**6. Glandular hair functions**

The function of glandular trichomes varies according to their location, the substances it secretes, and the moment of secretion. Structurally, similar trichomes can produce different materials, but even when similar materials are produced, when the trichomes are located differently, they may have different functions. For example, the functions of seed mucilage trichomes differ considerably from those of leaves.

Different types of glandular trichomes may differ according to the stage of development of the organ. In Lamiaceae, from a functional viewpoint and according to their mode of secretion, the secretion materials of certain types of capitate glands are released outside just after production, when the leaves are still young and developing, often with a porous cuticle or mechanical breakage. The lipophilic fraction acts as a repellent on young leaves even before being touched. In the peltate glands, the secretory material is gradually secreted from the glandular cells in a space formed by elevation of the cuticle. The cuticle can break through a predator, releasing secreted material, which can then act as repellents. The glandular hairs can also be classified in "short term" corresponding with the first group of capitate glands and "long term" corresponding with the peltate glands [3].


The epiphytic plants, which have no contact with the soil, as well as the plants that grow in xeric conditions, sometimes have other types of glands for the collection of fog water such as pineapple (*Ananas comosus*) [10].


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**Figure 2.**

*(protective hairs).*

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

play various roles during the growth of the plants.

*6.1.1 Histological study of glandular structures of Salvia officinalis*

**6.1 Histological variations of the glandular hairs of three aromatic and medicinal plants:** *Ocimum gratissimum***,** *Salvia officinalis***, and** *Pelargonium* **sp.**

The microscopic observations made on the *Salvia* plants (**Figure 2**) show the existence of two types of structures (secretion glands and protective hairs) which

*Scanning electron microscope observations of the central region of Salvia officinalis leaf (a), showing the presence of two types of glands: peltate glands (b, c) and capitate glands (d) with GP (peltate glands) and P*  *Essential Oils - Oils of Nature*

**6. Glandular hair functions**

of leaves.

In Lamiaceae, in addition to lipophilic substances, relatively large amounts of polysaccharides, which vary in amount between species, are secreted by certain

The function of glandular trichomes varies according to their location, the substances it secretes, and the moment of secretion. Structurally, similar trichomes can produce different materials, but even when similar materials are produced, when the trichomes are located differently, they may have different functions. For example, the functions of seed mucilage trichomes differ considerably from those

Different types of glandular trichomes may differ according to the stage of development of the organ. In Lamiaceae, from a functional viewpoint and according to their mode of secretion, the secretion materials of certain types of capitate glands are released outside just after production, when the leaves are still young and developing, often with a porous cuticle or mechanical breakage. The lipophilic fraction acts as a repellent on young leaves even before being touched. In the peltate glands, the secretory material is gradually secreted from the glandular cells in a space formed by elevation of the cuticle. The cuticle can break through a predator, releasing secreted material, which can then act as repellents. The glandular hairs can also be classified in "short term" corresponding with the first group of capitate

a.**Protection**: the plant needs protection against various external factors such as herbivores and pathogens, extreme temperatures, excessive loss of water, allelopathy against competing plants, etc. When nonglandular hairs are densely distributed (sometimes forming a carpet thicker than the leaf itself, found in many plants growing in arid conditions), they can serve as a mechanical barrier against most of the factors externally. A correlation between trichome density and pest resistance has been demonstrated by Levin [9]. The glands, which secrete lipophilic substances, can be used in chemical protection against various types of herbivores and pathogens by repulsing or poisoning them.

b.**Absorption of water**: ensure a permanent function of water absorption and soil minerals. They are usually short-lived, unicellular, unbranched, and well

The epiphytic plants, which have no contact with the soil, as well as the plants that grow in xeric conditions, sometimes have other types of glands for the

collection of fog water such as pineapple (*Ananas comosus*) [10].

c.**Salt secretion**: consist of cells that actively secrete a solution of different

d.**Seductive trichomes**: secretory glands of certain nectars, such as the unicellular glands of *Lonicera* and *Tropaeolum* and the multicellular glands of *Abutilon*. The most striking examples are carnivorous plants that have special requirements to catch their prey; they need to seduce and trap the various preys (sensory-triggering, digestion, and absorption devices). These functions are

capitate glands [8]. Many of these secreted materials are phytotoxic.

glands and "long term" corresponding with the peltate glands [3].

Secreted mucilage can trap insects by sticking.

elongated or with one or more swellings.

mainly fulfilled by trichomes [11].

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minerals.
