Land Plant Characteristics

• Land plants acquired traits that made it possible to colonize land and survive out of water
• All land plants share characteristics like alternation of generations, sporangium, gametangium, and apical meristem tissue
• Land plants appeared about 500 million years ago in the Ordovician period

Alternation of Generation

• The life cycle of land plants alternates between two multicellular stages: the gametophyte and sporophyte
• Gametophyte is haploid (n) and produces gametes through mitosis
• Fusion of two gametes creates a diploid (2n) zygote
• Zygote divides mitotically to produce a diploid sporophyte
• Sporophyte undergoes meiosis to produce haploid spores
• These spores divide mitotically, developing into a haploid gametophyte

Sporangia

• The sporophyte of seedless plants is diploid
• It results from the fusion of two gametes
• Sporophyte bears sporangia, which are organs that first appeared in the land plants
• Inside the multicellular sporangia, diploid sporocytes or mother cells produce haploid spores by meiosis
• Spores are released from the sporangia and disperse in the environment

Gametangia

• Gametangia are structures observed on multicellular haploid gametophytes
• Precursor cells in the gametangia give rise to gametes by mitosis
• Antheridium is the male gametangium that releases sperm
• Many seedless plants produce sperm equipped with flagella that enable swimming in a moist environment to the archegonia, the female gametangium
• The embryo develops inside the archegonium as the sporophyte

Apical Meristem

• Shoots and roots of plants increase in length through rapid cell division in the apical meristem
• Apical meristem is a small zone of cells found at the shoot tip or root tip
• It is made of undifferentiated cells that proliferate throughout the plant’s life
• Meristematic cells give rise to all the specialized tissues of the organism
• Shoots and roots can elongate allowing a plant to access additional light, water, and minerals

Plant Classification

• Plants can be classified into Charophytes, Streptophytes and Embryophytes
• Embryophytes are further categorized into vascular and non-vascular classes depending on whether or not they have vascular systems
• Vascular plants can be further categorized into seedless plants and seed plants

Vascular Plants

• The vascular system contains xylem and pholem tissues
• For plants to evolve larger forms, the evolution of vascular tissue for the distribution of water and solutes was needed
• Xylem conducts water and minerals absorbed from the soil up to the shoot
• Phloem transports food derived from photosynthesis throughout the entire plant
• Root systems take up water and minerals from the soil, and anchor taller shoes

Plant Protection

• A waxy, waterproof cover called a cuticle protects the leaves and stems from desiccation
• The cuticle also prevents the intake of carbon dioxide needed for the synthesis of carbohydrates through photosynthesis, so stomata/pores appeared in plants to regulate traffic gasses
• Plants evolved biosynthetic pathways to make protective flavonoids and other pigments that absorb UV wavelengths of light to protect the aerial parts of plants from photodynamic damage

Plants Coevolved with Animals

• Plants synthesize poisonous secondary metabolites to discourage predation
• Plants developed sweet and nutritious metabolites (i.e. fruit, nectar) to lure animals into providing valuable assistance in dispersing pollen grains, fruit, or seeds

Green Algae

• Green algae share more traits with land plants than other algae, according to structure and DNA analysis
• Charales form sporopollenin and precursors of lignin, phragmoplasts, and have flagellated sperm, but do not exhibit alternation of generations

Non-Vascular Plants

• Seedless nonvascular plants are small, having the gametophyte as the dominant stage of the lifecycle
• Lacking a vascular system and roots, they absorb water and nutrients on all their exposed surfaces
• Collectively known as bryophytes -- the three main groups include the liverworts, the hornworts, and the mosses

Liverworts

• Liverworts are the most primitive plants and are closely related to the first land plants

Hornworts

• Hornworts developed stomata and possess a single chloroplast per cell

Mosses

• Mosses have simple conductive cells and are attached to the substrate by rhizoids.
• They colonize harsh habitats and can regain moisture after drying out.
• The moss sporangium is a complex structure that allows release of spores away from the parent plant.

