bot 119- evolution of plant body

• The plant's complexity is owed to its evolution from algae ancestors.
• Plants are algae, bryophytes, angiosperms, and gymnosperms.
• The unique properties of plants are they contain chlorophyll, primary producer, and alternation of generation.
• Charophytes are evolutionary link beetwen simple green algae and more highly specialized plants.
• The characteristics of Charophytes are:

  
  

  1. Large-sized green algae
  2. Appear similar to land plants
  3. Stalked, leaf-like projections attached to the substrate
• Thallus or frond is not a true stem.
• Rhizoids are for anchorage but do not function for absorption,
• The structures of Charophytes are not specialized to perform different functions.
• The algae have stem and leafy branches that photosynthesize and absorb nutrients.
• Advantages and Disadvantages of Aquatic Environment:

  1. Water filters light
  2. Nutrients are readily available and absorbed
  3. Carbon dioxide not as plentiful and diffuses slower
  4. Offers mechanical support
• Water have buoyancy and turgor pressure for support.
• Advantages and Disadvantages of Terrestrial Environment:

  1. Unfiltered sunlight
  2. More carbon dioxide
  3. Little competition
  4. Few herbivores and pathogens
  5. Nutrient-rich soil
• Detriments of moving on land:

  1. Water loss
  2. No mechanisms for gas exchange, absorption and anchorage and transport of nutrients
  3. No structural support provided by the substrate (land and air)
  4. Mechanism for photosynthesis needed to be modified
• Water loss

  1. 3D rod like plant body decreases the area from which water can move out
  2. Waxy cuticle helps retain water due to its chemistry
  3. Sporopollenin is a very stable material resistant to chemical degradation by organic and inorganic chemicals
  4. Trichomes are hair-like structures that trap moisture
• Sporopollenin composes the tough outer covering of pollen grains and spores.
• Gas exchange

  1. Stomata and air pores aided in gas exchange
• Stomata- higher forms, guard cells

  Air pores- hole in surface, lower plants
• Absorption and anchorage

  1. Rhizoids (lower) aided in anchorage
  2. Roots (higher) are later forms of rhizoids that also function for absorption
• Transport (vascular)

  1. Xylem transport water
  2. Phloem transport photosynthates
• Transport (non-vascular)

  1. Hydroids transport water
  2. Leptoids transport photosynthates
• Structural

  1. Epidermis and turgor pressure
  2. Periderm
  3. Xylem
• Epidermis is the outer covering of the primary plant body composed of compactly arranged living cells.
• High turgor pressure presses the cell membrane to make the cell rigid.
• Periderm is the covering of the secondary plant body.
• Xylem have lignified cells that function for support.
• Photosynthesis

  1. Leaves are the primary photosynthetic organs of plants
• These adaptations would not have been possible if land plants were not multicellular.
• Multicellularity is not a pre-requisite for complex morphology.
• Compartmentalization is achieved through the presence of cell wall.
• Compartmentalization leads to physiological (functional) specialization since each compartment performs its own certain function.
• An organism can have an elaborate morphology and still be a unicellular organism (Ex. Caulerpa, Bryopsis)
• Cell theory states that multicellular plants evolved through the aggregation of unicellular organisms.
• Organismal Theory states that multicellular plants evolved through the subdivision of unicellular organisms.
• Plants needed to go on land because Charophytes are found in shallow aquatic environment and it occasionally dries out.
• Evidences of Cell Theory: Volvox and Pandorina (colonial algae)
• Evidences of Organismal Theory:

  1. Cuticle covering of zygote to prevent aggregation
  2. Nature of plant cell division (divide by forming metaphase plate)
  3. Plasmodesmata and pitfields