Seedless plants Section 22.2

What is algae? Seaweed is the most common form of algae Algae are not a single group of organisms Applied to any photosynthetic eukaryote that isn’t a land plant Some are protists, some are plants Those grouped with plants are called green algae

The first plants Green algae were the first plants on earth Large algal mats from the Cambrian – 550 million years ago Mostly aquatic Found in fresh and salt water, and some moist areas on land Photosynthetic pigment and cell wall composition shared with larger more complex plants Absorb moisture and nutrients directly from their surroundings Do not contain the specialized tissues found in other plants

Algae life cycle Life cycle switched between haploid and diploid Some do not alternate with every generation Chlamydomonas stays in the same haploid stage for multiple generations Haploid cell can reproduce asexually by mitosis Chlamydomonas can switch to asexual reproduction if conditions become unfavorable Gametes produced that fuse into a diploid zygote – a sporophyte Zygotes thick skin allows them to survive harsh conditions

Multicellularity Many green algae from colonies In some cases, (eg. Volvox) algae are connected through strands of cytoplasm and can communicate Allows movement of the colony Up to 50,000 cells can form a hollow sphere Straddles the fence between colonial and multicellular life

Mosses and Bryophytes Mosses are like natures carpet Thin waxy coating allows resistance to dry conditions Rhizoids anchor them to the soil and absorb water and minerals from surrounding soil Mosses are a bryophyte Bryophytes have specialized reproductive organs enclosed by other non reproductive cells Higher degree of cell specialization than algae Other examples are hornwarts and liverworts (Phylums)

Features of bryophytes Found in damp places where lots of water Because they do not contain vascular tissues to carry water Also means that they can’t grow above 1 meter in height They don’t produce lignin, meaning they can’t support large structures

Life cycle of bryophytes Like all land plants display alternation of generations Gametophyte is the dominant, recognizable stage of life cycle Also carries out most of plants photosynthesis Sporophyte is dependent on Gametophyte for water and nutrients Bryophytes produce sperm cells that swim using flagella Must be released in area with enough water for them to swim to an egg cell Need standing water for some of the year

Moss life cycle explained

What does vascular tissue do? Why is it important to plants? 420 million years ago plants suddenly got taller! They were the first plants to have a transport system Contain true vascular tissue Efficient at carrying water and nutrients

How did the transport system evolve? Vascular plants are also called tracheophytes, because they contain a specialized water conducting cell A tracheid – hollow tubelike cell with thick cell walls strengthened by lignin Massive evolutionary step in plant kingdom Tracheids are found in the xylem - a tissue that carries water upwards from roots to all parts of the plant Connected end to end Openings (pits) allow water to move more efficiently than simply by diffusion Phloem – transports both nutrients and carbohydrates produced by photosynthesis Fluids are moved throughout the plant against gravity

Seedless plants All seed bearing plants have a vascular system So do some seedless plants Common examples – Club mosses, horsetails and ferns Ferns are the most common- 11,000 species Can thrive in areas with little light and are abundant in seasonally wet habitats

Life cycle of ferns Ferns are actually diploid sporophytes This is the dominant stage for all vascular plants

Key summary questions What are the characteristics of green algae? What factors limit the size of bryophytes? How is vascular tissue important? What tissues to most plants contain? What function do they perform?