Vascular plants: Xylem

• Xylem tissue transports water and minerals
• Xylem transports water and minerals through vessel elements and tracheids, which are dead at maturity with primary and secondary cell walls
• Pits may have thin/missing secondary walls that allow water to low laterally

Pholem

• Phloem tissue transports sugars and proteins
• It transports sugars and other items, in angiosperms, sieve tube elements contain the sugar solution
• Sieve-tube cells are surrounded by various support cells

Roots

• Roots transfer water and minerals from the soil to the rest of the plant
• A large network of roots that penetrate deep into the soil stabilizes trees acting as a ballast or anchor
• Most roots establish a symbiotic relationship with fungi, forming mycorrhizae, which helps the plant by greatly increasing the surface area for water and soil mineral/nutrient absorbtion

Leaves

• Leaves act as large photosynthetic organs
• Small leaves are microphylls
• Large leaves with vein patterns are megaphylls
• Modified leaves that bear sporangia are sporophylls, with some sporophylls arranged in cone structures known as strobili

Seedless Vascular Plants

• Club mosses are the most primitive
• Whisk ferns lost leaves and roots by reductive evolution
• Horsetails and ferns exist
• Ferns are the most advanced group and have large leaves and sporangia that produce structures called sori found on the undersides of fronds

Evolution of Seed Plants

• Seed plants appeared about 360 million years ago.
• Seeds and pollen plants reproduce in the absence of water.
• The gametophytes shrank while the sporophytes became prominent and the diploid stage became the longest phase.
• Gymnosperms became the dominant group during the Triassic.
• Pollen grains and seeds protect against desiccation.
• The seed, unlike a spore, is a diploid embryo surrounded by storage tissue and protective layers and delays germination

Gymnosperms

• Gymnosperms have naked seeds, separate female and male gametes, and pollination by wind
• They have tracheids transport water and solutes in the vascular system
• Gymnosperm seeds are not enclosed in an ovary; they are exposed on cones or modified leaves

Coniferophyta

• The largest phylum, Coniferophyta, is conifers, the predominant plants at high altitude and latitude
• Water evaporation from leaves is reduced by their thin shape and the thick cuticle
• Snow slides easily off needle-shaped leaves, decreasing breaking of branches
• Female cones grow in the upper branches where they may be fertilized by pollen blown on the wind from the male cones
• Male cones grow in the lower branches
• A pollen tube forms, allowing the pollen to migrate toward the female gametophyte, after fertilization a diploid zygote forms

Cycadophyta

• Cycads (phylum Cycadophyta) resemble palm trees, grow in tropical climates, and may be pollinated by beetles
• Only a hundred or so species have persisted and face possible extinction

Gingkophyta

• Gingko biloba is the only representative of the phylum Gingkophyta

Gnetophytes

• Gnetophytes are the closest relative to modern angiosperms; they have broad leaves and produce elements in their wood

Evolution of Flowering Plants

• Angiosperms bear both flowers and fruit; these structures protect the gametes and the embyro
• They appeared during the Mesozoic era and have become the dominant plant life in terrestrial habitats

Angiosperms

• Angiosperms owe their success to two structures that protect reproduction from variability in the environment: the flower and the fruit
• Flowers are derived from modified leaves, with main parts being the sepal and petals that protect other reproductive parts like the stamens and carpels
• Stamens produce male gametes in pollen grains while carpels contain female gametes within the ovary of a carpel
• The walls of the ovary thicken after fertilization, ripening into fruit for dispersal by wind, water, or animals

Angiosperm Life Cycle

• Angiosperm life cycle is dominated by the sporophyte stage
• Double fertilization is unique to angiosperms, where one sperm in the pollen fertilizes the egg (diploid zygote) while the other combines with two polar nuclei, forming a triploid cell that develops into food storage tissue called the endosperm

Basal Angiosperms

• Basal angiosperms belong to an older lineage than other angiosperms

Monocots and Eudicots

• Flowering plants are divided into monocots and eudicots, according to the number of cotyledons in the seedlings

Herbivory

• Angiosperm diversity is due to multiple interactions with animals
• Herbivory favors the development of defense mechanisms in plants and the animals begin to avoid them

Pollination

• Pollination (transfer of pollen to a carpel) is carried out by wind and animals
• Angiosperms evolved numerous adaptations to capture wild and attract specific classes of animals

Plants in Ecosystems

• Plants are the source of food, medicinal compounds, and materials for many industries
• Rapid deforestation and indusrialization however, threaten plant biodiversity and its ecosystem