5.4 Plant Diversity, Structure, and Reproduction

A Plant Family Tree on the Praxis

Land plants share a common ancestor with green algae (specifically the charophytes). The major evolutionary innovations of land plants - cuticle, stomata, vascular tissue, seeds, flowers - appear in a predictable order, and the Praxis loves to test that order.

Bryophytes (Mosses, Liverworts, Hornworts)

• No vascular tissue - water moves cell-to-cell, limiting size to a few centimeters tall.
• Need free water for reproduction because sperm must swim to the egg.
• The gametophyte (haploid, 1n) is the dominant generation - the green moss carpet you see is gametophyte tissue. The sporophyte is a small stalk with a sporangium that sits on top of the gametophyte.

Seedless Vascular Plants (Ferns)

• First plants with xylem and phloem - allowed taller stature (tree ferns reach 25 m).
• Still disperse by spores; still need water for sperm to swim to the egg.
• Sporophyte (2n) is dominant - the fern fronds you see are sporophyte tissue. The gametophyte is a tiny separate heart-shaped prothallus.

Gymnosperms (Conifers, Cycads, Ginkgo)

• First group with seeds, allowing dispersal without free water.
• Pollen carried by wind replaces swimming sperm.
• "Naked" seeds sit exposed on the surface of cone scales (no fruit).
• Dominant in cold, dry biomes (boreal forests).

Angiosperms (Flowering Plants)

• Most diverse plant group (~300,000 species, 90% of living plants).
• Flowers attract animal pollinators (sometimes wind-pollinated).
• Fruit (mature ovary tissue) protects seeds and aids dispersal.
• Two classes: monocots (one cotyledon, parallel leaf veins, flower parts in 3s - grasses, lilies) and eudicots (two cotyledons, net-like leaf veins, flower parts in 4s or 5s - roses, oaks, sunflowers).

Plant Structure

Root, Stem, Leaf

• Roots anchor the plant and absorb water and minerals. Two main types:
  • Taproot system (one main vertical root, e.g., carrots, dandelions) - typical of eudicots.
  • Fibrous root system (many similar-size roots, e.g., grass) - typical of monocots.
• Stems support leaves, conduct fluids, and may store food.
• Leaves are the primary photosynthetic organ; gas exchange happens through pores called stomata, regulated by guard cells.

Vascular Tissue

Vascular plants have two specialized transport tissues that run continuously from root tip to leaf tip:

The cohesion-tension theory explains xylem flow: water evaporates from leaves (transpiration), pulling continuous columns of water upward thanks to hydrogen-bonded water cohesion.

The pressure-flow hypothesis explains phloem transport: sucrose is actively loaded at source tissues (mature leaves), drawing in water and creating high pressure that pushes phloem sap toward sinks (roots, fruits, growing tips), where sucrose is unloaded.

Meristems

Meristems are regions of undifferentiated, actively dividing cells - the plant equivalent of stem cells.

• Apical meristems at root and shoot tips drive primary growth (elongation).
• Lateral meristems (vascular cambium and cork cambium) drive secondary growth (thickening), which is how trees add wood and bark each year.
• Intercalary meristems at nodes between leaves allow grasses to regrow after mowing or grazing.

Alternation of Generations

All plants have a life cycle that alternates between two multicellular phases:

• Sporophyte (2n, diploid) - produces spores by meiosis.
• Gametophyte (1n, haploid) - produces gametes (sperm + egg) by mitosis.

Gametes fuse at fertilization to form a diploid zygote that grows into the next sporophyte. The relative size and prominence of these two phases differs across plant groups:

• Bryophytes: gametophyte dominant (large), sporophyte dependent (small).
• Vascular plants: sporophyte dominant (large), gametophyte reduced (tiny - sometimes microscopic, as in angiosperms).

Double Fertilization in Angiosperms

Angiosperms are unique in performing double fertilization - one of the most-tested topics in this chapter.

The female gametophyte (embryo sac) inside the ovule has eight nuclei in seven cells:

• 1 egg cell (1n)
• 2 synergids (1n) flanking the egg
• 3 antipodal cells (1n) at the opposite end
• 1 central cell containing 2 polar nuclei (each 1n)

A pollen grain produces a pollen tube that delivers two sperm cells into the embryo sac.

• Sperm 1 + egg → diploid (2n) zygote → embryo
• Sperm 2 + the two polar nuclei → triploid (3n) endosperm → nutritive tissue surrounding the embryo

The triploid endosperm is the energy-rich tissue that fuels the seedling on germination (and is the white starch of a corn kernel, the meat of a coconut, the bran/endosperm of a wheat grain).

Plant Hormones

Five classes of hormones coordinate plant growth and responses. Each appears regularly on the Praxis.

Phototropism in one sentence: light hits the shoot tip → auxin migrates to the shaded side → cells on the shaded side elongate more → the shoot bends toward the light.

Stomatal Control - A Praxis Favorite

Stomatal opening and closing is controlled by guard cell turgor:

• Open: light + low CO₂ → guard cells pump in K⁺ → water enters by osmosis → cells become turgid → curved shape opens the pore.
• Closed: darkness, drought, or ABA → guard cells lose K⁺ → water exits → cells become flaccid → pore closes.