Chapter 28 - Seedless Plants - Schools

Chapter 28 - Seedless Plants
Introduction
Plants are autotrophs; they make their own organic nutrients. The term
"organic" refers to compounds that contain carbon. Organic nutrients such
as sugars are made by photosynthesis.
Plants are adapted to living on land. For example, the above-ground parts
of most plants are covered by a waxy layer called a cuticle to prevent
water loss.
Aquatic plants are secondarily adapted to living in water.
Some evidence that suggests that plants evolved from the green algae is:
they both use chlorophyll a, chlorophyll b, and carotenoid
pigments during photosynthesis.
the primary food reserve of both is starch.
they both have cellulose cell walls.
Alternation of Generations
The basic alternation of generations life cycle is illustrated below.

The diploid plant that produces spores is called a sporophyte. The haploid
plant that produces gametes is called a gametophyte.
Some protists also have an alternation of generations life cycle but the
structures that produce gametes in protists are usually single cells. Plants
produce gametes in multicullar structures that have an outer protective
layer. Sperm are produced in structures called antheridia (sing.
antheridium), eggs are produced in archegonia (sing. archegonium),. As
in protists and fungi, spores of plants are produced in sporangia (sing.
sporangium).
A dependent sporophyte is a sporophyte that is small and grows attached
to the gametophyte. It obtains nutrients from the gametophyte. An
independent sporophyte grows separately from the gametophyte.
Similarly, a dependent gametophyte is small and grows attached to the
sporophyte while an independent gametophyte grows separately from the
sporophyte.
The evolutionary trend in plants has been from plants with a dominant
gametophyte and reduced, dependent sporophyte (ex. Mosses) to plants
with a dominant, independent sporophyte and a reduced, dependent
gametophyte (ex. Seed plants).
Classification of Plants
We will study 12 divisions (phyla) of plants.
Classification
Characteristics
Liverworts (Division Hepatophyta)
Mosses (Division Bryophyta)
Bryophytes (no vascular tissue)

Hornworts (Division Anthocerophyta
Whisk ferns (Division Psilotophyta)
Club mosses (Division Lycophyta)
Horsetails (Division Sphenophyta)
Ferns (Division Pterophyta)
Conifers (Division Coniferophyta)
Cycads (Division Cycadophyta)
Ginkgos (Division Ginkgophyta)
Gnetophytes (Division Gnetophyta)
Flowering Plants (Division
Anthophyta)
Monocots (Class Liliopsida)
Dicots (Class Magnoliopsida)
Seedless vascular plants
Gymnosperms (vascular, naked seeds)
Angiosperms (vascular, protected
seeds)
Bryophytes
General Characteristics
Mosses
Mosses, liverworts, and hornworts lack vascular tissue for transporting
water and nutrients. This restricts their size. The largest are less than 20
cm (8 in.).
Rhizoids are root-like structures that absorb water and nutrients. They do
not have true roots, stems, or leaves because they lack vascular tissue.
Bryophytes are generally restricted to moist areas because the sperm are
flagellated and therefore require at least a film of water to swim to the
egg.
The sporophyte of bryophytes is dependent on the gametophyte, that is, it
derives its nutrition from the gametophyte. The gametophyte is
independent.

Left: Moss growing on a rock
Left: The sporophytes are
attached to the gametophytes.
Reproduction
Spores germinate to produce a branching horizontal filament called a
protonema. The familiar green gametophyte body and rhizoids arise from
the protonema.

Left: Moss capsule containing spores.

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Liverworts
Mosses also reproduce asexually by fragmentation.
Marchantia is an example.
Reproduction
Some stalks contain antheridia and others contain archegonia. Sporophytes
are small and produce windblown spores.
Asexual Reproduction
The upper surface of the thallus (plant body) produces cup-shaped
structures called gemma cups. Groups of cells (called gemma) within the
cups are capable of breaking off and producing a new plant.

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Left: Marchantia preserved in
a plastic mount. Upper left:
gemma cups; Upper right:
antheridia; Lower: archegonia.
Hornworts
The gametophyte of hornworts resembles liverworts. The sporophytes are
horn-like projections that rise from the gametophyte.
Importance of Bryophytes
Bryophytes can colonize rocks and help initiate the soil-formation process.
Vascular Tissue
Vascular plants contain vascular tissue.
There are two kinds of vascular tissue:
Xylem conducts water and minerals up from the soil. The
cell walls of xylem cells help support the plant.

Phloem conducts organic nutrients from one part of the
plant to another.
True roots, stems, and leaves are found only in vascular plants because
these structures must contain vascular tissue.
The sporophyte of vascular plants is dominant.
Seedless vascular plants
General Characteristics
Ferns
Seedless vascular plants include ferns, whisk ferns, club mosses, and
horsetails.
The plants do not produce seeds so, like bryophytes, they are dispersed
(spread) by windblown spores.
The gametophyte and sporophyte are independent.
They are vascular plants and therefore have true roots, stems, and leaves.
The sperm are flagellated and require water for reproduction. These plants
are therefore limited to moist areas.
Many of the seedless vascular plants were once tree-sized. During the
carboniferous period (near the end of the Paleozoic), these plants were so
abundant that in some areas, their remains accumulated faster than they
decomposed. These accumulations produced our fossil fuels.
The earliest known vascular plant is Cooksonia. It's pattern of branching
increased the number of sporangia that could be produced compared to
bryophytes which do not branch. Leaves of later plants probably evolved
from webbing between the branches.

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Left: The sporophyte is the
dominant generation.
In most ferns, the stem is a horizontal, underground structure called a
rhizome. The leaves grow above-ground (see the photograph above).
A sorus (pl. sori) is a cluster of sporangia. Sori are located on the
underside of the leaves.

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Left: Close-up of a sorus.
Sporangia can be seen
underneath the cover
(indusium).
VIDEO - Fern sporangia discharge (X40) (4.13 MB)
The gametophyte is small and heart shaped.
Left: Fern gametophyte X 40
showing archegonia.

Fern Life Cycle
Whisk Ferns