Chapter 30- Green Plants
- Dominate terrestrial and freshwater habitats
- Green algae:
1. Closest living relative to land plants and form a monophyletic group with them
2. Transition from aquatic to terrestrial life occurred when land plants evolved from green algae.
30.1 Why Do Biologists Study Green Plants?
Plants Provide Ecosystem Services
Ecosystem: all organisms in a particular area along with physical components of the environment such as
atmosphere, precipitation, surface water, sunlight, soil and nutrients.
Plants add to quality of atmosphere, surface water, soil, and other physical components of an ecosystem.
Stated another way, plants alter the landscape in ways that benefit other organisms:
- Plants produce oxygen.
- Plants build soil.
- Plants hold soil. Fine roots produced by trees help hold particular soil particles in place. By
taking up nutrients in soil, plants prevent nutrients from being blown or washed away
- Plants hold water. Plant tissues take up and retain water.
- Plants moderate the local climate. By providing shade, reduce temperatures. Reduce impact of
winds that dry out landscapes.
- Green plants are the key to the carbon cycle on land. Take CO2 from atmosphere and reduce it t
- Plants Provide Humans with Food, Fuel, Fibre, Building Materials, and Medicines
30.2 How Do Biologists Study Green Plants?
Analyzing Morphological Traits
1. Non-vascular plants: Include Hepaticophyta (liverworts),Anthocerophyta (hornworts), and
Bryophyta (mosses). Lack vascular tissue (specialized tissue that conducts water or dissolved
nutrients from one part of the plant body to another). Abundant in areas that are small and grow
close to the ground.
2. Seedless vascular plants: Have well-developed vascular tissue but do not make seeds. Seed
consists of embryo and a store of nutritive tissue, surrounded by a tough protected layer. Relatively small in stature. Ex. Lycophyta, Psilophyta (whisk ferns), Sphenophyta (horse-tails),
and Pteridophyta (ferns)
3. Seed plants. Have vascular tissue and make seeds. Gymnosperms: Seeds do not develop in
enclosed structure. Angiosperms: flowering plants, seeds develop inside a protective structure
called carpel. Ex. Cycadophyta (cycads), Ginkgophyta (ginkgos), Gnetophyta (gnetophytes),
Pinophyta (pines, spruces, firs), other conifers (redwoods, junipers, yews),Anthophyta
(angiosperms or flowering plants).
Using the Fossil Record
- Green algae first green plants in rocks that formed 700-725 mya. First land plants in 475 mya.
- Land plants are derived from green algae.
- Evolution of green algae contributed to rise of oxygen levels
- Oldest interval (of fossil record) begins 475 mya. Spores (reproductive cell) and cuticles (waxy
coating) found. Several observations support the hypothesis that these fossils are of land plants.
First, cuticle is a watertight barrier that coats today’s land plants and helps them resist drying.
Second, fossilized spores are surrounded by a sheet-like coating. Coating material appears almost
identical to sporopollenin, which encases spores and pollen from modern land plants and helps
them resist drying. Third, fossilized spores that are 475 mya have recently been found in
association with sporangia.
- Second major interval – Silurian-Devonian interval – 445-459 mya. Fossils from most major
plant lineages. Stomata, vascular tissue, roots, leaves.
- Third interval – Carboniferous. Extensive coal-forming swamps.
- Fourth interval – gymnosperms abundant. Both wet and dry environments blanketed with green
plants for the first time.
- Fifth (to present) - Angiosperms abundant. Diversification of flowering plants.
Evaluating Molecular Phylogenies
- Land plants evolved from green algae
- The green algal group called Charaphycaea is the sister group to land plants- meaning that they
are their closest living relatives. Land plants evolved from multicellular ancestor that lived in
ponds or lakes.
- The green plants are monophyletic. In contrast, green algae is paraphyletic. Green algae include
some but not all of the descendents of a single common ancestor.
- Land plants are monophyletic. Supports hypothesis that transition from freshwater environments
to land occurred just once. - The non vascular plants – liverworts, hornworts and mosses – are the earliest-branching, or most
basal groups among land plants.
- Earliest land plants lacked water-conducting cells and vascular tissue.
- The seedless vascular plants form a grade – a sequence of lineages that are not monophyletic. In
contrast, vascular plants are monophyletic meaning that vascular tissue evolved once during the
diversification of land plants.
