Biology Chapter 3.2.2: Mosses

Mosses

• The gametophyte is the dominant stage in the moss life cycle, consisting of two phases: protonema and leafy stage.
• Protonema, originating from spores, is creeping, green, and branched, often filamentous.
• The leafy stage features upright axes with spirally arranged leaves, anchored to soil via branched rhizoids.
• This stage bears sex organs; vegetative reproduction occurs through fragmentation and budding.
• Sexual reproduction involves antheridia (male) and archegonia (female) at the apex of leafy shoots.
• After fertilization, the zygote develops into a sporophyte, comprising a foot, seta, and capsule.
• Sporophytes are more elaborate in mosses compared to liverworts, with capsules that contain spores formed by meiosis.
• Mosses possess an intricate spore dispersal mechanism.
• Common moss examples include Funaria, Polytrichum, and Sphagnum.

Pteridophytes

• Includes horsetails and ferns, used for medicinal purposes, soil-binding, and ornamental growth.
• First terrestrial plants with vascular tissues (xylem and phloem).
• Typically thrive in cool, damp, shady locations; some may grow in sandy soils.
• Dominant life cycle phase is the sporophyte, differentiated into true roots, stems, and leaves.
• Leaves can be microphylls (small, as in Selaginella) or macrophylls (large, as in ferns).
• Sporophytes bear sporangia supported by sporophylls, which can form strobili or cones.
• Spores are produced via meiosis, germinating into small, photosynthetic gametophytes called prothallus.
• Gametophytes grow in moist environments, requiring water for fertilization; they bear antheridia and archegonia.
• Male gametes (antherozoids) require water for transport, leading to zygote formation within archegonia.
• Most pteridophytes are homosporous; genera like Selaginella and Salvinia are heterosporous, producing both macro and micro spores.
• Development of zygotes within female gametophytes is a step towards seed evolution.
• Classifications: Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum), and Pteropsida (Dryopteris, Pteris, Adiantum).

Gymnosperms

• Gymnosperms (means "naked seeds") have ovules that are not enclosed by an ovary wall before and after fertilization.
• Seeds remain exposed post-fertilization; include medium to tall trees and shrubs.
• The giant redwood tree, Sequoia, is among the tallest tree species.
• Roots typically form tap roots; some have mycorrhizal associations, while others like Cycas possess coralloid roots that associate with N2-fixing cyanobacteria.
• Stems can be unbranched (Cycas) or branched (Pinus, Cedrus).
• Leaves in gymnosperms can be simple or compound; Cycas has long-lasting pinnate leaves.
• Adapted for harsh conditions: conifers have needle-like leaves that reduce surface area and exhibit thick cuticles and sunken stomata to withstand extreme temperatures and humidity.

Explore the fascinating life cycle of mosses, focusing on the predominant gametophyte stage. Learn about the protonema stage and the leafy stage, each with distinct characteristics. This quiz will help reinforce your understanding of moss biology.