Chapter 23.  Plantae:  Lower Vascular Plants
Note --- We will emphasize the Ferns

• Classifying the "land plants" (Kingdom Plantae, plants in the narrow sense, excluding the algae)
• Bryophytes: small, non vascular, gametophyte dominant, (mostly) restricted to moist habitats (require water for sperms to reach eggs). About 23,000 species.
• Vascular plants: well developed vascular systems (xylem and phloem), sporophyte dominant.  Sporophytes tolerant of dry conditions (vascular system, cuticle, stomata are adaptations permitting life in completely terrestrial conditions).
• Lower vascular plants:   Sporophytes well adapted to terrestrial conditions but gametophytes require moist habitat for sperms to reach eggs.  These plants do not produce true seeds.  About 14,000 extant species (more diverse in earlier geological periods). Division Pterophyta (ferns) is the largest group today.
• Higher vascular plants ("seed plants").  Heterosporous life cycle, heterothallic gametophytes. Gametophytes highly reduced, megagametophyte enclosed by tissue of parent sporophyte, microgametophyte develops inside microspore with resistant wall,  consequently there is no longer a need for moist habitat for fertilization.  These are the dominant plants today - most of the species and most of the vegetation.
• Gymnosperms (naked-seed plants).  About 900 species, all woody (trees, shrubs, lianas).  Includes the conifers and several other very ancient groups.
• Angiosperms (plants with enclosed seeds, the flowering plants).  About 250,000 species, very diverse in habit (herbs, shrubs, trees, annuals, perennials, etc.)
• Plant life on land
• A quick review of the history of life:
• The earth is about 4.6 billion years old, life appeared about 3.8 billion years ago (3.8BYA).
• For most of life's history only prokaryotes ("bacteria") have existed.  First eukaryotes appeared about 1.4BYA.
• For most of life's history only unicellular organisms have existed.  First multicellular organisms appeared about .6BYA (600MYA).
• Plants began to colonize the land about 500MYA.  Colonization by animals followed.
• Life on land presented a significant challenge:
• Aquatic environment: water is buoyant (no need for support tissues), nutrients can be absorbed directly from water (no need for roots or conducting tissues), gametes and spores and tissues need not be protected from desiccation (no need for cuticle, stomata).
• UV radiation was lethal on early earth.  Early atmosphere was transparent to UV (but sea water absorbs UV).  Low concentrations of oxygen (O2) in atmosphere meant low concentrations of protective ozone (O3).  (Ozone forms from oxygen). Berkner-Marshall hypothesis: colonization of land depended on accumulation of photosynthetically produced oxygen and formation of ozone layer.
• Major land plant radiation occurred during Silurian-Devonian periods (438-360 MYA).  This is the age of many rock formations in western and central New York.  See Figure 23.2.
• Earliest plants (e.g. Cooksonia, Rhynia) had very simple morphologies: branched stems, no leaves, terminal sporangia.
• Evolutionary innovations during this period included:
• Spores with resistant walls (like modern spores and pollen grains).  These are among the earliest fossils.
• Simple leaves (microphylls, develop as epidermal outgrowths, single vascular bundle).
• True leaves (megaphylls, developed from branched stem system, Fig. 23.7)
• Tracheids and other specialized cells of vascular system.
• Secondary growth (there were trees and forests)
• Heterospory and seeds.
• Division Psilophyta: The most primitive vascular plants.
• Two representatives, Psilotum, Tmesipteris.
• Psilotum: very simple morphology reminiscent of extinct plants.
• Dichotomously branched stem system
• Sporangia associated with non-vascular scales.
• No leaves.
• "Root-like" stem anatomy (no pith). Stem cortex is photosynthetic tissue.
• Very simple gametophytes, require mycorrhizal fungus.
• Division Lycophyta: Club mosses and spike mosses (they aren't mosses!).
• More complex morphology than Psilophyta: roots, microphylls with veins.  Some microphylls are specialized to support sporangia, these are called sporophylls.
• Lycopodium life cycle (Fig. 23.14). Conspicuous, leafy sporophyte; tiny gametophyte.  The life cycle is homosporous, with homothallic, free-living gametophytes. The sperms are flagellated and require a film of water to reach eggs.
• Selaginella life cycle (Fig. 23.15).  Conspicuous, leafy sporophyte, microscopic gametophytes develop inside spore wall.  The life cycle is heterosporous with heterothallic gametophytes.  Note - this life cycle is very like the angiosperm life cycle.
• Division Sphenophyta: Horsetails [will not be covered]
• Division Pterophyta: Ferns.  A diverse, actively evolving division.  (The other divisions are relicts, "living fossils").
• Most diverse of the lower vascular plant divisions.
• Typical morphology: stem is an underground rhizome, leaves (fronds) are usually compound with meristematic tips (fiddleheads).  These are true leaves, megaphylls. See Fig. 23.19.
• Other morphologies: tree ferns, aquatic ferns,
• Life cycle (Figure 23.23): homosporous in most species.  Spores produced by sporangia arranged in clusters (sori, singular sorus) on undersides of leaves.  Spores are dispersed, germinate to produce gametophytes.
• Gametophytes are free-living, tiny, homothallic (antheridia and archegonia produced by same gametophyte thallus).  Fertilization occurs when flagellate sperms swim to eggs in archegonia.

.