Leaves

  • plant leaves evolved independently several times

    • simple forms evolved during the early Devonian Period

      • gave rise to lycophytes

    • other forms evolved during the late Devonian Period through the early Carboniferous Period, then diversified

      • gave rise to the euphyllophytes

  • two major types:

    • lycophylls (a.k.a microphylls)

      • lack leaf stalks, simple in form, typically featuring a single, non-braching vein

    • euphylls (a.k.a megaphylls)

      • highly diverse in form and venation

  • enation theory

    • microphylls evolved from flap-like outgrowths of stems called enations

      • stems developed enations, then leaf traces, and later fully became vascularized

  • telome theory

    • megaphylls evolved from branching systems

      • dichotomous branching became unequal giving rise to a dominant stem bearing branches (overtopping)

      • branching systems flattened (planation)

      • lamina (flat. sheets of tissue) developed between branches to form leaves (webbing)

Morphological Diversity

  • eaves are extremely diverse morphologically

  • some basic structures include:

    • blade/lamina - the often broad and flatt structure that houses veins and photosynthetic tissues

      • simple - one bladed

      • compound - 2+ leaflets

    • veins housing xylem/phloem

      • midrib in some

    • some plants have expanded leaf bases that wrap around the stem forming a sheath

    • petiole - the stalk attaching the blade to the stem at a node

      • leaves without petioles are said to be sessile

    • axil - the angle formed between the stem and the petiole (this is because the stem at the axil are called axillary buds

    • directional terms for leaves based on this landmark

      • proximal - towards the axil

      • distal - away from the axil

      • adaxial - upper surface of a leaf (facing the axil)

      • abaxial - lower surface of a leaf (not facing the axil)

  • margins of leaves: smooth, toothed, or lobed

  • compund leaves may have leaflets arising from a common point at the end of the petiole (pamate) or leaflets arranged around a central axis that is an extension of the petiole (pinnate)

  • venation may be parallel (monocots) or netted (dicots)

    • pinnately netted or palmately netted

  • leaves may arise from stems in alternate, opposite, or whorled arrangments

Leaf Tissues

Mesophytes

  • mesophytes - plants adapted to moderate environments

    • epidermis

      • lower epidermis (abaxial) and upper epidermis (adaxial)

      • pavement cells - typically translucent; plastids produce fatty acids that are secreted to leaf surfaces to form the cuticle (hydrocarbon waxes and cutin)

        • cuticle prevents water loss → usually thicker on the adaxial side (faces sun)

      • trichomes - defensive hair cells; ome glandular (secrete oil repellants); can help in collecting water/minerals (as in epiphytes)

      • stomatal guard cells - change chape to open/close pores (stomata)

        • allows for gas exchange

        • open when full of water (turgid), which happens as ions dissolve in the surrounds water → water moves in via osmosis

          • water build up creates turgor pressure → cells bend and opens pore

          • ions more actively accumulated during daylight and actively pumped out at night

        • subsidiary cells provide mechanical, hydraulic, and osmotic support

        • mostly found on the lower epidermis

        • bulliform cells in grasses allow for folding and unfolding leaves

          • when full of water (turgid), blades unfold

          • when not full of water (flaccid), blade fold up