How Plants Colonized the Land and Evolution of Seed Plants

Selected Topics in Biology

Ariane B. Sogo-anMST Biology

Objectives

At the end of the lesson, the MST Biology students are expected to conduct the following with at least 80% level of accuracy:– Relate Evolution and Biodiversity of Plant Kingdom – Trace the evolutionary modification of plants

throughout time. – Determine the different plant group per

evolutionary modification and characteristics.

Biodiversity and Evolution

• There are more than 290,000 recorded species of plants known today to be inhibiting the planet earth and most of them adapted mechanisms to be able for them to inhabit various corners of the world such as deserts, grassland and forest.

• There are even terrestrial plants that seemingly shares characteristics from aquatic plants thus pushing the curiosity of mankind to try to explain the origin of plants and how it colonized the world since the beginning of time.

Biodiversity and Evolution

Biodiversity and Evolution

• The great diversity of life is the PRODUCT of evolution. It represents the many different ways in which the common elements of life’s organization have combined to provide new and successful ways to survive and reproduce. • Modification for survival to

certain environment• Ability to bear seeds

An Overview of Land Plant Evolution

• For more than the first 3 billion years of Earth’s history, the terrestrial surface was lifeless and plants first inhabited bodies of water.

Why??

• The movement onto land carried many benefits: including unfiltered sun, more plentiful CO2, nutrient-rich soil, and few herbivores or pathogens

Aquatic to Terrestrial plants

• Three (3) major obstacles.– Water retention (not drying

out),– Structural support (against

gravity), and– Dependence on water for

reproduction (getting gametes together).

Shared Traits of Algae and Plants

• Like brown, red, and some green algae, plants are multicellular, eukaryotic, photosynthetic autotrophs.

• Like green algae, plants have cellulose cell walls.

• Like green algae, euglenids, and some dinoflagellates, plants have chlorophylls a and b.

Charophytes

• Many species of Charophyte algae live in shallow water around the edges of lakes and ponds. How do they withstand that kind of situation? – Sporopollenin - a layer of a durable polymer that

prevents exposed zygotes from drying out.– It was traced scientists that a similar chemical

adaptation is found in the sporopollenin walls that encase plant spores.

EVIDENCES THAT SUPPORT THE PHYLOGENETIC CONNECTION BETWEEN LAND PLANTS AND GREEN ALGAE

• Homologous chloroplasts• presence of chlorophyll b and beta-carotene and

thylakoids stacked as grana. DNA comparison with terrestrial.

• Homologous cellulose walls– cellulose comprises 20-26% of the cell wall.

EVIDENCES THAT SUPPORT THE PHYLOGENETIC CONNECTION BETWEEN LAND PLANTS AND GREEN ALGAE

• Homologous peroxisomes– Both land plants and charophycean algae package

enzymes that minimize the costs of photorespiration in peroxisomes.

• Phagmoplasts– These plate-like structures occur during cell division

only in land plants and charopyceans.• Flagellated sperm– Many plants have flagellated sperm, which match

charophycean sperm closely in ultrastructure.

EVIDENCES THAT SUPPORT THE PHYLOGENETIC CONNECTION BETWEEN LAND PLANTS AND GREEN ALGAE

• Molecular systematics– In addition to similarities derived from

comparisons of chloroplast genes, analyses of several nuclear genes also provide evidence of a Charophycean ancestry of plants.

Terrestrial Plant Adaptation

• Alternation of generations and multicellular, dependent embryos

• Spores produced in sporangia• Multicellular gametangia• Apicalmeristem

Evolution of Roots, Stem and Leaves

Physiological modifications of plants to survive above sea level:

• root system. • shoot system • Stems grew and branches extensively only after plants

developed a biochemical capacity to synthesize and deposit lignin, an organic compound in cell wall.

• xylem (distributes water) and phloem (distributes dissolved sugars and other photosynthetic products).

• Cuticle• Stomata

Further adaptations of plants

• Cuticle• Mycorrhizae – important to plants without

roots. Nitrogen fixing agent. • Secondary compounds – ex. alkaloids,

terpenes, and tannins

From Haploid to Diploid Dominance

• In Algae, a haploid (n) phase in the form of gametophytes (gamete producing bodies), dominates their life cycles.

• The dipoid (2n) phase is the zygote, which forms when gametes fuse at fertilization.

Evolution of Pollen and Seeds

• Like some seedless species, seed bearing plants produce not one but two (2) types of spores.

• This condition is called heterospory, an opposed to homospory (only one type).

• pollen grains, which develop into a mature sperm bearing male gametophytes. – do not require free-standing water to reach the

egg

Pollen Grain

• The evolution of pollen grains contributed to the successful radiation of seed bearing plants into high and dry habitats

Seeds

• It was no coincidence that during the Permian time when the climate was extreme, seed plants rose dominant.

Plant Diversity

• Bryophytes • The Bryophytes lineage consists of about

18,600 species called mosses, liverworts and hornworts.