- The morphological simplicity of whisk ferns is a derived trait – meaning that complex traits have
- The seed plants consist of the gymnosperms and angiosperms and are a monophyletic group,
meaning that seeds evolved once.
- The gymnosperms are monophyletic.
- Land plant evolution began with nonvascular plants, and proceeded to seedless vascular plants,
and continued with the evolution of seed plants.
30.3 What Themes Occur in the Diversification of Green Plants?
- The most ancient groups in the lineage of green plants are dependent on wet habitats, while more
recently evolved groups are tolerant of dry – even desert – conditions.
- The story of land plants is the story of adaptations that allowed photosynthetic organisms to move from
aquatic to terrestrial environments.
The Transition to Land, 1: How Did PlantsAdapt to Dry Conditions?
Adaptations that helped solve the water on land problem: 1) Prevention of water loss from cells which
kept the cells from drying out and dying 2) transportation of water from tissues with direct access to water
to tissues without access.
Preventing Water Loss: Cuticle and Stomata
- Cuticle is a waxy, watertight sealant that covers the aboveground parts of plants and gives them
the ability to survive in dry environments. Covering surfaces with wax creates a problem
regarding exchange of gases across those surfaces.
- Stomata: opening surrounded by guard cells. The opening, called a pore, opens or closes as the
guard cells change shape. Pores are closed to limit water loss. When guard cells become taut, they
open the pore, not only allowing CO2 to diffuse into the interior of leaves and stems where cells
are actively photosynthesizing, but also allowing excess O2 to diffuse out.
- Stomata present in all land plants besides liverworts which have pores.
Transporting Water: Vascular Tissue and Upright Growth - Biologists hypothesize that the first land plants were small, or had a low sprawling growth habit.
Would have to obtain water through pores or through a few cells that lacked cuticle – meaning
they would have to grow in a way that kept most of their tissue in direct contact with moist soil.
- Sprawling growth hypothesis supported by the observation that most basal groups of land plants
living today (mosses, liverworts, hornworts) are all low-growing forms.
- Competition for space and light would have become intense soon after the first land plants
appeared. To escape competition, plants would have to grow upright.
- Two problems had to be overcome. 1) Transporting water from tissues that are in contact with wet
soil to those that are not in contact. 2) Becoming rigid enough to avoid falling over because of
wind and gravity. Vascular tissue helped solve both problems.
- The evolution of lignin rings gave stem tissues the strength to remain erect in the face of wind
and gravity. Today, the presence of lignin in the cell walls of water-conducting cells is considered
the defining feature of vascular tissue. The evolution of vascular tissue allowed early plants to
support water from roots to aboveground tissue.
- Tracheids: Long, thin tapering cells that have 1) a thickened lignin-containing secondary cell wall
in addition to a cellulose based primary cell wall. 2) Gaps in the secondary cell wall, in the sides
and ends of the cell, where water can flow efficiently from one tracheid to the next.
- Secondary cell walls gave tracheids the ability to provide better structural support, but water
could still move through the cells easily because of the gaps.
- Tracheids in all vascular plants
- Vessel elements: 250-270 mya. Most advanced type of water-conducting cells. Shorter and wider
than tracheids. Upper and lower ends have gaps in both the primary and secondary cell walls.
Makes water movement extremely efficient. In vascular tissue, vessel elements are lined up end
to end to form a continuous pipelike structure.
- In stems and branches of some vascular plant species, tracheids or combination of tracheids and
vessel elements make up wood. Extremely strong support material.
- Vascular tissue evolved in a series of gradual steps that provided increased structural support and
increased efficiency in water transport.
The Transition to Land, II: How Do Plants Reproduce in Dry Conditions?
- One of the key adaptations for reproducing on land: spores that resist drying because they are
encased in a tough case of sporopollenin. Sporopollenin-like compounds are found in the walls of
some green algal zygotes; thick walled, sporopollenin-rich spores appear early in the fossil record
of land plants and occur in all land plants living today.
- Sporopollenin-encased spores were one of the innovations that made early land colonization
possible - Two other innovations: 1) Gametes were produced in complex, multicellular structures 2) the
embryo was retained on the parent plant and was nourished by it.
Retaining and Nourishing Offspring: Land Plants as Embryophytes
- The fossilized gametophytes of early land plants contained specialized reproductive organ