*6.1.1 Histological study of glandular structures of Salvia officinalis*

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

#### **Figure 2.**

*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 (protective hairs).*

#### *Essential Oils - Oils of Nature*

Glandular hairs consist of a base cell, a short unicellular stem, and a secretion head generally composed of 12 cells (sometimes 16) arranged in the form of a shield of 4 central cells surrounded by 8 peripheral cells (**Figure 2a, b**). Moreover, **Figure 2c** shows a sessile gland with multiple cells of the head. These structures are called peltate glands.

The second category of glands or capitate glands have a variable morphology since one distinguishes two types of glands: glands consisting of a short stem with one or two cells (**Figures 3a** and **4a–c**) and a head that can be unicellular or bicellular. In this type of gland, the cuticle is thin, with the presence or absence of a small subcuticular space. The second type consists of glands observed in old leaves at the end of flowering (**Figure 5**) and has a thin cuticle with the presence of a large subcuticular space directly to the epidermis. This structure is characterized by the presence of an accumulation pocket which is used to store the secretory material suggesting the presence of secretory cells below this pocket and whose synthesis products would arrive at the accumulation pocket through a structure (channel or pore) located under this pocket. The absence of this structure in young leaves and their appearance in old leaves suggests that these structures have a specific function related to the age of the leaf and are therefore specialized in the accumulation of a different substance.

Capitate glands are usually characterized by a single-cell head (**Figure 4a, b**). However, the light microscopic observations of the *Salvia officinalis* leaves show the presence of capitate glands that can present a two-celled head (**Figure 4c**).

According to the classification proposed by Werker et al. [4], the capitate glands are short-term glands specialized in the synthesis of non-terpenic substances frequently found in young leaves to provide a protective function against predators.

#### **Figure 3.**

*Light microscopic observations of the different glands in Salvia officinalis. (a) Capitate gland, (b) detached peltate gland, (c) peltate gland in secretion phase, and (d) post-secretion peltate gland.*

**89**

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

in the synthesis of terpene substances.

*with single-cell head; (c) Gland with two-cell head.*

**Figure 4.**

mechanical events or at the end of gland life.

production via a porous cuticle (**Figure 7b**).

*6.1.2 Histological study of Pelargonium sp. glandular hairs*

The peltate glands are called long-term glands that are present in the leaves throughout the growth phases and whose number increases at flowering and are specialized

*Observations in light and scanning electron microscopy of capitate glands in Salvia officinalis*. *(a & b) Glands* 

Observations by the LM as well as SEM of the peltate glands (**Figure 3c, d**) do not reveal the presence of pores through which the secretory material can exude, which makes the material not released until the break of the cuticle, whether due to

As in the case of sage, light microscopy observations made on *Pelargonium* sp. leaves show that this plant is characterized by the presence of two types of secretory glands, the peltate glands or long-term glands and the capitate glands or short-term

Capitate glands (short-term glands) are divided into two categories according to their forms: capitate glands with a stem consisting of four cells (**Figure 7b**) and capitate glands with one head and unicellular stem (**Figure 7c**). At the level of these glands, the synthesized secretion material is ready to be released just after its

Moreover, we could only meet one type of peltate glands (long term), with a multicellular stem and a large head in which one finds the cells of synthesis as well as a storage pocket of essential oils. The appearance of these glands varies with their stage of maturity. **Figure 8a** shows a gland that has an empty aspect of any content and confirms the juvenile condition of the gland. These glands are gradually filled by the EO as they mature (**Figure 8b, c**), and their cuticle eventually breaks to

These peltate glands have a thick and rigid cuticle, but along the maturity stages

of these glands, the thickness of this cuticle decreases to facilitate bursting and

glands, as well as protective hairs or non-secretory glands (**Figure 6**).

release their contents at the end of their maturation (**Figure 8c**).

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

**Figure 4.**

*Essential Oils - Oils of Nature*

called peltate glands.

Glandular hairs consist of a base cell, a short unicellular stem, and a secretion head generally composed of 12 cells (sometimes 16) arranged in the form of a shield of 4 central cells surrounded by 8 peripheral cells (**Figure 2a, b**). Moreover, **Figure 2c** shows a sessile gland with multiple cells of the head. These structures are

The second category of glands or capitate glands have a variable morphology since one distinguishes two types of glands: glands consisting of a short stem with one or two cells (**Figures 3a** and **4a–c**) and a head that can be unicellular or bicellular. In this type of gland, the cuticle is thin, with the presence or absence of a small subcuticular space. The second type consists of glands observed in old leaves at the end of flowering (**Figure 5**) and has a thin cuticle with the presence of a large subcuticular space directly to the epidermis. This structure is characterized by the presence of an accumulation pocket which is used to store the secretory material suggesting the presence of secretory cells below this pocket and whose synthesis products would arrive at the accumulation pocket through a structure (channel or pore) located under this pocket. The absence of this structure in young leaves and their appearance in old leaves suggests that these structures have a specific function related to the age of the leaf and are therefore specialized in the accumulation of a different substance. Capitate glands are usually characterized by a single-cell head (**Figure 4a, b**). However, the light microscopic observations of the *Salvia officinalis* leaves show the

presence of capitate glands that can present a two-celled head (**Figure 4c**).

are short-term glands specialized in the synthesis of non-terpenic substances frequently found in young leaves to provide a protective function against predators.