• These nonvascular plants are mostly well adapted to grow in fully or seasonally moist habitats although there is also other rare types of mosses thriving in deserts and windswept plateaus of Antartica.

Examples of BryophytesLiverworts (Phylum Hephaeophyta)

Hornworts(Phylum Anthoceraphyta)

MossesPhylum Bryophyta

Physical Characteristics of Bryophytes

• All known Bryophytes are less than twenty (20) centimeters (eight inches tall). They have leaflike, stemlike and rootlike parts but these do not contain xylem and phloem.

• Most have rhizoids, which are elongated cells or threadlike structures that attach gametophytes to the soil and serve as absorptive structures.

Evolution of Bryophytes

• Presence of Cuticle • Cellular Jacket • Large gametophytes

Mode of Reproduction of Bryophytes

SEEDLESS VASCULAR PLANTS

• Descendants of seedless plants lineage still exist today such as whisk ferns, lycophytes, horsetails and ferns.

How does Vascular plants differ from Bryophytes?

• Sporophytes does not remain attached to gametophyte

• It has true vascular tissues• Seedless Vascular plants is larger and have

longer lived phase life cycle

Characteristics of Seedless Vascular plants

• Most seedless vascular plants live in wet, humid places, and their gametophytes lack vascular tissues. Water droplets clinging to the plants are the only means by which flagellated sperm can reach the eggs.

Life Cycle of Vascular Seedless plants

Examples of Seedless Vascular PlantsWhisk Ferns (Psilophyta)

Lycophytes (Lycophyta)

Examples of Seedless Vascular PlantsHorsetail(Sphenophyta)

Ferns (Pterophyta)

RISE OF THE SEED-BEARING PLANTS

• In terms of diversity, numbers and distribution, they would become the most successful groups of the plant kingdom.

• Seed Ferns, Gymnospserm and much later the angiosperm were the dominant groups.

How do they differ from Seedless Vascular Plants?

• Besides microspores, seed-bearing plants also reproduce megaspores – these develop within ovules the female reproductive structures which at maturity are seeds.

• Each ovule consists of female gametophytes (with its egg cell), nutrient-rich tissue, and a jacket of cell layers which develops into seed coats. A zygote will form inside the ovule.

• Compared with the seedless vascular plants, gymnosperms had water conserving traits, including thick cuticles and stomata recessed below the surface of the leaf.

How do Pines Reproduce?

• Pine tress produces pine cones. • The scales are parts of a mature female cone which

bears ovules in which megaspores formed and developed into female gametophytes.

• Pine trees also produce male cones, in which microspores forms and develop into pollen grains.

• Pollination is completed when some land on ovules of female cones.

• For pines, fertilization occurs months or a year after pollination.

• Unlike the seeds of flowering plant, which are enclosed in a reproductive chamber (an ovary), gymnosperm seeds grows, in an exposed location, on top of a spore-producing structure.

Gymnosperm Diversity

• Conifers

Gymnosperm Diversity

• Cycads

Gymnosperm Diversity

• Ginkgos

Angiosperms – The Flowering, Seed Bearing plant

• Only the Angiosperm produce specialized reproductive structures called Flowers.

• Angeion, which means vessel, refers to the female reproductive parts at the center of the flower. The enlarged base of the “vessel” is the floral ovary, where ovules and seeds develop.

Embryo Development of Angiosperms

Embryo Development in Angiosperm

• The first mitotic division of the zygote splits the fertilized egg into a basal cell and a terminal cell

• The basal cell produces a multicellular suspensor, which anchors the embryo to the parent plant

• The terminal cell gives rise to most of the embryo • The cotyledons form and the embryo elongates

Structure of the Mature Seed

• The embryo and its food supply are enclosed by a hard, protective seed coat

• Below the cotyledons the embryonic axis is called the hypocotyl and terminates in the radicle (embryonic root); above the cotyledons it is called the epicotyl

• The plumule comprises the epicotyl, young leaves, and shoot apical meristem

Derivation from seed to plant

• Seeds gives rise to mature plants when their dormancy is disrupted in the presence of water. Hypocotyl gives rise to shoot system of the plant, Cotyledon diminishes once the plant is already able to have stable transport of nourishments.

• There are two (2) classes of flowering plants called the dicots and monocots.

• The monocots are grass and "grass-like" angiosperms (flowering plants). Particularly, the embryos of monocots have only a single (mono-) first leaf (a.k.a., seed leaves or cotyledon), vascular bundles are arranged throughout the stem’s ground tissue and leaf venation projects in parallel unlike in Dicots which has netted venation and their vascular bundles are arranged in a ring.

Characteristics Monocot Dicot

Number of cotyledons

One Cotyledon Two Cotyledons

Number of floral parts

Floral parts in three Floral parts in four or five

Leaf venation Parallel Netted

Number of pores in their pollen grain

Pollen grain has one pore or furrow

Pollen grain has three pore or

furrows

Arrangement of the vascular bundles

Vascular bundles are arranged

throughout stem’s ground tissue

Vascular bundles arranged in ring

END OF REPORT