Protection tissues are located in the more external part of the plant organs and are usually in contact with the environment. There are two main protection tissues: epidermis and periderm. The epidermis is found covering the organs with primary growth and periderm covers the organs with secondary growth. Some authors propose the hypodermis and endodermis as protecton tissues. Hypodermis is located under the epidermis in some species, whereas the endodermis is found in the roots protecting the vascular tissues.
The epidermis is the protecting outer layer of plant, but it is not present in the apical tip of roots, under the calyptra, and it is not differentiated in aerial meristems. Furthermore, it disappears from those organs that undergo secondary grow, where it is replaced by the peridemis (see below). In plants showing only primary grow, the epidermis is maintained all along the life of the plant, excepting some monocots that substitute the epidermis by a kind of periderm. In the stem, the epidermis is differentiated from the outermost layer of the apical meristem. This layer is known as protodermis. In roots, the epidermis is differentiated from the same meristem cells that give rise to the cells of the calyptra, or from the more superficial layers of the cortical parenchyma. These differences in the origin of epidermis between shoots and roots lead some authors to name the root epidermis as rhizodermis. In addition, during the embryo development, a superficial layer known as protoderm is differentiated, even before that the formation of the apical shoot and apical root meristems, and gives rise to protodermis, which in turn gives rise to the first functional epidermis.
Epidermis is the protecting layer of shoots, roots, leaves, flowers and seeds. The protection role is against pathogens and mechanical insults. Another essential functions of the epidermis are: regulation of the transpiration or water lost, gas interchange, storing some substances, secretion, repelling herbivores, attracting pollinating insects, absorption of water by the root epidermis, and keep the integrity of some organs.
Epidermis is commonly a single row of cells. Some exceptions are found with stratified organization, as is the case of some aerial roots, xerophytes plants, or some leaves of oleander, ficus. Stratified or multiple-layered epidermis is called velamen. In the velamen, the superficial layers function as a typical epidermis, showing thick cell walls and superficial cuticle, whereas the deeper layers usually store water.
Several types of cells can be found in the epidermis: pavement cells, bulliform cells, glandular cells, pigmented cells, and cells that form stomata, thrycomes and root hairs. Some of these cell types are only found in some places of the plant like pigmented cells in petals, which also contain volatile essences, and the epidermal cells of the stigma that are involved in the reception of the pollen grains.
Pavement epidermal cells are the more abundant and not highly specialized. They are tightly joined with almost no intercellular spaces. Morphology and size are variable depending on of the structure where they are located. For example, they are elongated in stem, but show star-like shape in some leaves. In dicots, they have wavy cell walls, whereas in monocots they show more straight cell walls. They contain plastids, but not chloroplasts, a large vacuole, a well-developed endoplasmic reticulum and Golgi apparatus. Generally, the cell wall is only primary, although with variable thickness. A secondary cell wall is observed in just a few cases, such as in seeds. Lignin is not frequent in these secondary cell walls, but it can be found in some gymnosperm leaves. Epidermal cells show primary pit fields and plasmodesmata in the inner cell walls. Some times, the free cell wall (superficial) shows small channels known as ectodesms that allow the communication between the cytoplasm and the cuticle (see below) and the secretion of substances to the external surface of the organ.
In the aerial parts, epidermal cells synthesize and secrete cutin, a lipidic impermeable substance deposited outside of the cell wall to form a continuous layer known as cuticle. This external layer prevents the water lost and is a barrier against pathogens. Suberin is a molecule found in the cuticle of root epidermis, including the root hair epidermis. The cuticle thickness varies depending on the cell function and cellular localization. Over the surface of the cuticle, other lipid substances such as certain waxes are sometimes deposited, which can be in crystalline form or dissolved as oils. Cuticle makes epithelial cells asymmetric or polarized, with inner and apical domains.
Intermingled with the pavement epidermal cells, there are other cell types that sometimes are used as a taxonomic character. Thus, some epidermal cells specialize in storing water, as do bulliform cells in the leaves of grasses and other monocots. These cells are much larger than the rest of the epidermal cells, do not contain chloroplasts, show a large vacuole mostly containing water, and they have a very thin cuticle. They seem to be involved in the folding and unfolding mechanisms of leaves by changes in hydration. Bulliform cells are organized in rows parallel to the vascular bundles or in groups located in the hinged points of the folds.
