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Articles published in our open access journals are peer-reviewed and made free for everyone to read and download. Permitted reuse is defined by the authors’ choice of user license. Please forward this error screen to 216. Diagram of a simple leaf. Furthermore, several kinds of leaf-like structures found in vascular plants are not totally homologous with them. File:3D rendering of a micro CT scan of a piece of dried leaf. Leaves are the most important organs of most vascular plants.
The plant must therefore bring these three ingredients together in the leaf for photosynthesis to take place. The phloem and xylem are parallel to each other but the transport of materials is usually in opposite directions. They are arranged on the plant so as to expose their surfaces to light as efficiently as possible without shading each other, but there are many exceptions and complications. Many gymnosperms have thin needle-like or scale-like leaves that can be advantageous in cold climates with frequent snow and frost. Vein skeleton of a leaf. Veins contain lignin that make them harder to degrade for microorganisms.
In any square centimeter of a plant leaf there may be from 1,000 to 100,000 stomata. Other factors include the need to balance water loss at high temperature and low humidity against the need to absorb atmospheric carbon dioxide. In either case the shed leaves may be expected to contribute their retained nutrients to the soil where they fall. Not every species produces leaves with all of these structural components.
The petiole mechanically links the leaf to the plant and provides the route for transfer of water and sugars to and from the leaf. The lamina is typically the location of the majority of photosynthesis. Structures located there are called “axillary”. Leaves almost always have determinate growth. They grow to a specific pattern and shape and then stop. Other plant parts like stems or roots have non-determinate growth, and will usually continue to grow as long as they have the resources to do so. Note the developing buds in the axils of these leaves.
One leaf, branch, or flower part attaches at each point or node on the stem, and leaves alternate direction, to a greater or lesser degree, along the stem. Arising from the base of the stem. Arising from the aerial stem. Two leaves, branches, or flower parts attach at each point or node on the stem. Three or more leaves, branches, or flower parts attach at each point or node on the stem. Opposite leaves may appear whorled near the tip of the stem.
Leaves in this arrangement may be alternate or opposite in their attachment. 8 leaves in 3 gyres. The leaflets may have petiolules and stipels, the equivalents of the petioles and stipules of leaves. Because each leaflet can appear to be a simple leaf, it is important to recognize where the petiole occurs to identify a compound leaf. Leaves have the leaflets arranged along the main or mid-vein. Leaves are twice divided: the leaflets are arranged along a secondary vein that is one of several branching off the rachis.
Subpetiolate leaves are nearly petiolate or have an extremely short petiole and may appear to be sessile. In peltate leaves, the petiole attaches to the blade inside the blade margin. There may or may not be normal pinnate leaves at the tip of the phyllode. The situation, arrangement, and structure of the stipules is called the “stipulation”. They also play a role in the mechanical support of the leaf.
The vein or veins entering the leaf from the petiole are called primary or first order veins. The veins branching from these are secondary or second order veins. These primary and secondary veins are considered major veins or lower order veins, though some authors include third order. Each subsequent branching is sequentially numbered, and these are the higher order veins, each branching being associated with a narrower vein diameter. Usually many smaller minor veins interconnect these primary veins, but may terminate with very fine vein endings in the mesophyll.
Minor veins are more typical of angiosperms, which may have as many as four higher orders. The midrib then branches to a number of smaller secondary veins, also known as second order veins, that extend toward the leaf margins. These minor veins act as the sites of exchange between the mesophyll and the plant’s vascular system. At the same time water is being transported in the opposite direction.