Plant Evolution: Characteristics and Classifications

Plant Characteristics

• Approximately 300,000 species of described land plants produce seeds.
• Land plants include mosses, ferns, conifers, and flowering plants.
• Most Plantae members are photosynthetic; some parasitic forms have lost the ability to photosynthesize.
• All plants exhibit growth and possess cell walls, meaning they continue to grow body mass until they die and do not have a final body form.

Land Challenges

• Desiccation is a constant danger for organisms exposed to air.
• Land removes buoyancy provided by water, necessitating structural support tissues.
• Delivering sperm to egg requires new strategies without available water.
• Gametes and zygotes must be protected from desiccation.

Land Advantages

• Light is abundant and not filtered by water or competing photosynthetic organisms.
• Carbon dioxide is readily available due to its higher concentration in air than in water.
• Early land plants had no herbivores to threaten their well-being.

Land Strategies

• The first land plants required strategies to survive droughts near water sources.
• Mosses can be drought tolerant, drying up and soaking up water when available.
• Ferns colonize high humidity environments where droughts are uncommon.

Terrestrial Adaptations

• Four major adaptations are alternation of generations, sporangium (seedless plants), gametangium (seedless plants), and apical meristem tissue.

Alternation of Generations

• Life cycle where an organism has both haploid and diploid stages.
• Most plants exhibit the haplodiplontic life cycle.
• The haploid stage is the gametophyte, while the diploid stage is the sporophyte.
• The gametophyte gives rise to haploids by mitosis, while the sporophyte produces haploids by meiosis.

Sporangia in Seedless Plants

• Sporophytes produce multicellular spore-producing structures containing haploid spores made by meiosis.
• Upon release the spores disperse into the environment.
• Seedless non-vascular plants have one spore type, called homospory.
• These spores will grow into bisexual gametophytes.

Sporangia Protection

• Land plant spores and seed plant pollen have thick cell walls containing a tough polymer.
• Protecting spores and pollen from desiccation is essential for land plants.
• This ensures the survival of spores for gametophyte growth and sperm for fertilization.

Gametangia in Seedless Plants

• Gametangia are multicellular structures on gametophytes that produce gametes by mitosis.
• Antheridium is the male gametangium that produces and releases sperm.
• The female gametangium produces a single egg.
• A zygote develops into the sporophyte inside the archegonium.

Apical Meristems

• Shoots and roots lengthen through quick cell division in the apical meristem tissue.
• The apical meristem is composed of undifferentiated cells at the shoot or root tip, that divide throughout the plant's life.
• Apical meristems enable plants to grow up for sunlight and down for water and minerals, essential for survival.

Additional Land Plant Adaptations

• New organs and structures appeared as plants adapted to dry land.
• Early land plants were tiny, remaining inches off the ground to maintain moisture.
• Vascular tissue evolved for structural support and moving water, minerals, and nutrients.
• Waterproof substance evolved to coat plant aerial portions, preventing water loss.
• Stomata pores in the leaves evolved to facilitate oxygen and carbon dioxide exchange.
• Plants evolved poisonous secondary compounds to deter evolving terrestrial herbivores.
• Plants evolved nutritious compounds to lure animals for pollen and seed dispersal assistance.

Bryophytes

• Informal grouping of the three phyla of nonvascular plants including liverworts, hornworts, and mosses.

First Bryophytes

• The first bryophytes appeared approximately 490 million years ago.
• The fossil record is sparse as these plants lack lignin, the tough polymer found in vascular plants.
• Around 18,000 species can be found in damp habitats and they lack specialized tissues for water, mineral and nutrient conduction.

Closest Relatives - Bryophytes

• Considered the closest relatives to ancestors that first moved onto the land.

Bryophytes - Liverworts

• Over 6,000 described species.
• Consisting of leafy and thallose liverworts.

Bryophytes - Hornworts

• Small phylum of only around 100 described species.
• Colonized a variety of habitats, but are always close to a source of moisture.
• The blue-green gametophyte resembles a thallose liverwort.
• A long and narrow pipe-like structure defines the sporophyte.

Bryophytes - Mosses

• The phylum includes more than 12,000 described species of mosses.
• Instrumental in slowing down soil erosion, storing moisture, and providing shelter for small animals as well as providing food for larger ones in a variety of terrestrial habitats.

Vascular Plants

• Comprise the remainder of the living species of land plants and is the dominant and most conspicuous group, commonly referred to as Tracheophytes
• Around 275,000 vascular plant species account for over 90% of Earth’s vegetation.
• The primary characteristic is possessing vascular ______ tissue.
• Evolved structures that are commonly called xylem and pholem.

Xylem

• Vascular responsible for long-distance water and mineral transport, water-soluble growth factors transfer, and water and nutrient storage.

Pholem

• Vascular tissue that transports sugars, proteins, and other solutes through the plant.

Vascular Roots

• Roots are not well-preserved, however, seem to appear after vascular tissue evolution
• Vascular plant roots function well in water and mineral uptake and anchoring the plant in soil, while thin rhizoids that bryophytes use are flimsy and not useful.
• Most vascular plant roots have associated mycorrhizae.

Leaves

• The evolution of true leaves accompanied sporophyte prominence and vascular tissue development.
• This greatly improved photosynthetic efficiency by capturing more sunlight with their increased surface area.
• Vascular plants bear modified leaves called sporophylls that bear sporangia to evolve leaves for reproductive capacity.

