First stage General plant Dr. Huda Altameme Lab -7- The Leaf A leaf is an aerial and lateral outgrowth of the stem of a usually flat and dorsiventral anatomy. It functions mainly to manufacture food by photosynthesis and consists typically of a stalk also called petiole, a flattened blade, the lamina, and the leaf base. Strands of conducting and strengthening tissues, the veins, run through it. Their pattern, also called leaf venation, is a feature of characterization. Leaves may be simple, i.e. undivided or compound (composed of several parts called leaflets). The blade margin and the leaf arrangement at the stem are further features of characterization. Simple Leaves :- Simple leaves have one leaf blade. The leaf may be irregularly shaped, but it is still one blade with one leaf stalk. Compound Leaf :- Compound leaves are made of many individual leaflets that are all attached to the same leaf stalk Modified Leaves :- Some leaves have many unusual shapes and sizes. This allows them to adapt to protect the plant, to store food, or to adapt to the environment in which they grow. Leaf Arrangement Alternate:- In an alternate arrangement, single leaves are attached spirally along the stem. Opposite:- Two leaves emerge opposite each other on a stem. Whorled:- In a whorled arrangement, several leaves emerge together around a stem node. Leaf Tissues The leaf blade’s outer surface is epidermal tissue and the ground tissue within is photosynthetic mesophyll, interspersed with “veins” (vascular bundles) of conducting (vascular) tissue. Vascular connections between blade and stem pass through the petiole. Dicot Leaf Epidermis:- There may be one or more layers of epidermal cells with cutinized outer walls that form a waterproof outer surface. Stomata of guard cells are present on the upper and lower surface or only on the lower surface. Various hairs and glands may also be present . Ground Tissue:- The parenchyma cells of the ground tissue are divided into palisade and spongy mesophyll. Usually the columnar cells of the palisade layer lie under the upper leaf epidermis and contain most of the chloroplasts of the ground tissue. The spongy parenchyma has irregular-shaped cells with air spaces between and lies above the lower leaf epidermis. Vascular Tissue:- Four small vein transections are shown which are part of the vast, interconnecting network between major veins. A leaf vein (vascular bundle) usually has xylem on the upper side and phloem on the lower side. The reverse may be true in major veins. A layer of parenchyma cells called a bundle sheath , which functions in First stage General plant Dr. Huda Altameme moving materials between the vein and the mesophyll tissue, usually encloses small veins. Monocot Leaf (Grass) Epidermis:- Along with the epidermal cells , guard cells , hairs, silica, and cork cells, grass leaves have bulliform cells . The bubble-shaped, water-filled bulliform cells control leaf rolling (when the leaf is dry) and unrolling (when the leaf is turgid or filled with water). Under drought conditions, bulliform cells collapse and the leaf rolls up. Ground Tissue:- In the temperate region, grass mesophyll consists of only spongy parenchyma tissue whose cells contain chloroplasts—sites of photosynthesis. Vascular Tissue:- Grass leaves have bundle sheaths surrounding each vein. Sclerenchyma fiber strands may be associated with the veins. Conifer Leaf Epidermis:- The usually small, triangular or flat “needle” leaves of conifers are adapted to dry conditions. Epidermal cells have thick walls of cuticle, and sunken stomatal guard cells overlapped on the surface by subsidiary cells . Stomata may be on one or all sides of the leaf surface. Under the epidermis may be a layer of thick- walled sclerenchyma cells called hypodermis . Ground Tissue. Mesophyll tissue is not usually differentiated into palisade and spongy layers. Resin ducts are present. Vascular Tissue. There are one or two veins in the center surrounded by transfusion tissue and endodermis containing tannins and resins. Practical parts: 1- Observe the characters of leaves then draw the simple , compound and modified leaves in Nerium, Rosa and Allium respectively. Nerium Rosa Allium First stage General plant Dr. Huda Altameme 2- Observe the Leaf Arrangement in stem and draw it . Alternate Opposite Whorled 3- Examine the permanent slide of Corn (Zea mays) stem in 10x magnification, Showing : cuticle, upper epidermis, mesophyll, vein (or vascular bundle), xylem, phloem, bundle sheath cells, lower epidermis, guard cells, bulliform cells. First stage General plant Dr. Huda Altameme 4- Examine the permanent slide of Helianthus or Ligustrum stem in 10x magnification. You should be able to recognize: cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, vein (or vascular bundle of a leaf), xylem, phloem, bundle sheath cells, lower epidermis, guard cells. 5- Examine the permanent slide of Pinus (Conifer Leaf) in 10x magnification. You should be able to recognize: Epidermis , Stomata, Resin canal and Vascular bundle. .