Archegonium and antheridium
The egg is protected in the specialised structure, archegonium.
The sperm are produced and protected in the antheridium.
Three lineages of bryophytes and characteristics of bryophytes.
Hornworts, liverworts, mosses (liverworts and mosses are always sisters).
- non-vascular, absorb water over body surface
- small plants that inhabit moist environment
- young sporophyte (diploid) depends on gametophyte (haploid) for nutrition and water
Structures of liverworts
- leafy (not actual leaf) or thallose (flat sheets)
- specialised hairs (paraphyses) do not occur amongst archegonia and antheridia
- capsule lacks stomata (pores always open), operculum, peristome and calyptra
- have elators (spring-like structures which assist in spore dispersal) among spores
Structures of mosses
- leafy gametophyte
- rhizoids are multicellular, leaves usually have a costa
- paraphyses (filament-like structures) present amongst archegonia and antheridia
- have capsule (may have stomata), calyptra and operculum, not all have peristome
- conducting tissue: hydroids (move water) and leptoids (move photosynthate)
Structures of hornworts
- thallose gametophyte, rhizoids are unicellular
- cells within thallus have large chloroplast with a pyrenoid (structure associated with chloroplast in algae, that grabs CO2)
- sex organs embedded in thallus
- sporophytes lack a seta; capsule forms tall cylindrical structure with stomata and lacks operculum, peristome and calyptra
- pseudoelators are present amongst the spores
Polysporangiophytes and their features
Includes all land plants except bryophytes.
- have branched sporophytes (bryophytes sporophytes are unbranched), in which each branch is terminated by a sporangium (makes spores)
- have dominant, independent sporophyte generation that all have vasculature (tracheophytes)
- Note: Some extinct ones do not have vasculature.
Three lineages of polysporangiophytes
- protracheophytes: had anatomical feaatures intermediate between those of bryophytes and vascular plants (not tracheophytes)
- paratracheophytes: had anatomical features more advanced than bryophytes, include some living vascular plants
- eutracheophytes - all living vascular plants (also includes the fossil Cooksonia)
Three types of vasculature
- hydroid-like: cells with uniform cell walls, protracheophytes (like moss hydroids that lack wall thickenings and non-lignified)
- S-type: cells have prominent wall thickenings, lignified, paratracheophytes
- G-type: cells have prominent wall thickenings, lignified (most extinct and all extant tracheophytes)
Three types of xylem cell in living eutracheophytes
- tracheids: less specialised than vessel elements
- vessel elements: evolved independently
- vascular tissue: found in vascular cylinders (steles) of roots and stems
Three types of vasculature in living eutracheophytes
- protostele: phloem surrounds a central core of xylem, no leaf gaps (in extinct eutracheophytes and living club mosses)
- siphonostele: central pith surrounded by vasculature (xylem and phloem), leaf gaps can be present (found in most seedless plants e.g. ferns)
- eustele: vascular cylinder comprises a ring of vascular strands around pith, leaf gaps are present (almost all seed plants)
Microphyll and megaphyll
Microphyll is a type of plant leaf with one single unbranched leaf vein (enation theory).
Megaphylls have multiple veins within leaf and leaf gaps above them in stem (telome theory).
Trends in eutracheophytes
- lignin adds rigidity to cells -> upright, taller plants
- tall sporophytes hav advantage of moving spores further by passive wind dispersal
- but have problem in moving water and photosynthates around, and minimising desiccation while gas exchange
Vascular plants are…
- haplo-diplontic with sporic meiosis
- heteromorphic (gamete is morphologically different - smaller and less complex; sporophyte large and more complex). Sporophyte is persistent and independent.
- oogamous: union of mobile male and immobile female (eggs are large, sperm are small)
Lycophytes examples and features
Spike mosses, club mosses and quillworts
- can be homosporic (spores of one kind, not differentiated by sex) or heterosporic (each spore is morphologically different)
- strobilus (contains reproductive structures): collection of sporophylls and associated sporangium on modified leaves (where spores are produced)
Ferns and their friends
*Note: Leptosporangiate ferns have developed ways to protect sporangia (e.g. sori and indusia) and to enhance spore dispersal.
Microspores and megaspores
Microspores are male spores that give rise to male gametophytes.
Megaspores are female spores that give rise to female gametophytes.
Two kinds of sporangia: eusporangium and leptosporangium
Spore producing structures in vascular plants formed on sporophyll (specialised leaf).
- Eusporangium (primitive): developed from GROUP of superficial initial cells, where outer layer produces sporangium wall and inner cells produce mother spore that will produce spores (nutritive cells called tapetum can be between those layers).
