containing male
gametophytes. A stamen consists of a stalk called the filament and a terminal sac, the anther, where pollen is
produced.
o Carpels: are megasporophylls, which make megaspores and their products are female gametophytes. Flowers
can have several carpels. Carpel parts are the stigma, style and ovary.
 Fruits: consists of a mature ovary and sometimes other flower parts.
Fruits develop after fertilization triggers hormonal changes that cause the ovary to grow.
The wall of the ovary develops into a pericarp which is the thickened wall of the fruit.
 Angiosperm Life Cycle:
The flower of the sporophyte produces microspores that form male gametophyte and megaspores that form female
gametophyte.
o The male gametophytes are in pollen grains. Each pollen grain has 2 haploid cells: a generative cell that
divides to form 2 sperm cells and a tube cell that produces a pollen tube.
o Female gametophytes are found in ovaries. Female gametophytes are also known as the sac embryo (consists
of few cells one of which is the egg).
Pollen is released from the anther to the sticky stigma at the tip of the carpel.
o Cross pollination: occurs when the pollen from one anther of a flower on one plant is transferred to the
stigma of a flower of another plant of the same species.
A tube forms down the style of the carpel once the tube reaches the ovary 2 sperm cells are released into the female
gametophyte.
o One sperm fertilizes the egg to form a diploid zygote and the other fuses with the two nuclei in the large
central cell of the female gametophyte. This is called double fertilization. Double fertilization is responsible
for the development of feed storage in the seed and the embryo development.
The ovulate matures into a seed and the zygote develops into a sporophyte embryo with a rudimentary root and one
or two cotyledons (seed leaves).
The nucleus of the central cell of the female gametophyte divides repeatedly and develops into endosperm: a tissue
rich in starch and other food reserves.
A seed consists of an embryo, endosperm, remnants of the sporangium and a seed coat derived from the integuments.
 Angiosperm Evolution:
Fossil Angiosperms: the major branches of the clade diverged from their common ancestor during the late Mesozoic
period.
Evo-Devo Hypothesis: states that pollen-producing and ovule-producing structures were combined into a single
flower.
 Angiosperm Diversity:
Monocots: species with one cotyledon, parallel veined leaves, scattered vascular tissue, fibrous roots, pollen grains
with one opening and floral organs in groups of 3.
Eudicots: species with two cotyledons, netlike veined leaves, vascular tissue arranged in a ring, taproot, pollen grains
with 3 openings and floral organs in groups of 4 or 5.
o Monocots and eudicots are the two main groups of angiosperms.
Basal Angiosperms: a less derived group of angiosperms. It includes flowering plants from older lineages.
Magnoliids: share some characteristics with basal angiosperms but are more related to monocots and eudicots.
 Evolutionary links between angiosperms and gymnosperms:
Both animals and angiosperms benefit from each other. Animals use angiosperms as a source of food and angiosperms
use animals to disperse their seeds.
Concept 30.4 – Human welfare depends greatly on seed plants
 Seeds are a key source of food, fuel, wood products and medicine.
 Products from seed plants:
Food: flowers (cloves), fruits, seeds (vanilla, black peppers), leaves (mint) and bark (cinnamon).
Wood: source of fuel and wood pulp is used to make papers.
Medicine: willow leaves and bark were used since ancient times in pain-relieving remedies.
 Threats to plant Diversity:
Humans destroy vast areas of forests for agricultural use thus leading to the extinction of certain plants and therefore
certain animals.