Chapter 25 Seedless Plants

Introduction: Seedless plants

• Kingdom Plantae

• more than 300,000 species of plants

• 260,000 are seed plants. Seedless plants represent a small fraction

• Most seedless plants require a moist environment for reproduction

• Mosses, ferns, conifers and flowering plants belong to the plant kingdom

Plant Adaptations to life on land

• tolerance to dryness: Mosses are very tolerant

• Structural support that is not just buoyancy by water

• Gametes are protected from desiccation

• Colonization of environments with high humidity: Ferns are seen at the bottom of the temperate forests

• Resistance to desiccation: Cacti

Adaptation Continued: 4 major adaptation for success

• alternation of generation (sporophyte and gametophyte)

• apical meristem tissue in roots and shoots

• waxy cuticle to resist desiccation (not all mosses have it)

• cell walls with lignin to support structures off the ground

Alternation of Generation

• All sexually reproductive organisms have both haploid and diploid cells in their life cycles

• Haplontic - dominant haploid stage

• Diplontic - dominant diploid stage

• Alternation of Generation describes a life cycle in which an organism has both haploid and diploid multicellular stages.

• Plants are haplodiplodontic

• sporophyte stage is barely noticeable in seedless plants: liverworts and ferns

• Alternate name for land plants is embryophytes

Sporangia in Seedless plants

• sporophyte of seedless plants is diploid: results from syngamy which is the fusion of 2 gametes

• Sporangia means spore in a vessel (reproductive sac in which spores are formed)

• Diploid sporocytes undergo meiosis to produce haploid spores

• Spores are released and spread across the environment, where the haploid spore could germinate in a hospitable environment

• It will generate a multicellular gametophyte by mitosis

Homosporous

plants produce only one type of spore, and the resultant gametophyte produces both male and female gametes on the same individual (Nonvascular plants are homosporous, with the gametophytes being dominant)

Heterosporous

plants produce 2 types of spores , (males are called microspores and females are called megaspores). Seedless vascular plants and seed plants are heterosporous

Sporopollenin

is a tough polymer that protects the spores of seedless plants - resistant to chemicals and biological degradation

Spore

producing sacs called sporangia grow at the ends of long, thin stalks in this photo of the moss Bryum capillare

Gametangia in Seedless plants

• found in the multicellular haploid gametophytes

• they give rise to the gametes by mitosis

• male gametangium (antheridium) produces flagellated sperm

• Female gametangium (archegonium)

• embryo develops inside the archegonium as the sporophyte

• gametangia are very prominent in seedless plants

Apical Meristems

• increase in length of shoots and roots through rapid cell division in a tissue known as apical meristem - a small mitotically active zone found at the shoot tip or root tip

• is made of undifferentiated cells that continue to differentiate throughout the life of the plant

• Elongation of shoots and roots allows plants to have more access to light, water, and minerals

• Lateral meristem produces cells for the increase in diameter of tree trunks

Additional Land plant adaptations

• Evolution of vascular tissues - Xylem and Phloem

• Xylem conducts water and minerals absorbed from the soil up to the shoot

• Phloem transports food derived from photosynthesis

• development of shoots

• root system for taking water and minerals and anchoring the plants

• Waxy, waterproof cover known as cuticle protects the leaves and stems from desiccation

• Stomata for the intake of carbon dioxide

• Biosynthetic pathways gave rise to Flavonoids and other pigments to absorb UV wavelengths of light

• Secondary metabolites: complex organic molecules such as alkaloids with noxious smells and unpleasant taste to deter animals

• We have exploited the secondary metabolites for drugs, medication, or spices

• Sweet and nutritious metabolites lured animals into providing valuable assistance in dispersing pollen grains, fruits or seeds

Evolution of Land Plants

• Early era - Paleozoic has 6 periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian

• Ordovician: 500 mya when the land was colonized by the ancestors of the modern land plants

• Oldest known vascular plants date back to the Devonian period

• End of the Devonian era was marked by the emergence of ferns, horsetails and seed plants which populated the landscape, giving rise to trees and forest

• This luxuriant vegetation helped to enrich the atmosphere with oxygen

• Plants established early relationship with fungi forming mycorrhizae