Seedless Vascular Plants

• The sporophyte evolved as the dominant stage.
• Water is needed for sperm to swim to and fertilize the egg; most prefer a moist environment.
• Club mosses, horsetails, ferns, and whisk ferns are modern-day seedless vascular plants.

Seedless Vascular Plants - Club Mosses

• Represent the earliest branching lineage of seedless vascular plants.
• During the Carboniferous Period they grew into tall trees, but are now small evergreen plants containing a branched stem with microphylls.
• Some 1,000 species of quillworts, club mosses and spike mosses are not true mosses.

Seedless Vascular Plants - Horsetails

• Along with ferns and whisk ferns belong to the same phylum.
• Botanists classify them within the Class Equisetopsida, Phylum Pterophyta, although they previously formed a separate phylum.
• Prevalent lineage in the Carboniferous Period like the club mosses.

Seedless Vascular Plants - Whisk Ferns

• Whisk ferns are more closely related to the ferns than to the horsetails.
• Morphology has undergone extensive evolutionary reduction.
• Lack roots and leaves, with photosynthesis occurring in the external tissues of the green stem.

Seedless Vascular Plants - Ferns

• True ferns are the most advanced seedless vascular plants.
• Have well-developed leaves and extensively branched roots.
• Recognizable seedless vascular plants.
• Commonly grow in moist and shaded environments with roughly 12,000 species.

Seed Adaptations

• Seeds and pollen are adaptations to drought.
• Critical for the above plants to colonize terrestrial environments by breaking their dependence on water for reproduction.

Seed Function

• Allows plants to disperse the next generation through space and time.
• Offers nourishment to the embryo.
• Mechanism to maintain dormancy for tens or even thousands of years.

Pollen Function

• Carries the male gametes encased in a protective coat that prevents out and damage.
• Can travel far from the sporophyte, spreading the plant's genes and avoiding competition.

Gymnosperm Groups

• The gymnosperms do not include all the descendants of their common ancestor, making them paraphyletic.
• Characteristics include naked seeds, separate male and female gametes, wind pollination, and a single type of water and mineral xylem transport cell, call the trancheid.

Phylum Classification

• Living gymnosperms have four classified phyla, that are hypothesized to share a single common ancestor.
• Relationships among them are unresolved, remaining a topic of intense taxonomic research.

Gymnosperm - Coniferophyta

• Conifers stand as the most dominant gymnosperm phylum.

Plant Type

• Most tall trees are bearing scale-like or -like leaves, where thin shapes reduce limit water loss allowing snow to easily slide off.
• Adapted to the high altitudes and cold climates.
• Many trees are harvested for paper pulp and timber.

Gymnosperm - Cycadophyta

• _____ is the are often mistaken for palm trees because of their large, compound leaves and presence in warmer climates.
• Unusual large cones and pollination done by beetles rather than wind.
• 100 species and face possible extinction.

Gymnosperm - Ginkgophyta

• Single living species which is ginkgo biloba.
• Unique -shaped leaves that display branching veins, turn yellow in autumn and fall off the plant.
• Buddhist monks had cultivated plants centuries ago ensured their preservation.

Gymnosperm: Gnetophyta

• Plants are the closet gymnosperm linage to flowering plants that are comprised of 3 dissimilar genera.
• Gnetum mostly grows as tropical and subtropical vines and shrubs, which provides a food source.
• Is found in arid areas of North America, used in medicine and provide a potent decongestant
• Is represented by Welwitschia, having an unusual low growing plant that can only be found in the deserts of Namibia and Angola that can live over 2,000 years old.

Angiosperm Traits

• Possess flowers and fruit.
• Undergo insect dispersal.

Flower and Fruit Adaptive Traits

• Flowers enable cooperative evolutionary relationships with animals to disperse their pollen,
• Fruits protect the seed and promote dispersal.

Complete Flowers

• Flowers that have all 4 floral organs

Incomplete Flowers

• Flowers that are missing at least one floral organ.

Female Part (Pistil/Gynoecium) Traits

• The ovary, style and stigma make the female part.
• The carpel is located in the flower center and houses with the female gametophytes, and more than one carpel can be exist.
• Ovules are then contained through the ovary and an egg cell developing into a seed after fertilization.

Male Part (Stamens/Androecia)

• Is comprised of a thin _______ which supports a sac-like structure in which the pollen grains produced are eventually released the anther structure.

Fruit Formation

• After fertilization, the ovule develops into a seed.
• The ovary walls thicken and ripen to form the fruit.
• Serve as a protective barrier.

Seed Dispersal

• The hooks/teeth will ____ the passing fur of animals or the clothing of people so as to cause seedling of the new plant.
• Currents that get created aid in the transportation from the feathery tissue so as to establish the new seedlings of _______.
• Highly nutritious to make seedling dispersal.

Flower Varieties

• Flowers which contain complete qualities for an appropriate _______.

Plant varieties

• _______ exhibit male and female flowering.
• Male and female flowering exhibited by plants of _______.

Floral Lineage

• In _________ the species is classified as the plants as per its lineage.

Angiosperm Phylum

• Has a lineage for its unique qualities for the __________.

Angiosperm: Monocots and Eudicots

• Monocots include familiar plants such as the true lilies, grasses, orchids, and palms.
• _______ include over two thirds of all flowering species, is either herbacious or woody which all fall under the phylum of angiosermae.