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Leaves (External Structure of Foliage Leaves (sheathing leaf base…

- - - - light harvesting portion is called leaf blade
    - - also called the dorsal surface, large veins abaxial side, upper side ventral surface
  - - - long, thin flexible petioles allow blade to flutter in wind, cooling and bringing fresh air
        
        if there's no air, CO2 is absorbed from the vicinity of the leaf faster, which depletes CO2 and decreases photosynthesis
      - leaf flutters also make it difficult for insects to land on leaf and knock those that have already landed
  - - - perpendicular to the petiole, cells involved in cutting off the leaf when its useful life is over.
- - - - water lost through epidermis is called transpiration
      - a serious problem if the soil is to dry and roots cannot replace lost water
    - - large percentage of flat tabular ordinary epidermis cells
      - guard cells and trichomes, may be abundant
      - leaf epidermises are often remarkably hairy, trichomes can affect leaf biology
        
        provide shad on upper surface but prevent rapid air movement and slow water loss from stomata #
        
        makes it difficult for insects to walk on plant, sometimes makes them vulnerable
        
        can even recreate stinging compounds which prevents animals from eating
        
        if large enough can protect small insects from predators so they can reproduce on leaf
      - like stem, leaf epidermis contain a coating of cutin and usually wax on their outer walls
    - - sunny day, leaf is warmer then surrounding air, thus heats air and convection currents rise
      - if stomata in upper epidermis are open and loosing water, the water molecules are swept away by convection
      - however, air tends to be trapped on the underside of the leaf, so water lose from stomata there is not significant
      - water molecules are trapped in quiet air and may diffuse back to stomata, stomata is greater in lower epidermis than upper.
      - stomata being concentrated on bottom has its benefits,fungi, and bacteria typically land on the top of the leaf
        
        leaf stomata are usually sunken into epidermal cavities, creates a region of nonmoving air
  - - - main photosynthetic tissue of plants
      - cells are separated slightly so that each cell has most of its surface exposed to intercellular spaces
      - cells being separated helps with CO2 absorption
      - typically only one layer thick but it places with intense sunlight can be multiple layers thick
    - - open loose aerenchyma that permits CO2 to diffuse rapidly away from stomata
    - - leaves held horizontally(most leaves) sun is overhead
        
        palisade parenchyma near the upper surface permits max absorption of light and photosynthesis
      - leaves held vertically, both sides are equally illuminated, and palisade parenchyma is functional on both sides
  - - - an eudicot leaf usually has one large midrib, lateral veins emerge into narrow minor veins
      - minor veins are important for releasing water from xylem and loading sugar into phloem
      - midrib and lateral veins are involved in conduction
      - vein structure size varies
        
        midrib and lateral veins are both primary xylem on upper side, and phloem on lower side
        
        because they both conduct and support leaf blade, they may have fibers arranged as a sheath
        
        a bundle sheath is around the vascular tissue, makes it hard for insects to chew on vascular tissue
        
        minor veins are sites of material exchange and must have a large surface area in contact with palisade and spongy mesophyll
        
        they do not contain fibers or other nonconducting cells
        
        some endings consist of only xylem and in others only phloem, but typically have both
    - - help give rigidity to blades
      - believed to provide an additional means by which water moves from bundle out to mesophyll #
  - - - arrangement of tissues differs at the two ends
        
        epidermis contains fewer stomata and trichomes then lamina
        
        petiole mesophyll is rather like cortex, somewhat compact, not especially aerenchymatous
        
        considerable collenchyma is present if petiole supports a heavy lamina
      - Vascular tissue are the most variable
        
        one, three, five or more vascular bundles are called leaf traces
        
        branch from stem vascular bundles and diverge towards the petiole
        
        may remain distinct or they fuse into a single trace near or in petiole
      - in many species, petiole bears two small flaps of tissue at its base called stipules
        
        they may protect the shoot apical meristem
        
        contribute a significant amount of photosynthesis in some plants
        
        usually when leaf is mature they are very small and die early
- - - - some leaves are cylindrical or even spherical
      - spherical is the optimal surface-to-volume shape
      - reduction in surface area is advantageous for water retention
      - has an automatic consequence of reducing the capacity of CO2 uptake
    - - reduces internal evaporative surface area and water loss through stomata
      - lack of air spaces, makes mesophyll more transparent, allowing light to penetrate further
      - photosynthesis occurs more deeply then foliage leaves
  - - - live more then 2 years
      - extended lifetime and prolonged productivity
      - result makes leaf hard, which makes it more resistant to animals, fungi, freezing temps, and UV light
      - such plants are sclerophyllous and leaves are sclerophylls
        
        sclerenchyma is often present as a layer just below epidermis and in bundle sheaths
        
        cuticle is usually very thick and waxes are abundant
    - - they have thick cuticles, epidermis and hypodermic cells have thick walls
      - leaves are always simple, have only few forms
      - most leaves are perennial, consequently the plants are evergreens
  - - - in perennial plants, dominant shoot apical meristems are protected from low temps and drying wind
      - bud scales protect these by forming a tight layer around stem tip
        
        are tougher and waxer then regular leaves, can produce a thin layer of corky bark
      - are small and rarly compound
      - petiole is either very small or absent because the bud scale must remain close to stem
    - - spines are their axillary buds
      - are needle shaped, inhibit lamina formation
      - no mesophyll parenchyma or vascular tissue, instead has closely packed fibers
        
        after fiber mature, they deposit lignin, which makes them hard and resistant to decay
  - - - they grow indefinitely and contain cells that are capable of sensing contact
      - when they touch something the side facing the object stops growing and the opposite side continues to grow
      - they use the object for support as it continues to elongate
    - - this happens because the habitats lack nitrogen and ammonia
      - although the leaf appears highly modified it is similar to foliage leaves
        
        it is thin, parenchymatous, and capable of photosynthesis
        
        it has numerous stomata and vascular bundles, and mesophyll that contains aerenchyma and chlorenchyma
        
        most significant difference is the lamina is tubular not flat
        
        epidermis must be absorptive, and throat of the pitcher contains numerous trichomes
- - - - extends upward as a narrow cone,
      - grows rapidly that it becomes taller than the shoot apical meristem
      - the primordium consist of leaf protoderm and leaf ground meristem
      - all cells are meristematic, with dense cytoplasm and small vacuoles
      - a strand of cells in the center differentiates into pro vascular tissue and then primary xylem and phloem
      - form a connection with the young bundles in the stem
      - as it grows upwards it increases in thickness, establishing the midrib
      - row of cells grow outwards initiating the lamina, resulting in a young leaf with a midrib and two small wings
        
        all cells in wings are meristematic, their division and expansion enlarge the lamina rapidly
        
        during lamina expansion, stomata, trichomes and vascular bundles differentiate, petiole becomes distinct from midrib
  - - - outer surface of tube is abaxial epidermis, inner is adaxial epidermis
      - shoot apex enlarges, forms new stem tissue and next leaf primordium
        
        which will develop inside the previous leafs sheathing tube
    - - the lamina grows a meristem located at its base, which attaches to the top of sheathing leaf base
      - the lamina meristematic cells remain active mitotically