Lecture 8 (pt1) - Spore-bearing vascular plants: lycophytes&monilophytes

Question 1

Vascular System of Plants

Answer 1

1. Sieve elements are the conducting cells of phloem (food transport), and have soft walls that collapse when they die and do not preserve well in fossils
2. Tracheary elements are the conducting cells of xylem (water transport and rigid) and consist of tracheids and maybe vessels
   - have secondary cell wall layers strengthened by lignin -> preserve well after death

Question 2

construction of vascular systems

Answer 2

• Located in a central cylinder = stele (sometimes surrounding pith)
  
  • Protostele (lycophyta), siphonostele (monilophyta), eustele (almost all seed plants)
    ○ Vascular system not the same in all plants, evolved from single system
    - Oldest plant lineage have the oldest simplest system

Question 3

lignin

Answer 3

• Most complex natural polymer known
  
  • Ester-linked and cross-linked polymer of p-hydroxycinnamyl alcohols
  • “gluing” together cellulose fibers = rigidity
  • Rigid and highly decay-resistant
  • Decomposition resistant
    ○ Done using non enzymatically - peroxidase enzyme to create hydrogen peroxide
    
    • Chemical structures similar to those in wood

Question 4

Tracheids (xylem)

Answer 4

• Slender with tapering ends
  
  • Ring like (annular) or spiral (helical) thickenings
  • May or may not have pits, but have no perforations fund in vessels (holes through both primary and secondary cell walls)
• Tracheids came first from late Silurian-Devonian (ie. Cooksonia)

Question 5

Vessels

Answer 5

• Tubular with angular end-plates containing perforations for continuous vertical connection between cells
  
  • Vessels may have annular and helical (etc.) thickenings on their inner walls
  • Vessels are known only from the angiosperms and Gnetophyta
• More efficient than tracheids

Question 6

Lycophyte (clubmosses)

Answer 6

• late Silurian-Devonian (414-380 MYA, ex. Drepanophycus and Baragwanathia)
  
  • microphylls and axillary sporangia
  • Lycopods became trees 390-290 MYA and were the dominant components of Carboniferous vegetation (340 MYA)
    ○ Really ecologically important 300 MYA and dominant but have now disappeared and are uncommon
  • Tree forms disappearing during Permian and replaced by Lycopodium (200 spp.), Selaginella (700 spp.) and Isoetes
• Formed basis of a lot of coal

Question 7

Lycophyta

Answer 7

• 10-15 living genera
  
  • 1000 described species ww
  • In Carboniferous, many more genera and this phylum was one of the dominant components of vegetation; not any more
  • protostele, microphylls with axillary sporangia, a branching rhizome (underground stem) and true roots (not rhizoids)
  • Photosynthetic parts are tiny micro scales = microphylls = tiny leaves
    ○ Not a true leaf
    ○ Microphylls ancestral state
  • Rhizome -> underground stem that can have buds and branch off new stems that come up and have true roots that non vascular byrophytes did not have they have rhizoids instead
  • Oldest vascular plant living have roots but the oldest non vascular did not
    = Lycopodiophyta

Question 8

Lycophyte reproduction

Answer 8

• Sporophytes (2n)
  
  • In Lycopodiaceae and Selaginellaceae the sporangia are located in upper axils of modified microphylls called sporophylls (clustered at the ends of branches as “cones” called strobili)
    ○ Axil = armpit of plant
    ○ Sporophylls = modified leaves
• In Isoetaceae, in axils of microphylls on the corm (bottom of plant)

Question 9

Family Lycopodiaceae

Answer 9

• one type of spore is produced (homosporous) -> 1n
  
  • Gametophytes are bisexual (but usually outcrossing) and slow to develop (6-15 years to produce antheridia and archegonia)
    ○ Outcrossing = sperm of one gametophytes are not compatible with the eggs on itself so has to swim
  • Fertilization is aquatic (sperm swim from antheridium to archegonium)
  • Main genus is Lycopodium (and its segregates), the club mosses
• Aquatic live in wet places because their sperm has to swim through the soil to get to the next archegonia

Question 10

life cycle of Lycopodiaceae

Answer 10

• Sporangium (2n) -> meiosis -> spores (1n) -> bisexual gametophyte (1n)
  ○ Archegonia -> egg (1n) -> immature archegonium -> mature archegonium
  ○ Antheridia -> spermatogenous tissue (1n) -> immature antheridium -> mature antheridium
• Sperm (1n) -> reaching mature sites -> fertilization -> zygote (2n) -> young embryo -> young sporophyte -> mature sporophyte (2n)

Question 11

Family Selaginellaceae

Answer 11

• 2 types of spores (heterosporous)
  
  • Microspores (small) producing male gametophytes
    ○ Microgametophytes (male gametophytes) develop within the microspores
    ○ But liberate sperm that must swim (aquatic fert.) to the archegonium of the female gametophyte
  • Megaspores (big) producing female gametophytes
    ○ Small
    ○ Non-photosynthetic
    ○ Dependent on food reserves of the megaspore
  • Only genus is Selaginella - includes tropical “resurrection plant” and temperate “spike mosses”

Question 12

life cycle of Selaginellaceae

Answer 12

• Only difference is that it produces either micro or mega spores (n), rest of cycle is the same as Lycopodiaceae
  
  • Mature sporophyte (2n) -> meiosis -> microspores (n) or megaspore (n)
    ○ Microspore (n) -> immature microgametophyte inside microspore wall -> spermatogenous tissue (n) -> mature microgametophyte (n) -> sperm (n)
    ○ Megaspore (n) -> megagametophyte protruding from megaspore wall -> mature megagametophyte inside megaspore wall (n) -> immature archegonium -> egg (n) -> mature archegonium
    The rest is the same as Lycopodiaceae

Question 13

Family Isoetaceae

Answer 13

• One genus Isoetes remains, consists of aquatic or semi-aquatic plants (muddy bottoms of lake margins)
  
  • roots, microphylls and heterosporous, but instead of stem, have a basal swelling = “corm” where specialized cambium produces secondary growth (different from gymnosperms)
    ○ Corm = short “stem”, swollen, that produces leaves without growing up
  • Evolutionary interesting but minor group, not economically or ecologically important
    ○ Nearest living relatives of tree lycophytes
    ○ Secondary growth
• CAM photosynthesis in some species (advanced mechanism that some plants in hot and dry environments deal with stress)