The protection tissues cover the surface of the plant organs, and are in contact with the environment. There are two main protection tissues: epidermis in organs with primary growth and periderm in organs with secondary growth.
1. Epidermis
The epidermis is the outer layer of plant organs showing primary growth, except for meristems. Although protection is a main function of the epidermis, it performs other functions, such as transpiration, gas exchange, storage, secretion, herbivorous repellent, attracting pollinators, absorption of water in roots, and so on. The epidermis is a tissue usually made up of a one-cell-thick layer of cells. The most abundant cell type is the epidermal cell proper, but other cell types are also present.
The epidermal cell proper is the more abundant and less specialized cell type of the epidermis. These cells are strongly attached to each other, with no intercellular spaces. Their shape and size are variable, with scarce or no chloroplasts, and generally develop a primary cell wall. On the surface of the free cell wall, the region in contact with the environment, a layer known as cuticle is usually formed.
Stomata are found in the epidermis. They consist of two guard cells that leave a pore, or ostiole, between them to let gases exchange with the environment. Below the guard cells, there is an empty space referred to as the sub-stomatal chamber. Subsidiary cells surround the guard cells. All these components form the stomatal complex. Trichomes, or hairs, are specialized epidermal cells that form long structures protruding from the epidermis. They can be for protection or for secretion (see the next page). In the root epidermis, some cells grow perpendicular to the surface and form root hairs for water and mineral absorption.
2. Periderm
The epidermis is replaced by the periderm in those organs with secondary growth. For instance, it becomes the bark of tree stems. The periderm is formed by the activity of the cork cambium meristem, or phelogen. It gives rise to cork outward and pheloderm (parenchyma) inward.
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Bibliography ↷
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Campilho A, Nieminen K, Ragni L. 2020. The development of the periderm: the final frontier between a plant and its environment. Current opinion in plant biology.53: 10-14.
Carpenter KJ. 2005. Stomatal architecture and evolution in basal angiosperms. American journal of botany 92: 1595-1615.
Javelle M, Vernoud V, Rogowsky PM, Ingram GC. 2010. Epidermis: the formation and functions of a fundamental plant tissue. New phytologist 189: 17-39.
Yeats TH, Rose JKC. 2013. The formation and function of plant cuticles. Plant physiology 163: 5-20
Xue D, Zhang X, Lu X, Ghen G, Chen Z-H. 2017. Molecular evolutionary mechanisms of cuticular wax for plant drought tolerance. Frontiers in plant sciences 8: 261
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Vascular 