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Leaf is a part of the plant, which is connected to the branches and carries out photosynthesis. Plants possessing true leaves are called the vascular plants, whereas those just possessing structures like leaf are the non-vascular plants. Vascular plants contain two types of tissues, they are xylem and phloem, which is absent in non-vascular plants as they lack true leaves.
Vascular flowering plants like angiosperms and seed plant gymnosperms both possess xylem and phloem in their leaves. Thus, the vascular plants contain true leaf, other than adaptations and various characteristic to carry out photosynthesis.
Due to the presence of the pigment chlorophyll, leaf appears green in color. This chlorophyll is a photosynthetic pigment which captivates light energy and converts it into chemical energy. This chlorophyll in plants is preset in an organelle called the chloroplast.
Except the chlorophyll, there are other pigments as well like the carotenoids. These are the accessory pigments because they harvest light and then pass it to chlorophyll.
Pigments are also found in plastids, which are not green in color, but their color is camouflaged by the chlorophyll as its present in larger amount and are called the chromoplast. When the leaf is about to die, the carotenoid color becomes visible, as the chlorophyll has died or is on the verge of getting degraded.
The parts of the leaves are the blade also known as the lamina, which rests or lies on the petiole or the leaf stalk and when continued further from the blade consists of a midrib. On either side of the base, petiole consist of appendages, which are known as stipule. The leaf is coated with a thin layer of epidermis on which are tiny openings, which are not visible to the eye but are called as stomata.
Leaves are usually flat and wide and have quite less thickness. They grow laterally and these characteristics of leaf helps them to gather as much light possible from the sun. These are the modifications made by some plants, however other plants to thrive have undergone other adaptations.
For example, the cactus leaves have been adapted to spines to prevent water loss and to protect from attackers. Thus, being able to survive in extremely humid conditions. Conifers possess leaves in the form of needles, to protect from snowy as well as changing climates.