Chapter 30
Evolutionary Groupings and Plant Classification
The classification of plants is organized into groups based on major adaptational developments and evolutionary transitions.
In the biological classification hierarchy, the structure proceeds from the largest to the smallest groups as follows:
Domain (the most inclusive group).
Super groups.
Kingdoms.
Phyla (singular: Phylum).
As of the current textbook edition, the Kingdom Plantae is divided into official classification groupings referred to as phyla.
These phyla are studied one by one, beginning with the most primitive extant plants and moving toward those with advanced terrestrial adaptations.
Nonvascular Plants: The Most Primitive Terrestrial Plants
Nonvascular plants are considered the most primitive plants that still exist today.
They are barely able to deal with terrestrial environments and are restricted to very moist, damp habitats.
These plants lack a complex internal transport system, which limits their size and reproductive strategies.
Adaptation Restrictions:
They must live in environments with high wetness to facilitate reproduction.
They do not possess true roots, stems, or leaves because these structures, by definition, require vascular tissues.
Structural Terminology:
In place of roots, they have filament-like structures called rhizoids which serve to anchor the plant but are less efficient at absorption than true vascular roots.
In place of leaves, the structures are often called blades or a thallus (derived from the Latin word for leaf).
Most lack a cuticle covering, allowing them to absorb water and nutrients directly across the entire surface of the individual.
Moss Life Cycle and Anatomy
To represent nonvascular plants, mosses are used as the primary example.
Alternation of Generations: Mosses follow a life cycle where the plant exists in two distinct forms:
Gametophyte: The most noticeable, long-lived, and dominant individual in the life cycle. It is the structure seen growing on the ground. It is the gamete-producing individual and is haploid ().
Sporophyte: A very short-lived, non-distinct structure that grows directly out of the female gametophyte. It is the spore-producing individual and is diploid ().
Reproductive Process:
Male and female gametophytes produce gametes (sperm and eggs).
Sperm must swim through a film of wetness to reach the archegonium (female structure) to fertilize the egg.
The resulting zygote (the first diploid cell) develops into the sporophyte individual.
Gametophytes specifically develop from haploid, single-cell spores.
Anatomy of the Sporophyte:
Seta: The stalk-like section.
Sporangia: The spore-producing structure at the top.
Capsid: A covering over the sporangia.
Liverworts and Hornworts
Hepatophyta (Liverworts):
Each individual is a single leaf-like structure, often referred to as a lobe or blade with a wavy edge.
They utilize rhizoids underneath to hold the plant in place.
Gamete-producing structures (archegonia and antheridia) are often elevated on little stock-like structures.
The sporophyte is very non-distinct and grows hanging under the archegonia, comparing them to "little coconuts on a palm tree."
Anthocerotophyta (Hornworts):
These are characterized by sporophytes that look like blades of grass sticking up from the flat, leaf-like gametophytes on the ground.
Seedless Vascular Plants: The First Transition to Verticality
Fossil evidence suggests the transition from algae to primitive plants occurred approximately years ago.
For roughly years, plants were largely restricted to damp environments at the edge of aquatic systems.
Evolutionary Pressure for Height: There was a selective advantage to growing taller to avoid being shaded by neighbors. This necessitated the development of vascular tissues for structural support and fluid transport.
Key Transitions in Seedless Vascular Plants:
The appearance of true roots, true stems, and true leaves.
The development of lignin, a material incorporated into cell walls to stiffen them and support the plant body.
The sporophyte becomes the more prominent, dominant, and long-lived individual, while the gametophyte becomes less noticeable and short-lived.
The introduction of branching, which allows a single individual to have multiple sporangia (spore-producing structures), increasing reproductive fitness.
Leaf Evolution and Anatomy
Microphylls: Small leaves with just one single bit (strand) of vascular tissue. These are characteristic of the Lycophyta.
Megaphylls: Larger leaves with multiple, branched vascular tissues (veins). These provide much more surface area for photosynthesis and are characteristic of ferns (Monilophyta).
Terminology Note: Any scientific term ending in "-phyll" relates to a leaf (e.g., microphyll, megaphyll, sporophyll).
Rhizome: A main stem that grows horizontally along the ground surface, with roots growing downward and vertical stems/sporangia growing upward.
Fern Life Cycle and the Carboniferous Period
Ferns (belonging to Phylum Monilophyta) are the classic example of seedless vascular plants.