Stomata are found in the epidermis. Stomatal guard cells are specialized epidermal cells organized to form an opening or pore, known as ostiole, through which the internal tissue of plant and the external environment are communicated. There is an air chamber under the pore called substomatal air chamber. Air chamber, ostiole, and guard cells form what is typically known as stomal complex. The guard cells are usually kidney-shaped, have chloroplasts and a non-uniformly thickened cell wall that makes possible turgidity for changing cell morphology, and therefore the increase or decrease the diameter of the pore.
Trichomes are specialized epidermal cells that outgrow from the epidermal layer. They can be protective or glandular (see next section), and sometimes are used as a trait in taxonomy. The protective trichomes can be unicellular or multicellular. They protect against intense light and create a superficial air layer over the epidermis that keeps a less fluctuating environment. These trichomes are particularly abundant in young plant structures, and may disappear when they become adults.
Trychomes perform many functions like avoiding herbivores, guiding polination insects, regulate temperature, preventing water lost in leaves, protection against ultraviolet radiation. Trychomes are different from root hairs because the set of genes that are activated during the differentiation of both types of cells are different. Plants with many trychomes are known as pubescent. Most of the trychomes are made up of living cells, although many thrychomes are dead cells.
In the roots, there are specialized epidermal cell known as root hairs, which are involved in water and soluble minerals absorption. Root hairs grow perpendicular to the root surface (similar to trichomes). Associated to root hairs there are symbiotic organisms such as fungi and nitrogen-fixing bacteria. Root hairs are located in the maturation zone of the apical root and are differentiated from undifferentiated epidermal cells called trichoblasts. The pattern (number and distribution) of root hairs depends on the plant species, but it also is influenced by the soil conditions.
Peridermis is found in those parts of roots and shoots undergoing secondary grow, normally during the first year of secondary growing. However, some plant species develope periderm several years after the secondary grow started. Periderm is not usually found in fruits and leaves. It separates the epidermis from the cortical parenchyma leading to the death of the epidermal cells, which usually detaches and falls as the root or shoot increases in diameter.
Peridermis is produced by the activity of the cork cambium or phellogen, a lateral and secondary meristem that can be originated several times during the life of the plant. During the first year of secondary grow, cork cambium is originated from the differentiation of parenchyma or collenchyma cells located underneath the epidermis. Sometimes, epidermis and primary phloem can also contribute to form the cork cambium. In this way, the cork cambium may be a continuous or discontinuous circular layer. Depending on the species, the first cork cambium may last several years (for example, it lasts for more than 20 years in the apple tree). Later, a second cork cambium can be originated in deeper areas of the stem, mostly from parenchyma cells of the secondary phloem. In the roots, the cork cambium is differentiated from the pericycle. Cells of the cork cambium divide by periclinal divisions (the divison plane parallels the surface of the stem) producing rows of cells outward and inward. The outer cells are more numerous and their cell walls get suberin, some of them lignin as well, and after that they die they form the bark of the trees. The inward pushed new cells are living cells and get organized in piles that form the phelloderm. Although these cells are similar to parenchyma cortical cells, they are however distributed in radial rows.
During the second year of secondary growing, the cork cambium is newly formed in deeper areas of the stems and roots. The parts that are superficial to this new cork cambium may contain secondary phloem, parenchyma cells, and old periderm. They become the so-called rhytidome, which is the bark detaching from the trees.
Mainly because of the cork, periderm is a barrier preventing the interchange of gases between the air and the inner tissues of stems and roots. This can be overcome by structures known as lencitels. They are circular or lenticular structures that protrude slightly over the surface and interrupt the normal organization of the periderm. They contain a large central pore that connect the external air with parenchyma cells that have thin cell walls and leave relatively large extracellular spaces. Lenticels are produced when the first periderm is generated, in locations where the cork cambium is more active and originates tissue with more intercellular spaces. As the stems and roots increase in diameter, new lenticels are formed.