Vascular Plants: Tracheophytes

Vascular Plants (Tracheophytes)

• Includes lycophytes, pteridophytes, and seed plants

Key Characteristics of Vascular Plants

• Water and nutrients are transported through xylem and phloem (vascular tissues)
• Roots provide transport and support
• Leaves increase surface area for photosynthesis and evolved twice
• Dominated by a larger, more complex sporophyte generation

Xylem and Phloem

• Tissues used to transport water and nutrients
• Xylem transports water in one direction, cells are dead
• Pholem transports sugar and other, more in two directions
• The cohesion-tension mechanism works by cohesion, where water molecules move from high to low concentration

Cohesion-Tension Theory

• Water travels from roots to evaporating surfaces through the continuous liquid columns that form
• Water evaporates, and evaporated water is replaced by water in the xylem
• Water is pulled out of root cells, and water is pulled out of the soil

Evolution of Vascular Tissue

• Vascular tissue evolved in gradual steps
• Plants gained increased structural support, which allowed them to grow upright

Types of Xylem Cells

• Two cell walls exist in tracheids
• Primary cell wall is made of cellulose and is flexible
• Secondary cell wall is made of lignin, a tough, woody substance; it helps plants stay upright
• Vessel elements exist in angiosperms, gnetophytes, and ferns
• Vessel elements are shorter and wider than tracheids, with water flowing faster through them
• They contain large openings called perforation plates
• These structures evolved independently in different plant groups (convergent evolution/homoplasy)

Translocation

• "Source cells" load sugar into phloem via active transport
• Water moves into phloem via osmosis
• Pressure increases inside phloem
• "Sap" flows along phloem down a pressure gradient
• "Sink" cells near phloem remove sugars via active transport, water leaving phloem
• Sap flow direction is dependent on time of year

Phloem

• Vascular plant tissue transports sugar and nutrients in both directions
• It is also dependent on where it is needed, and requires ATP
• Sugars made/stored in source cells (leaves during photosynthesis)
• Sugars taken/stored in sink cells (roots, flowers)
• Sugars load into phloem cells at the source with ATP, water from the xylem enters the pholem
• Incoming water causes a pressure build which pushes sugary sap
• In spring/summer, source cells produce sugar and send to sink
• In winter, store carbohydrates that are found in roots and also become the source

Tree Rings

• Tree ring dating allows us to check the support to the tree, and see active transport of water and nutrients
• Latewood is formed in late summer/fall with smaller, dense, darker cells due to less water
• Earlywood is formed in spring with larger, lighter cells

Seedless Vascular Plants

• Have vascular tissue, however, do not make seeds
• They include ferns, horsetails and club mossses
• they are dependent on water for reproduction,
• the sporophyte is the dominant generation
• The sporophyte developed from a spore is diploid(2n) and develops into a gametophyte
• First showed up in the Devonian period and Carboniferous period

Club Mosses (Lycophytes)

• Are distinct from mosses
• Ancient club mosses
  • Massive (100ft tall, 3 ft wide)
  • Dominated the Carboniferous period
• Modern club mosses vs modern mosses
  • Are tiny: knee height or below
  • Are like ground covers
  • Have true roots, stems, leaves, and vascular tissue
• Ancient swampy environments allowed them to compress into coal deposits
• Spores are used for reproduction and are flammable, made of oils
• Are among the earliest vascular plants, from around 400-300 million years ago

Carboniferous Period

• Time of vast swampy forests full of primitive plants; became today's coal deposits
• Lycophytes/club mosses could grow upto 100 feet tall and 3 feet wide
• The upper-canopy trees include the extinct tree lycopsid sigillaria, the cordaites, the Marattialean tree ferns, and the noeggerathiales

Pteridophyta (Ferns)

• Produce large and well-developed leaves (fronds), including invasive plants
• They germinate on sorus
• They are highly variable, and can live in dry regions
• Ferns are the only seedless vascular plants that have large, well-developed leaves
• Produce spores that use meiosis in sori
• A new genus of ferns (19 species) are named Gaga due to resemblances
• They do not produce seeds and use spores to develop fronds
• Ferns are grouped in two life stages: sporophyte and gametophyte
  • Sporophyte stage is diploid, produces spores with two sets of chromosomes through meiosis in structures called sori
  • Gametophyte stage is haploid