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Plant
Diversity I
How Plants
Colonized Land
Overview: The Greening of Earth
Looking at a lush landscape
It is difficult to imagine the land without any
plants or other organisms
For more than the first 3 billion years of Earths
history
The terrestrial surface was lifeless
Since colonizing land
Plants have diversified into roughly 290,000
living species
Land plants evolved from green algae
Researchers have identified green algae called charophyceans as the closest relatives of land plants
Plants
Plants appeared on land about 425 million years
ago, and the evolutionary history of the plant kingdom reflects increasing
adaptation to the terrestrial environment.
There are about
290,000 known plant species. (Food agriculture is based on only
about two dozen species.)
. The
Invasion of the Land is really the Invasion of the Atmosphere!!!
The
Protoplasm of Individual Plant Cells is
surrounded by a Cellulose Wall.
While Cellulose is strong and prevents mechanical damage to the cell contents,
it is extremely hydrophilic and readily absorbs water.However, Cellulose
easily loses water via evaporation.
Land
plants
Land
plants
Land
plants
Nontrcheophytes
Mosses
Mosses
Nontrcheophytes
Tracheophytes
Vascular
system
Plants
A. General Characteristics of Plants
Plants are multicellular
eukaryotes that are photosynthetic autotrophs. They
share the following characteristics with their green algal ancestors:
· Chloroplasts with
the photosynthetic pigments: chlorophyll a, chlorophyll b, and carotenoids.
· Cell walls
containing cellulose.
Food reserve is starch that is stored in plastids.
Plants
As plants adapted to terrestrial life, they
evolved complex bodies with cell specialization for different functions.
· Aerial plant parts
are coated with a waxy cuticle that helps prevent desiccation.
Though gas exchange cannot occur across the waxy
cuticle, CO2 and 02 can diffuse between the leafs interior and the surrounding air through stomata,
microscopic pores on the leafs surface.
Plants
With the move from an aquatic to terrestrial
environment, a new mode of reproduction was necessary to solve two problems:
1. Gametes must be dispersed in a nonaquatic environment. Plants produce gametes within gametangia, organs with protective jackets of
sterile (nonreproductive) cells that prevent gametes
from drying out. The egg is fertilized within the female organ.
Plants
2. Embryos must be protected against
desiccation. The
zygote develops into an embryo that is retained for awhile within the female gametangia's jacket of protective cells. Emphasizing this
terrestrial adaptation, plants are often referred to as embryophytes.
. Cellulose
is like a sponge. If you drop a sponge in water, it saturates instantaneously.
A wet sponge readily loses water when it is placed on a dry substrate. In order for an isolated plant cell, like
a unicellular alga, to survive, it must be in constant contact with water.
In
order to withstand periodic dry spells, plant cells needed a water protective
coating.
. One
of the most important plant adaptations is the Cuticle. It is a waxy material
that is secreted to the outside of the plasma membrane. It fills in the spaces
between cellulose fibrils and forms a continuous external waxy layer to the
outside of the cell wall. This makes the cell watertight!
. This
cell can be called an "all purpose" cell because it Regulates its water balance and performs
Photosynthesis.
The
Cuticle keeps water inside but it also prevents water uptake. The Cuticle
is usually thicker on the side of the cell facing the light. Consequently,
water could enter the bottom of the cell where the cuticle is
thin and where water is more
. abundant, and be retained within the cell by the
thick cuticle on its upper side. This could lead to the formation of colonies.
The first multicellular forms could be filaments.
These might be followed by flat sheets.
