**1. Introduction**

Essential oil presents a complicated and heterogenic composition, with some molecules which can cause an injury to the plant itself, as there is evidence that many terpenoids are potentially toxic to plant tissues, when monoterpenes are released to proximate cells [1]. Injury has also been found when some sesquiterpenes are artificially deposited on leaves during tests of their ability to deter herbivores [2]. Therefore, the sequestration of terpenoid in specific compartments by sensitive metabolic processes may be essential to avoid adverse effects. The morphology of these structures varies according to the conditions of irrigation and also according to the toxicity of intracuticular contents [3].

#### **Figure 1.** *Glandular hairs structure (Iriti et al. [12]).*

These risks implicate the presence of specialized structures for the storage and the secretion of these compounds, those structures changed with the content and function. Largely named glandular trichomes which are multicellular epidermal hairs, found in some families such as Lamiaceae, Asteraceae, and Solanaceae, which secrete terpenes in an extracellular cavity located at the apex of the trichome [4, 5].

Previous studies have been able to observe these structures as well as its different constituents (**Figure 1**). The glandular trichome is composed of several cells performing different functions: the terpenes synthesized by the secretory cells pass into a subcuticular space located at the apex of the trichome for accumulation, and the basal cells ensure the attachment of the structure to the epidermis. These glands vary morphologically, biochemically, and secretionally.

The secretory glands of aromatic plants come in different shapes and sizes, in order to ensure a specific function. This function consists mainly in the protection of the different organs of the plant and the attraction of pollinators. These glands are subdivided into peltate and capitate hairs. Werker et al. [4] classified glands into short-term glands and long-term glands; short-term glands are glands that secrete rapidly to protect young organs. The long-term glands are glands in which the secretory substance accumulates gradually in the subcuticular space and play a role in the protection of mature organs such as the flower, as well as in pollination. According to this definition, he inferred that the capitate hairs are short-term glands, while the peltate hairs are long-term glands. The difference between these two types of glands consists of several aspects like structure, mode of secretion, and timing of secretion.

Capitate usually consists of a single or bicellular head and rarely more than two cells in some species with lipophilic content. This content is ready to be released just after its production via a porous cuticle [3].

#### **2. The glands' location depends on the function**

The glands of secretions are localized in all the plant organs, leaves, and stems and even at the root. The location of these structures depends on the organ in which they are located and the function and nature of the substances stored and secreted by these glands.

Due to the enormous diversity of these structures, morphology, origin, size, location, microstructure of the head, secretory capacity, secretion mode, function, etc., their classification is made difficult. The use of one of these criteria renders the classification incomplete which requires a method based on different types of criteria to classify these glands.

The major classification divides the glands into two groups: glandular hair and nonglandular hair. The main characteristic of distinction between these two types is the morphology as well as the nature of the substances to be secreted.

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*Essential Oil and Glandular Hairs: Diversity and Roles DOI: http://dx.doi.org/10.5772/intechopen.86571*

Nonglandular trichomes differ in morphology, anatomy, and microstructure. Essentially, they are classified according to their morphology. They can be unicellular or multicellular and branched or unbranched. The unbranched multicellular trichomes can be uniseriate, biseriate, or multiseriate. They can be distinctly articulated between cells or transverse walls which make the distinction of these structures impossible on the surface. They may differ in length, size, and shape of cells and may be symmetrical or asymmetrical. This type of gland provides a protective function as a mechanical barrier and does not ensure secretion of any

The term "glandular hair" refers to a wide variety of glands. They differ according to the chemical composition of the substances they secrete, accumulate, or absorb; according to their mode of production, structure, and location; and according to their functions. All of these differences serve a certain level, which overlaps

In many of the Lamiaceae, two main types of glandular trichomes are encountered, the capitate and the peltate. They differ according to the shape of their secretory head and according to the morphology as well as the nature of the substances to be secreted. The head of the capitate glands consists of 1–4 more or less rounded secretory cells, generally oriented horizontally; a stem, one to several long cells; and a basal cell. The head cell can sometimes be very large, as in some species of *Salvia* [5]. The peltate hairs' head consists of 4–18 more flattened cells on a horizontal plane, a stem cell, and a basal cell. Thus, intermediate shapes

There is great variability in the secreted materials of glandular trichomes: polysaccharides, sugars, salts, lipids, essential oils, resins, proteins, etc. Uphof [6] proposed to classify glandular trichomes according to the nature of their secretory products. Fahn [7] classified secretory substances in plants in general into two

1.Unmodified or slightly modified substances, such as salts secreted by certain

The difficulty with this type of classification is that some glands secrete more than one type of substance. For example, the glands of *Inula viscosa* secrete lipophilic substances, polysaccharides, and proteins at different stages of life and by different organelles [8]. In some carnivorous plants, the same glandular hairs produce

2.Substances synthesized by secretory cells. This can be hydrophilic (as in trichomes which secrete mucilage and glandular trichomes of digestive carnivorous plants) or lipophilic (such as in the glandular trichomes of Lamiaceae,

Asteraceae, Geraniaceae, Solanaceae, and Cannabaceae).

both seductive (nectar) and digestive substances (enzymes).

**3. Non-glandular trichome**

substance.

**4. Glandular trichome**

the classification.

can be encountered.

groups:

**5. Variability and classification**

glands, as well as nectar.
