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Plant Evolution The Emergence of Vascular Seed Plants by mikeholy

VIEWS: 12 PAGES: 5

									Plant Evolution: The Emergence of Vascular, Seed Plants
-    seed plants originated about 360 mya;
Seed
        = embryo + food supply surrounded by protective coat;
        key adaptation that propelled seed plants into becoming dominant producers on land
         WHY?
        enabled plants to better resist harsh environments, AND,
        disperse offspring more widely
        make up vast majority of plant biodiversity
        huge impact on human society; seed plants began to be cultivated for wheat, figs, maize,
         bananas, etc. ... lead to transformation of human society from roving hunters, to permanent
         settlers anchored by agriculture

-    gametophytes of seed plants are microscopic; Why is this NB?
-    b/c gametophytes could now be developed even within sporangia of parental sporophyte
-    benefits: 1. protects female gametophyte from UV radiation & from drying out etc.
               2. gametophytes obtain nutrients from sporophyte

Traits Derived in Seed Plants
1. Vascular Seed Plants are Heterosporous
2. Ovules & Egg Production
3. Pollen and Production of Sperm


1. Vascular Seed Plants are Heterosporous
     = 2 types of sporophylls that produce 2 kinds of spores...
                   Megasporophylls → megasporangia → megapsores → female gametophyte
                             (each megaspornagium has a single functional megaspore)
                                                         vs.
                    Microsporophylls → microsporangia → microspores → male gametophyte
                             (each microsporangium produces large #s of microspores)


2.    Ovules & Egg Production
-    megaspore develops within megasporangium within parent sporophyte
-    megasporangium is surrounded and protected by layers of tissue called “integuments”...
     gymnosperms have 1 integument vs. angiosperms have 2 integuments


     OVULE = megasporangium + integument(s) + megaspores
-    inside ovule... megaspore divides & develops into female gametophyte.. eventually 1 or 2 egg cells made
3. Pollen and Production of Sperm
-   Microspores → pollen grains → male gametophytes
-   b/c of sporopollenin covering, pollen grains can be carried away from parent plant by wind or pollinator
-   pollen grain germinating = pollen tube
                                 
          discharges 2 sperm into female gametophyte within ovule




RECALL:
Seedless Plants                Seed Plants
free living gametophytes       female gametophyte NEVER leaves
release..                      sporophyte ovule
flagellated sperm that         male gametophytes in pollen grains
                               can be carried for long distances
travel through film of water
over short distance (cm)
                               ** sperm of gymnosperm are
                               flagellated, BUT, not in angiosperms


                               ∴ do not require water or motility
                               since pollen is carried to pollen tube
                               & then, sperm transfer to ovule
                               occurs
GYMNOSPERMS
-   appeared ~ 360 mya
-   warmer and drier climates favoured spread of gymnosperms
-   on land, amphibian diversity declined while reptilian diversity increased
-   served as food source for many giant herbivorous dinosaurs
-   gymnosperms exploded because of adaptations that met challenges of
    conserving water

    1. needle-shaped leaves
    2. thick cuticles
    3. small surface area

    General Features
-   seeds exposed on sporophylls that form cones (strobili)... vs. angiosperms, in which seeds are enclosed in
    fruits (i.e. mature ovaries)
-   e.g. conifers = cone-bearing trees like pine, redwoord, fir

4 phyla:

           1. Cycadophyta (thrived in Age of Dinosaurs)

           2. Ginkgophyta

           3. Gnetophyta

           4. Coniferphyta (most well known; largest group)



A Case-Study Look at the Life Cycle of the Pine Tree
   pine tree is sporophyte
   scale-like structure are packed together into cones
   on these scale-like structure are the sporangia
   heterosporous... ∴ separate cones with separate sporangia produce different spores & thus, different
    gametophytes:

a) small pollen cones.. the spore mother cells undergo meiosis = microspores (n)
                                      
      each microspore produces pollen grain with male gametophyte

b) large ovulate cones ... spore mother cells undergo meiosis & produce megaspores (n)
                                       
   each megaspore produces female gametophyte within megasproangium

NOTE:
   takes 3 years for male & female gametophytes to be: i) produced ii) brought together iii) fertilized ovule to
    results and iv) mature seeds to form
                                                          
scales of each ovulate cone separate & seeds travel on wind; if it lands on habitable place, it germinates &
emerges as pine seedling
ANGIOSPERMS
   MOST diverse and widespread of all plants; > 250 000 species (90% of all plant species)

Flower = specialized for sexual reproduction;
        have evolved to promote directed pollination via pollinators (compared to wind pollination)

Sepals = green, enclose flower before it opens; above sepal are petals
Petals = brightly coloured; aid in pollination
2 types of sporophylls are:
1. Stamens = microsporophylls that produce microscopes → pollen grains with male gametophyte
                   = consists of stalk called filament & anther
2. Carpels = megasporophylls that produce megaspores → female gametophyte
            = many angiosperms have flowers with multiple carpels
            = tip consists of sticky stigma that receives pollen; style leads to ovary at base of carpel
            = each carpel contains 1 or more ovules (upon fertilization, ovule becomes seed)
Receptacle = part of stem that is attached to: sepals + petals + stamens + carpels
Fruit = a mature ovary that protects dormant seeds & aid in dispersal
How does a fruit develop?
                                   ovule fertilized
                                            
                                   seed develops
                                            
                                   wall of ovary thickens in response to hormonal changes

                                 becomes pericarp = thickened wall of fruit

meanwhile, other flower parts wither away
(IF POLLINATION DOES NOT OCCUR.... fruit does not develop; entire fruit withers & falls)
Mature fruits can be fleshy or dry like beans, nuts, grains.

Angiosperm Life Cycle

   each pollen grain has 2 haploid cells:
    i) generative cell that divides & forms 2 sperm
    ii) tube cell that produces pollen tube

 each ovule contains female gametophyte = “embryo sac”
 embryo sac consists of only a few cells & one of them is the egg
Process of Pollination
 cross pollination enhances genetic variation
 self-pollination is sometimes not possible because of flowers may mature at different times and are
   arranged in physical arrangement that does not support self-pollination

                            pollen from anther carried to sticky stigma at tip of carpel
                                                        
                                 pollen grain germinates upon adhering to stigma
                                                        
               mature male gametophyte extends pollen tube as it grows down style & reaches ovary
                                                        
                         pollen tube penetrates micropyle, pore in integuments of ovule
                                                       
                    2 sperm cells are discharged into female gametophyte (embryo sac)
                                                       
                                  1st sperm fertilizes egg = diploid zygote
                  2nd sperm fuses with 2 nuclei in large central cell of female gametophyte

                    ∴ double fertilization takes place!!! This is unique to angiosperms.
                                                      
                                           ovule matures into seed
                                                      
 Zygote develops into sporophyte embryo with rudimentary root & 1 or 2 seed leaves called cotyledons;
nucleus of central cell of female gametophyte divides repeatedly & develops into endosperm = tissue rich
                                      in starch & other food reserves.

SEED = EMBRYO + ENDOSPERM + REMNANTS OF SPORANGIUM + SEED COAT FROM INTEGUMENT

As an ovaries’ ovules develop into seeds, it matures into fruit.

After being dispersed, seed may germinate if environmental conditions are favourable!
                                              
                                     Seed coat ruptures
                                              
                 Embryo emerges as seedling using food stored in endosperm & cotyledons

								
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