Sporophyte Anatomy:
Fronds: Large megaphyll leaves.
Sori: Clusters of sporangia found on the underside of the leaves.
Sporophyll: A leaf that specifically supports sporangia.
Gametophyte Anatomy:
A tiny, heart-shaped, single leaf-like structure.
Monoecious: Standard for ferns, meaning "one house"; both male (antheridia) and female (archegonia) reproductive structures are on the same individual.
The Carboniferous Period:
Occurred as the result of massive forests of lycophytes and monilophytes growing and dying in tropical, damp conditions.
Huge amounts of organic material were produced and buried under sediment.
Extreme pressure transformed this material into carbon-based fossil fuels, such as coal and oil.
Development of Seeds and Pollen
As the supercontinent drifted and environments became drier and cooler, swimming sperm became less viable.
Pollen: Carries male gametes and protects them against harsh, dry environments. It eliminates the need for external water for fertilization.
Seeds: A complex structure encasing a developing embryo with a package of food resources and a tough, environmentally resistant seed coat.
Seeds can remain dormant until conditions improve.
Example: Archaeologists found date palm seeds in a -year-old Middle Eastern tomb that successfully grew when planted.
Life Cycle Shift: The separate, free-living gametophyte disappears. Instead, haploid gamete-producing tissues exist within the sporophyte body.
Heterospory and Reproductive Terminology
Primitive plants are homosporous (producing one type of spore).
Advanced plants (Gymnosperms and Angiosperms) are heterosporous, producing two types of "spore" tissues:
Micro- (Prefix): Associated with male structures and the production of pollen.
Microsporangium: Tissues that produce microspores via meiosis.
Microspores: Develop into pollen grains (microgametophytes).
Mega- (Prefix): Associated with female structures and the production of ovules.
Megasporangium: Tissues that produce megaspores via meiosis.
Megaspores: Develop into the ovule/egg-producing structure (megagametophyte).
Note on Terminology: Botanists use the word "spore" in these advanced terms (like microspore) even though the plants no longer release independent spores like mosses do.
Gymnosperms: The "Naked Seed" Plants
Etymology: Derived from the Greek Gymnos (naked) and Sperm (seed).
The name refers to the fact that the ovule/seed sits "naked" on a leaf-like structure within a cone, exposed to the environment for pollination.
Historical Context: The word gymnasium also comes from gymnos because the Greeks practiced sports naked.
All gymnosperms produce some form of cone structure.
Pollination vs. Fertilization:
Pollination: The arrival of a pollen grain at the female structure (the micropyle is the gap in the ovule's protective covering).
Fertilization: The growth of a pollen tube and the fusion of a haploid nucleus with the egg. This occurs after pollination.
The Four Phyla of Gymnosperms
1. Cycadophyta (Cycads): Look like palm trees but are shorter (often head-tall). Very common in landscaping.
2. Ginkgophyta: Only one extant species, Ginkgo biloba. Thought to be extinct until rediscovered in China.
3. Gnetophyta: Includes plants like Ephedra (a shrubby plant found in the US Southwest).
4. Coniferophyta: The most familiar group, including pines and similar trees.
Pine Tree Life Cycle (Coniferophyta)
Male Cones (Staminate Cones): Small cones that produce microspores which develop into pollen. They release massive amounts of yellow pollen into the wind to compensate for the low probability of reaching a female cone.
Female Cones (Ovulate Cones): The large, woody pine cones familiar to most. They contain the megasporangia and ovules.
Dispersal: Pine seeds often have "wings" to be dispersed by the wind or may be carried by animals.
Questions & Discussion
Student Question: "Is the female gametophyte haploid or diploid?"
Instructor Response: The individual structure on the bottom left of the diagram is the female gametophyte, which is haploid (). The brown structure growing out of it is the sporophyte, which is diploid (). Only the zygote/sporophyte cells are diploid; the gametophyte body itself is not.
Student Question: "Is that specifically the female one?"
Instructor Response: Yes, the one on the very bottom left. The brown structure coming up out of it is the sporophyte.
Discussion on Common Names: Common names are often misleading.
Spanish Moss is not a moss; it is a flowering plant.
Various plants called "mosses" (like club moss) are actually lycophytes.
Using scientific names is essential for effective global communication in botany.