*Light microscopic observations of the different glands in Salvia officinalis. (a) Capitate gland, (b) detached* 

*peltate gland, (c) peltate gland in secretion phase, and (d) post-secretion peltate gland.*

According to the classification proposed by Werker et al. [4], the capitate glands

**88**

**Figure 3.**

*Observations in light and scanning electron microscopy of capitate glands in Salvia officinalis*. *(a & b) Glands with single-cell head; (c) Gland with two-cell head.*

The peltate glands are called long-term glands that are present in the leaves throughout the growth phases and whose number increases at flowering and are specialized in the synthesis of terpene substances.

Observations by the LM as well as SEM of the peltate glands (**Figure 3c, d**) do not reveal the presence of pores through which the secretory material can exude, which makes the material not released until the break of the cuticle, whether due to mechanical events or at the end of gland life.

#### *6.1.2 Histological study of Pelargonium sp. glandular hairs*

As in the case of sage, light microscopy observations made on *Pelargonium* sp. leaves show that this plant is characterized by the presence of two types of secretory glands, the peltate glands or long-term glands and the capitate glands or short-term glands, as well as protective hairs or non-secretory glands (**Figure 6**).

Capitate glands (short-term glands) are divided into two categories according to their forms: capitate glands with a stem consisting of four cells (**Figure 7b**) and capitate glands with one head and unicellular stem (**Figure 7c**). At the level of these glands, the synthesized secretion material is ready to be released just after its production via a porous cuticle (**Figure 7b**).

Moreover, we could only meet one type of peltate glands (long term), with a multicellular stem and a large head in which one finds the cells of synthesis as well as a storage pocket of essential oils. The appearance of these glands varies with their stage of maturity. **Figure 8a** shows a gland that has an empty aspect of any content and confirms the juvenile condition of the gland. These glands are gradually filled by the EO as they mature (**Figure 8b, c**), and their cuticle eventually breaks to release their contents at the end of their maturation (**Figure 8c**).

These peltate glands have a thick and rigid cuticle, but along the maturity stages of these glands, the thickness of this cuticle decreases to facilitate bursting and

#### **Figure 5.**

*Observations with light microscopy and scanning electron microscopy of capitate glands in Salvia officinalis. (a) Distribution of the pocket; (b&d) empty pocket; (c&e) pocket fill with secretory material.*

release of contents toward the end of the maturity of the gland or by contact with some insects (**Figure 8**).

#### *6.1.3 Histological study of Ocimum gratissimum glandular hairs*

Environmental scanning electron microscopy observations of basil leaves show the presence of two types of glands (**Figure 9**), peltate gland and capitate gland. The peltate glands have a round shape and a fairly large diameter that can exceed 70 microns. The capitate glands are small not exceeding 40 microns and come in two forms, glands capitate with one head (CH) and two bilobed heads (C2H).

The observations made by the light microscopy of the capitate glands confirm those made by scanning electron microscopy, with the presence of capitate glands with a single head (**Figure 10a**) and capitate glands with two heads (**Figure 11b**). On the other hand, the glands with a single head can be unicellular (**Figure 10a**) or bicellular (**Figure 10b**).

In basil plants, the peltate glands show a variability in the number of secretory cells. There are large glands with four cells (**Figure 12a**). In addition, we have been able to highlight the presence of glands that have eight secretory cells (**Figure 12b, c**). A thick cuticle covers both types of glands. With the filling of the essential oil structure, this cuticle becomes thinner to facilitate the release of the contents.

**Figure 13** shows peltate glands of basil after bursting and release of their terpenic contents; according to this figure, we notice the existence of four secretory cells

**91**

**Figure 7.**

*and unicellular stem.*

**Figure 6.**

*(c) and capitate (with red arrow).*

*Scanning electron microscopic observations of the ventro-central part of a* Pelargonium *sp. leaf, showing the presence of two types of secretion glands, peltate. (a): observation of non glandular hairs and glandular hairs*

*Observations made by light microscopy on Pelargonium sp. leaf showing the two types of capitate glands. (a) Global vision; (b) capitate glands with a stem consisting of four cells; (c) capitate glands with one head* 

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

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

*Essential Oils - Oils of Nature*

some insects (**Figure 8**).

