Kingdom PlantaePhylum TracheophytaClass Psilophytes and Ferns

by Jaime Crosby CHS

• Tracheophytes are “true” land plants because they have evolved ways of freeing themselves from dependence upon wet environments

Adaptations for land

• Vascular tissue– Xylem contains tracheid cell (thus

“tracheophytes”!) which carry water from the roots to all parts of the plant.

• Thick, strong cell walls of tracheid cells strengthen stems of plant and support primary growth

– Phloem for downward sugar transfer from site of photosynthesis in leaves

• True Roots and True Leaves– Roots contain a vascular cylinder which is a

central area of the root that houses xylem and phloem

– Veins house xylem and phloem in leaves– Cuticle retains water in leaves

Take a step back to: The first vascular plants

• Psilophytes– Discovered in fossils and dated back to 400

Million years• Small, creeping plants that had primitive xylem

and phloem• No true roots or leaves• Some scientists believe they are extinct, others

believe there are still two living Psilophytes

Are they Psilophytes or do they belong in another classification?

• Lycopodium common name: Club Moss– Looks like a mini-pine tree (9 cm tall)– Small, scale-like leaves that cling to the stem

• Horsetails (Sphenophytes) common name: Equisetum– Grow 1 meter tall– Have true roots, stems and leaves (vasc.)– Contain abrasive silica crystals– Used in colonial times to scour pans

Club Moss Horsetail

Tracheophytes: Ferns

• Evolved about 400 MYA

• 11,000 species exist

• Have vascular tissue, strong roots and creeping underground stems called Rhizomes

• Most abundant in wet or seasonally wet areas like the Pacific Northwest

• Can grow as large as small trees

• Have large leaves called Fronds

Alternation of Generations in Ferns

• Actual Fern is Sporophyte (2N) generation

• Ferns still depend on water for the sperm to swim to the egg

• Haploid spores are produced in sporangia which are grouped into SORI on the underside of the frond

• When ripe, spores get released from the sporangia and carried away by wind or water

• If conditions are right, spores will germinate into haploid gametophytes– Grow root-like rhizoids– Flattens out into Prothallus, the heart-shaped

gametophyte

Prothallus

• Archaegonia and Antheridia are located on underside– Mature antheridium release sperm

• Fertilization occurs when ground and prothallus are covered with water so sperm can swim to egg in archaegonia

• Diploid zygote immediately begins to grow into a new sporophyte plant (fiddlehead) with rhizomes and a frond.

• Fronds die in winter, but rhizomes live through the winter to sprout again.

Summary of Fern Life Cycle

• Alternation of Generations is present: Sporophyte is dominant, gametophyte is dependent.

• Sporophyte is well developed with true vascular tissues

• Gametophyte is tiny, lacks vascular tissues and can only live for a short time in moist areas

• Sexual reproduction still requires water

A final thought…

• Why are ferns better adapted to life on land than mosses?