- Leptosporangium: develops from a SINGLE initial cell, stalked, thin walled (also has annulus - thick walled portion of sporangium wall).
Heterospory and endospory
Heterospory: male and female spores of different size and function (microspores and megaspores).
Endospory: gametophyte develops within the spore, not externally.
Developments and evolution of seed
- Note: heterospory is precursor to development of seed
- microspore is referred to as a pollen grain
- another cell in tube cell will elongate and carry it to the egg (sperm aren’t made until pollen tube is going down into egg)
How is the megaspore developed in seed plants? (e.g. conifers)
- 1 cell divides by meiosis
- 1 of 4 of those cells develops into megaspore ‘mother’ cell
- megaspore ‘mother’ cell divides by mitosis into female gametophyte
- female gametophyte nourished by nucellus (2n) megaspore wall
- several archegonia produced
The ovule of seed plants include…
- nucellus (megasporangium; central part of ovule containing embryo sac)
- megaspore wall
- Note: Eventually pollen will deliver sperm to female part, opening micropyle. Sperm has to get delivered to archegonia with egg in it.
How do conifers pollinate?
- pollen is released via wind dispersal and lands on ovule cone (sperm not flagellate)
- ovule produces pollination droplet (viscous liquid), once dehydrated, will draw pollen down into chamber (don’t need free water)
- pollen tubes start to grow down into female gametophyte (gametophyte only start developing once there is a chance of fertilisation)
- sperm will then get to egg
Advantages of the seed habit
- offers potential for mate choice
- male gametophyte is reduced in size and complexity
- male gametophyte protected by exine (pollen wall)
- sperm is not released until the gametophyte reaches the egg
- female gametophyte nourishment for developing sporophyte
Outline evolution of ovule up until gymnosperms (naked seed plants such as conifers)
- homosporous ancestor
- reduction to one megaspore and endospory
- sporangium enclosed by integuments
List five lineages of extant seed plant (including gymnosperms and angiosperms)
Gymnosperms (naked seed): - ginkgo - cycads - conifers - gnetophytes Angiosperms (enclsoed seeds): - flowering plants
Features of Cycadophyta
Cycads thought to be last survivors of Triassic and Jurassic plants that existed on Gondwana.
- leaves are pinnately or bipinnately compound
- male and female cones are produced on separate plants (dioecious)
- flagellate sperm
- coralloid roots (endosymbiotic cyanobacteria, N2 fixation)
Features of Ginkgophyta (only one genus and species, Ginkgo biloba)
- leaves, wood, ect of living plants are identical to fossil Ginkgo
- separate male and female parts (dioecious)
- pollen borne in axillary spike-like clusters of sporangiophores that look like stamens
- usually two naked ovules on tip of forked peduncle (stalk bearing fruit or flower)
- flagellate sperm
Features of Pinophyta
- monoecious or dioecious
- pollen often have sack-like appendages
- microsporangia on lower surface of cone-scales (microsporophylls)
- receptive ovules on upper surface of cone scales (megasporangia)
- sperm not flagellate
Features of Gnetophyta
- monophyletic (molecular evidence)
- have many flowering plant-like structures:
- > strobili clustered like inflorescences
- > vessels (not tracheids) in xylem
- > lack of archegonia in some species
- > double fertilisation in some
- > simple, pinnately-veined leaves in Gnetum
Androecium and gynoecium (pistil)
Androecium (male parts) is the stamen, which consists of anthers and filaments.
Gynoecium (female parts) is the carpel, which consists of stigma, style and ovary.
*Note: number of pistil corresponds to number of carpels.
Combined reference for corrolla (petals) and calyx (sepals - protective outer layer outside petals)
Formation of microspores (pollen grains) inside the microsporangia (pollen sac) of seed plants. A pollen mother cell can produce 4 pollen grains via meiosis (which will grow via mitosis).
*Note: exine (strong material) is outermost wall of pollen grain, intine is innermost wall.
Explain double fertilisation in angiosperms
After penetrating the degenerated synergid cell, pollen tube releases 2 sperm into embryo sac, where one fuses with egg (zygote) and the other fuses with two polar nuclei of central cell (forms endosperm nucleus).
- zygote has full chromosomes (diploid)
- endosperm nucleus is triploid (divides mitotically to form endosperm of seed as food-storage tissue)
List 3 early angiosperm lineages
Two mating strategies applied by angiosperms to reduce sexual interference
Strategies that reduce self pollination (goes on neighbouring flower)
Herkogamy: spatial separation of anther and stigma function
Dichogamy: temporal separation of anther and stigma (protandry - stamens mature first; protogyny - stigmas mature first)