Lecture #26—Plant Kingdom—Bryophytes—Mosses and their friends
Plants are eukaryotic, multicellular, and photosynthetic using chlorophylls a & b to capture light. DNA evidence suggests the first land plants evolved from filamentous green algae living along the shore 500-600 mya, although the earliest yet discovered are fossils at 470 mya.
Recall that eukaryotic cells evolved much earlier (2.5 billion years ago) so that invasion of land took a long time, possibly because O2 levels were not high and ozone layer not well developed.
Bryophytes are the first land plants: mosses, liverworts and hornworts. DNA suggests that present-day liverworts are most similar to their ancestors. They survive by living in moist environments or survive drying by becoming dormant. They are all small and lack vascular tissue to conduct water.
Tracheophytes evolved later. The first Tracheophytes were simply stems, lacking roots and leaves. The Tracheophytes had several evolutionary advances over bryophytes allowing them to be better suited to terrestrial life:
- xylem and phloem
- lignin = wood
- thick waxy cuticle
- fully functional stomates (open and close with turgor pressure)
- profuse branching
- increased young tissues for photosynthesis
- increased areas for sporangia (as opposed to only one sporangium per sporophyte in the Bryophytes)
- highly differentiated plant tissues and organs
Vascular Tissue: Specialized tubular cells conducting water and nutrients throughout the plant body. Allows the plant to grow large.
Xylem= cells specialized to carry water and minerals. They function when they are hollowed out and dead carrying water from the roots to the leaves.
Xylem growth produces annual rings in trees.
Phloem=cells specialized to carry water and sugars. They function when they are alive carrying materials from the leaves to the roots and flowers.
Life Cycles.
Moss life cycle. Notice that in the Moss cycle the dominant (obvious) part of the cycle is the green leafy stuff. The cells of the leaves have only one set of chromosomes (n) and we call this the gametophyte stage because this plant forms gametes (sperm and eggs) in special little organs called gametangia. The male gametangium is the antheridium (forms sperm) and the female gametangium is the archegonium forms eggs. The sperm swim to the egg when it is wet or is carried by insects and fertilize it. Then an embryo forms in the archegonium and this is the sporophyte generation the cells of which have 2 sets of chromosomes (2n). The sporophyte depends on the maternal tissue to grow just like a human baby. When it is mature, it produces spores (single cells that have a protective covering) which are released into the environment.
Terms/concepts to define:
Bryophytes
Tracheophytes
Moss
Liverwort
Xylem
Phloem
Gamete
Alternation of Generations
Sporophyte
Sporamgium
Gametophyte
Diploid
Haploid
Gametangium
Antheridium
Archegonium
Can you answer these questions?
- At one time, bacteria and fungi were classified in the Plant Kingdom because their cells had cell walls. Why have we changed our mind about this?
- Even before we had DNA analysis, green algae were considered to be the likely ancestors of land plants. Why was this the case?
- Why is it that Bryophytes are restricted to moist environments?
- Why is it that Bryophytes are rather small plants?
- Why is it that Tracheophytes require xylem and phloem in order to succeed?
- Both animals and plants have both haploid and diploid stages in their life cycle, yet we say that plants have an alternation of generations but animals don’t. Why is this?
- How do mosses deal with the three major challenges of living on land?
- Diagram the life cycle of a moss.