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General Biology 101
Chapter Outlines
Starr & Taggert
10th edition
Chapter 23
Introduction – Pioneers in a New World
It is thought that plants first colonized the land around 700 million years ago. There are
fossils of these first pioneers. Today cyanobacteria (photosynthetic prokaryotes) and
green algae grow as mats in near-shore waters and freshwater streams, similar areas to
what ancestral forms of all modern plants may have first colonized.
Plants are multi-celled photoautotrophs, containing the pigments chlorophyll a and b,
similar to the green algae that they share a common ancestor with.
There are currently 295,000 species of known plants.
Section 23.1 Trends in Plant Evolution
Types of plants:
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Vascular – those that have internal conducting tissues.
Bryophytes – nonvascular including mosses and liverworts.
Seedless vascular – including horsetails and ferns.
Gymnosperms – cone bearing seed plants
Angiosperms – flowering plants
Characteristics of land plants
Root systems – for absorbing water and anchoring the plant in place.
Shoot systems – energy producing leaves that absorb sunlight and undergo
photosynthesis.
Special features:
Lignin – organic compound in cell walls that gives support to stems.
Vascular tissues within stems
Xylem – conducts water and dissolved ions and minerals.
Phloem – distributes sugars made during photosynthesis.
Cuticle – waxy covering on leaves and stems that conserves water.
Stomata – openings in the leaves that allow passage of gases and water vapor.
Life Cycles
Gametophyte – gamete producing phase in a plant’s life cycle. Haploid. Dominant form
in bryophytes.
Sporophytes – a multi-celled, diploid plant body that gives rise to spores. They are the
dominant form in most land plants. By mitosis spores produce gametophytes.
Advantage of a dominant sporophyte was fertilization and dispersal of new/next
generation timed with environmental conditions.
Pollen grains in seed bearing plants contain spores that when mature become the male
gametophyte. Pollen helped plants colonize dryer areas on land.
The female gametophyte is associated with the seed bearing tissues e.g. flowers or cones
that get pollinated and produce seeds containing the embryo that will start the next
generation of plants.
Section 23.2
The Bryophytes
18,600 species
Include:
Mosses * most common
Liverworts
Hornworts
Characteristics:
Nonvascular plants
Grow in moist habitats
Short – less than 20 cm i.e. 8 inches tall
Rhizoids – which are absorptive structures that attach gametophytes to soil.
Land plant derived structures include:
 Cuticle
 Cellular jacket around reproductive structures around the shoot tip of the
gametophyte
 Gametophyte stage dominant and don’t get nutrients from the sporophyte.
Peat mosses grow very fast and in some parts of the world are used for a fuel source.
Peat is high in acids which hamper the growth of bacterial and fungal decomposers.
Peat bogs – accumulations of compressed, moist peat. Communities in peat bogs include
acid loving plants like cranberries and blueberries.
Liverworts are also gametophytes which can reproduce sexually (with separate male and
female plants) as well as asexually in gemma cups.
Section 23.3
Existing Seedless Vascular Plants
Seedless Vascular plants differ from bryophytes in that:
1) Sporophyte does not stay attached to the gametophyte
2) Has true vascular tissues
3) Sporophyte is the more dominant and longer lived portion of the life cycle.
Most reside in wet, humid places.
Flagellated sperm swim to the eggs
I.
Lycophytes: Club mosses (found from the Arctic to the tropics). Aka
ground pines
- Spores form in hollow chambers in the leaves. Chamber called the
sporangium. Spores form via meiosis.
- Strobilus- a cluster of spore producing structures.
II.
Whisk Ferns (Psilophyta)
- Have rhizomes which are short, branched, horizontal absorptive stems that
grow underground.
- All have vascular tissues i.e. xylem and phloem.
- Sporophtyes have no roots.
III.
Horsetails (Sphenophyta)
Very primitive plant that has been around for the past 300 million years.
Commonly found along streams and roadsides
Free-living gametophyte (very small).
Reinforced stems have silica within them (makes for sandpaper like
texture to the plant).
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IV.
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Ferns (Pterophyta)
Largest and most diverse group of vascular, seedless plants.
Have vascularized rhizomes, give rise to roots and leaves.
Young leaves are called fronds. (coiled initially and then unfold).
On the underside there are typically brown dots called sori, containing a
cluster of sporangia. At dispersal the sporangia snap open and catapult the
spores out into the nearby environment (usually when it is moist – like
after a rain).