The
Chlorophyta (Green Algae) is algal group which
probably gave rise to land plants. The genus Coleochaete
is regarded as the
Plant
Evolution
Adaptations
Cuticle
alternation of generations
specialized tissues
Cuticle
Waxy coating on surfaces
resists drying out
stomata exist to allow necessary gas exchange
Morphological
and Biochemical Evidence
Many characteristics of land plants
Also appear in a variety of algal clades
There are four key traits that land plants share
only with charophyceans
Rose-shaped complexes for cellulose synthesis
Peroxisome enzymes
Structure of flagellated sperm
Formation of a phragmoplast
Genetic
Evidence
Comparisons of both nuclear and chloroplast genes
Point to charophyceans
as the closest living relatives of land plants
Adaptations
Enabling the Move to Land
In charophyceans
A layer of a durable polymer called sporopollenin prevents exposed zygotes from drying
out
The accumulation of traits that facilitated
survival on land
May have opened the way to its colonization by
plants
: Land plants possess a set of derived terrestrial
adaptations
Many adaptations
Emerged after land plants diverged from their charophycean relatives
Defining
the
Systematists
Are currently debating the boundaries of the plant
kingdom
Some biologists think that the plant kingdom
Should be expanded to include some or all green
algae
Until this debate is resolved
This textbook retains the embryophyte
definition of kingdom Plantae
Derived
Traits of Plants
Five key traits appear in nearly all land plants
but are absent in the charophyceans
Apical meristems
Alternation of generations
Walled spores produced in sporangia
Multicellular gametangia
Multicellular dependent embryo
Apical meristems and
alternation of generations
Walled spores; multicellular
gametangia; and multicellular,
dependent embryos
Additional derived units
Such as a cuticle and secondary compounds, evolved
in many plant species
The
Origin and Diversification of Plants
Fossil evidence
Indicates that plants were on land at least 475
million years ago
Fossilized spores and tissues
Have been extracted from 475-million-year-old
rocks
Whatever
the age of the first land plants
Those ancestral species gave rise to a vast
diversity of modern plants
Land plants can be informally grouped
Based on the presence or absence of vascular
tissue
An overview of land plant evolution
: The life cycles of mosses and other bryophytes
are dominated by the gametophyte stage
Bryophytes are represented today by three phyla of
small herbaceous (nonwoody) plants
Liverworts, phylum Hepatophyta
Hornworts, phylum Anthocerophyta
Mosses, phylum Bryophyta
Debate continues over the sequence of bryophyte
evolution
Mosses are most closely related to vascular plants
Bryophyte
Gametophytes
In all three bryophyte phyla
Gametophytes are larger and longer-living than sporophytes
The life cycle of a moss
Bryophyte gametophytes
Produce flagellated sperm in antheridia
Produce ova in archegonia
Generally form ground-hugging carpets and are at
most only a few cells thick
Some mosses
Have conducting tissues in the center of their
stems and may grow vertically
Bryophyte
Sporophytes
Bryophyte sporophytes
Grow out of archegonia
Are the smallest and simplest of all extant plant
groups
Consist of a foot, a seta, and a sporangium
Hornwort and moss sporophytes
Have stomata
Bryophyte diversity
Ecological
and Economic Importance of Mosses
Sphagnum, or peat moss
Forms extensive deposits of partially decayed
organic material known as peat
Plays an important role in the Earths carbon
cycle
: Ferns and other seedless vascular plants formed
the first forests
Bryophytes and bryophyte-like plants
Were the prevalent vegetation during the first 100
million years of plant evolution
Vascular plants
Began to evolve during the Carboniferous period
Origins
and Traits of Vascular Plants
Fossils of the forerunners of vascular plants
Date back about 420 million years
These early tiny plants
Had independent, branching sporophytes
Lacked other derived traits of vascular plants
Life
Cycles with Dominant Sporophytes
In contrast with bryophytes
Sporophytes of seedless vascular
plants are the larger generation, as in the familiar leafy fern
The gametophytes are tiny plants that grow on or
below the soil surface
The life cycle of a fern
Transport
in Xylem and Phloem
Vascular plants have two types of vascular tissue
Xylem and phloem
Xylem
Conducts most of the water and minerals
Includes dead cells called tracheids
Phloem
Distributes sugars, amino acids, and other organic
products
Consists of living cells
Evolution
of Roots
Roots
Are organs that anchor vascular plants
Enable vascular plants to absorb water and nutrients
from the soil
May have evolved from subterranean stems
Evolution
of Leaves
Leaves
Are organs that increase the surface area of
vascular plants, thereby capturing more solar energy for photosynthesis
Leaves are categorized by two types
Microphylls, leaves with a single vein
Megaphylls, leaves with a highly
branched vascular system
According to one model of evolution
Microphylls evolved first, as
outgrowths of stems
Sporophylls and Spore Variations
Sporophylls
Are modified leaves with sporangia
Most seedless vascular plants
Are homosporous,
producing one type of spore that develops into a bisexual gametophyte
All seed plants and some seedless vascular plants
Are heterosporous,
having two types of spores that give rise to male and female gametophytes
Classification
of Seedless Vascular Plants
Seedless vascular plants form two phyla
Lycophyta, including club mosses,
spike mosses, and quillworts
Pterophyta, including ferns,
horsetails, and whisk ferns and their relatives
The general groups of seedless vascular plants
Phylum
Lycophyta: Club Mosses, Spike Mosses, and Quillworts
Modern species of lycophytes
Are relics from a far more eminent past
Are small herbaceous plants
Phylum
Pterophyta: Ferns, Horsetails, and Whisk Ferns and
Relatives
Ferns
Are the most diverse seedless vascular plants
The
Significance of Seedless Vascular Plants
The ancestors of modern lycophytes,
horsetails, and ferns
Grew to great heights during the Carboniferous,
forming the first forests
The growth of these early forests
May have helped produce the major global cooling
that characterized the end of the Carboniferous period
Decayed and eventually became coal
Enough for now!