**Figure 5.**

bicellular (**Figure 10b**).

release of contents toward the end of the maturity of the gland or by contact with

*Observations with light microscopy and scanning electron microscopy of capitate glands in Salvia officinalis.* 

*(a) Distribution of the pocket; (b&d) empty pocket; (c&e) pocket fill with secretory material.*

Environmental scanning electron microscopy observations of basil leaves show the presence of two types of glands (**Figure 9**), peltate gland and capitate gland. The peltate glands have a round shape and a fairly large diameter that can exceed 70 microns. The capitate glands are small not exceeding 40 microns and come in two

The observations made by the light microscopy of the capitate glands confirm those made by scanning electron microscopy, with the presence of capitate glands with a single head (**Figure 10a**) and capitate glands with two heads (**Figure 11b**). On the other hand, the glands with a single head can be unicellular (**Figure 10a**) or

In basil plants, the peltate glands show a variability in the number of secretory cells. There are large glands with four cells (**Figure 12a**). In addition, we have been able to highlight the presence of glands that have eight secretory cells (**Figure 12b, c**). A thick cuticle covers both types of glands. With the filling of the essential oil struc-

**Figure 13** shows peltate glands of basil after bursting and release of their terpenic contents; according to this figure, we notice the existence of four secretory cells

forms, glands capitate with one head (CH) and two bilobed heads (C2H).

ture, this cuticle becomes thinner to facilitate the release of the contents.

*6.1.3 Histological study of Ocimum gratissimum glandular hairs*

**90**

#### **Figure 6.**

*Scanning electron microscopic observations of the ventro-central part of a* Pelargonium *sp. leaf, showing the presence of two types of secretion glands, peltate. (a): observation of non glandular hairs and glandular hairs (c) and capitate (with red arrow).*

#### **Figure 7.**

*Observations made by light microscopy on Pelargonium sp. leaf showing the two types of capitate glands. (a) Global vision; (b) capitate glands with a stem consisting of four cells; (c) capitate glands with one head and unicellular stem.*

#### **Figure 8.**

*Observations in light microscopy of peltate glands, secreting essential oils at different stages of maturity in*  Pelargonium *sp. (a) Beginning of maturation, (b, c) filling with the EO, and (d) bursting of the gland. C, cuticle; EO, essential oil (Gx 400).*

#### **Figure 9.**

*Observation by scanning electron microscopy of basil leaf showing the presence of pelt glands (P) and capitate glands (CH: capitate gland with one head and capitates gland with head bilobed C2H).*

**93**

**Figure 12.**

**Figure 10.**

**Figure 11.**

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

*(b). C, cuticle; E, epidermis; CG, glandular cell (GX400).*

*Light microscopy observations of a capitate gland with a single head and a single cell (a) and with two cells* 

*Observations made by light microscopy and scanning electron microscopy of a double-headed gland in basil plants.*

*Light microscopic observations of a cross section (central part of a basil leaf* O. gratissimum*) showing two types of peltate glands: glands with four secretory cells (a) and glands with eight secretory cells (b, c).*

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

#### **Figure 10.**

*Essential Oils - Oils of Nature*

**92**

**Figure 9.**

**Figure 8.**

*C, cuticle; EO, essential oil (Gx 400).*

*Observation by scanning electron microscopy of basil leaf showing the presence of pelt glands (P) and capitate* 

*Observations in light microscopy of peltate glands, secreting essential oils at different stages of maturity in*  Pelargonium *sp. (a) Beginning of maturation, (b, c) filling with the EO, and (d) bursting of the gland.* 

*glands (CH: capitate gland with one head and capitates gland with head bilobed C2H).*

*Light microscopy observations of a capitate gland with a single head and a single cell (a) and with two cells (b). C, cuticle; E, epidermis; CG, glandular cell (GX400).*

#### **Figure 11.**

*Observations made by light microscopy and scanning electron microscopy of a double-headed gland in basil plants.*

#### **Figure 12.**

*Light microscopic observations of a cross section (central part of a basil leaf* O. gratissimum*) showing two types of peltate glands: glands with four secretory cells (a) and glands with eight secretory cells (b, c).*

**Figure 13.** *Observation by scanning electron microscopy of a pelted basil gland after natural bursting (a) and mechanics (b).*

(**Figure 13a**) with an internal diameter (without the cuticle which encompasses the gland) that touches 62 μm. Furthermore, **Figure 13b** shows an immature gland, but burst under pressure (exerted by SEM), and shows a thick cuticle which confirms the state of non-maturity that has already been reported.
