Leaf Morphology: Structure, Development, and Types
Leaf Structure: Stem body, limited growth, dorsiventral symmetry, specialization to perform photosynthesis.
Leaf Parts
Pod: Widening rather than union with the stem. Petiole: Called up, leaf blade joins the stem when present. Lamina (Limbo Foliar): Simple leaf (pulvinus motor, rachis, rachilla, leaflets or pinnae, foliólulos or pinnules).
Foliar Succession (Phyllome)
Used to refer to all types of leaves of a plant:
- Cataphylls (Scales): Appear in the winter buds of underground stems, called Perula together.
- Cotyledon: Embryonic leaves (scutellum in Liliopsida).
- Eophylls: First pair of juvenile leaves that appear after the cotyledons.
- Phyllodes: Base form of leaf lamina.
- Hypsophylls: Bracts accompanying red flowers.
- Metaphylls: Nomophylls, dry broadleaf; leaves typical of the adult plant.
- Prophylls: First leaves of a lateral branch, usually small (one leaf per node in Liliopsida, two leaves in Magnoliopsida).
- Protophylls: Corresponding to the juvenile leaves of the plant.
Foliar Development
Generally determined except in Meliaceae.
Steps:
The position of the initial divisions of phyllotaxis depends on the stem and leaf size.
Training Extrusion:
Periclinal divisions (one or more subepidermal layers), then anticlinal divisions are performed.
Formation of the Leaf Shaft:
The apical meristem elongates and produces a conical leaf axis. In species with sheathing leaves, the divisions extend to the sides. The lateral expansion of the leaf base originates spicules or leaf sheath. In Poaceae, the ligule forms a division by divisions of the protodermis periclinares.
Formation of the Leaf Blade:
Through the activity of marginal meristems located on both sides of the leaf base, which is about two cells deep. In petiolate leaves, the petiole arises between the lamina and leaf base. Often the midrib and petiole increase in thickness in the adaxial direction due to the adaxial meristem.
Types of Development:
- Homoblastic: The shape of the leaf remains uniform during its ontogeny.
- Heteroblastic: The leaf shape changes in a single plant.
Leaf Types:
- Unifacial Leaves: Radially extending both photosynthetic and vascular tissue.
- Bifacial Leaves: Starts with anticlinal elongation of the palisade parenchyma cells. The spongy parenchyma also has anticlinal divisions, but with a lower frequency.
Vascular Tissue:
Begins with the differentiation of procambium in the bud, which differentiates acropetally. The lateral nerves originate from the derivatives of the marginal meristem; minor veins occur during the growth period. Interlayer interconnections between veins sometimes establish bidirectional differentiation.
Pinophyta Leaf Structure
Variables in structure, most are evergreen and the leaves remain attached to the branches. In cross-section, the center is crossed by one or two vascular bundles, surrounded by a special fabric called transfusion tissue, a thick endodermis. Outside the endodermis is the hypodermis, protected by a very cutinized epidermis. Stomata in longitudinal lines are parallel to the vascular sheath and are sunken. There is no differentiation between palisade and spongy parenchyma. It contains resin ducts in the mesophyll.
Magnoliopsida Leaf Structure
Presents adaxial and abaxial epidermis, mesophyll, and different types of vascular tissue distributed throughout the leaf, petiole, and midrib system of veins. The larger veins are surrounded by parenchyma cells with few chloroplasts, extending to the termination of the ribs. There are also sheets or bands of cells extending into the epidermis, called sheath extensions and may contain lignin or suberin. The minor veins occur in the mesophyll. Both the sheath and the extensions would be involved in the conduction of water and solutes.
Petiole Structure
Has continuous epidermis, parenchyma chlorophyll supporting tissue is composed of collenchyma, sclerenchyma, or both. The vascular bundles may be collateral or concentric bicolaterales. The vascular tissue in the petiole differs among plant groups. In cross-section, it may appear as a crescent (Nicotiana nerium), as a continuous ring (Ricinus communis), or as a ring with additional external and internal beams (Vitis, Pelargonium). If only one side makes the phloem, it is located at the abaxial face when the vascular bundles are arranged in a ring with external phloem.