The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll.

[11] Some window plants such as Fenestraria species and some Haworthia species such as Haworthia tesselata and Haworthia truncata are examples of xerophytes. Veins (sometimes referred to as nerves) constitute one of the more visible leaf traits or characteristics.

Their surfaces are waterproofed by the plant cuticle and gas exchange between the mesophyll cells and the atmosphere is controlled by minute (length and width measured in tens of µm) openings called stomata which open or close to regulate the rate exchange of carbon dioxide, oxygen, and water vapor into and out of the internal intercellular space system. In a typical leaf, the stomata are more numerous over the abaxial (lower) epidermis than the adaxial (upper) epidermis and are more numerous in plants from cooler climates.

A plant has whorled leaves when there are three or…. One such clue is the leaf arrangement on the stem, which is formally known as phyllotaxy in botany. The stomatal pores perforate the epidermis and are surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts, forming a specialized cell group known as the stomatal complex.

A stipule, present on the leaves of many dicotyledons, is an appendage on each side at the base of the petiole, resembling a small leaf.

Both the mechanics and architecture of the leaf reflect the need for transportation and support. The opening and closing of the stomatal aperture is controlled by the stomatal complex and regulates the exchange of gases and water vapor between the outside air and the interior of the leaf.

In some Acacia species, such as the koa tree (Acacia koa), the petioles are expanded or broadened and function like leaf blades; these are called phyllodes.
Many gymnosperms have thin needle-like or scale-like leaves that can be advantageous in cold climates with frequent snow and frost.

[14] Compound leaves are closer to shoots than simple leaves. True leaves or euphylls of larger size and with more complex venation did not become widespread in other groups until the Devonian period, by which time the carbon dioxide concentration in the atmosphere had dropped significantly.

In many aquatic species, the leaves are submerged in water. It is covered with a waxy cuticle which is impermeable to liquid water and water vapor and forms the boundary separating the plant's inner cells from the external world. [8] Green plants are autotrophic, meaning that they do not obtain food from other living things but instead create their own food by photosynthesis. [16] Other factors include the need to balance water loss at high temperature and low humidity against the need to absorb atmospheric carbon dioxide.

These tend to be in leaves with smooth outlines, and are characteristic of monocotyledons. Examples include the phyllids of mosses and liverworts.

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