**7. Discussion**

The microscopic observations made in this work show the presence of two major groups of secretory glands in the three plants (sage, pelargonium, and basil). The first group is peltate glands or long-term glands, and the second group is capitate glands or short-term glands. Within the same group, we can see a morphological variation from one plant to another, depending on the content, the role, and the phenological stage of the plant, which makes the classification difficult. But to ensure a credible classification, various authors are mainly based on the morphological criterion, the content, as well as the mode of secretion [4–8, 12].

In this context, scientists have given a definition to these two names; they have described a short-term gland as any gland whose secretion is rapid to protect young organs and a long-term gland as any gland in which the secretory substance accumulates gradually in the subcuticular space and serves for the protection of mature organs in the flower, as well as for pollination. The difference between the two types consists of several aspects, namely, the structure, the mode of secretion, and the moment of secretion.

The microscopic observations made on our plants go in the direction of this classification and show the two groups of glands, peltate and capitate. Moreover, two major types of peltate glands could be determined in this study, peltate glands with stem consisting of a single-cell and peltate glands glued directly to the epidermis. In the latter group, there is a major difference in the number of cells of each gland, since there are 4 and 8 cells in *O. gratissimum* and up to 12 in *Salvia*. These results are confirmed by the work of Tissier [13], Werker [8], and Gang et al. [14]. According to these authors, the peltate glands can have a large number of cells that can reach 16 cells.

These peltate glands are generally characterized by a large subcuticular space and a rigid cuticle which thins during the maturation of the gland which facilitates bursting at maturity or after mechanical contact with predators.

**95**

it has only 4 cells.

been classified as a capitate gland.

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

forms:

cells.

cells) and a head

The captioned glands are subdivided into several categories according to their

• Capitate glands possessing a basal cell and a united or multicellular stem (1–3

• Capitate glands with a stem cell and a united or bicellular head

• Capitate glands with a basal cell and a head with one or two cells

• Capitate glands in the form of a pocket glued directly to the epidermis

These capitate glands are characterized by a thin cuticle with the synthesized secretory material which is ready to be released just after its production via a porous cuticle and usually consist of a united or bicellular head with lipophilic content. Similar observations have been found in *Salvia* [15], since according to these authors the capitate glands are morphologically very variable and four types can be distinguished. Type I has a single or double bicellular stem and a united or bicellular head. The cuticle is thin and there is no subcuticular space. The secretory material is released slowly through the cuticle and can be released suddenly if the cuticle is broken. Type II is very small, possessing a unicellular stem and a small subcuticular space. The secretory material is probably secreted by a pore. Type III is a large gland with a long stem made up of several cells (about three or more), a neck cell, and a unicellular head, which probably releases the secretory material often collected as a drop on the head. Type IV is a large gland with a long thin stem, made up by four

According to these authors, each type of gland is responsible for the synthesis of a distinct substance. Venkatachalam et al. [16] found a correlation between the increased number of peltate glands and increased camphor and thujane synthesis. These observations are in agreement with those of Pedro et al. [17] who deduced

the presence of three types of glands, a peltate type and two capitate types. According to these authors, the glands with a unicellular stem represent a distinct capitate gland with a specific content different from that of the peltate structure. According to the same work, these three types of glands ensure the synthesis of

three types of different substances, terpenoids, phenols, and flavonoids.

In other works by Ko et al. [18], *Pelargonium*'s peltate and capitate glands all have a single basal cell. These two types of glands differ in the number of stem cells and in the diameter of the head depending on the stage of maturity [3]. According to these authors, the glands defined in our study as glands capitate to a head and a unicellular stem represent only one stage of the formation of the peltate glands. However, observations made in *Salvia* leaves show structures identical to this structure, possessing a small unicellular head and a single-celled stem. This structure has

These kinds of peltate and capitate glands are found in several species (*Mentha piperita*, *Origanum dictamnus*, *Monarda fistulosa*, *Pogostemon cablin*, etc.) and have been described in ancient works [16]. In mint, the peltate glands have eight head cells, and the capitate glands have a single-stem cell and a head cell. In *Origanum dictamnus*, the head of the peltate glands has 12 cells, whereas in *Pogostemon cablin*,

According to Werker [8], young basil leaves (*O. basilicum*) that are highly vulnerable to predators are heavily covered by capitate glands that are characterized by rapid secretion to repel herbivores. When the plants become older, the rate of the *Essential Oils - Oils of Nature*

**7. Discussion**

**Figure 13.**

moment of secretion.

can reach 16 cells.

(**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

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

The microscopic observations made in this work show the presence of two major groups of secretory glands in the three plants (sage, pelargonium, and basil). The first group is peltate glands or long-term glands, and the second group is capitate glands or short-term glands. Within the same group, we can see a morphological variation from one plant to another, depending on the content, the role, and the phenological stage of the plant, which makes the classification difficult. But to ensure a credible classification, various authors are mainly based on the morpho-

In this context, scientists have given a definition to these two names; they have described a short-term gland as any gland whose secretion is rapid to protect young organs and a long-term gland as any gland in which the secretory substance accumulates gradually in the subcuticular space and serves for the protection of mature organs in the flower, as well as for pollination. The difference between the two types consists of several aspects, namely, the structure, the mode of secretion, and the

The microscopic observations made on our plants go in the direction of this classification and show the two groups of glands, peltate and capitate. Moreover, two major types of peltate glands could be determined in this study, peltate glands with stem consisting of a single-cell and peltate glands glued directly to the epidermis. In the latter group, there is a major difference in the number of cells of each gland, since there are 4 and 8 cells in *O. gratissimum* and up to 12 in *Salvia*. These results are confirmed by the work of Tissier [13], Werker [8], and Gang et al. [14]. According to these authors, the peltate glands can have a large number of cells that

These peltate glands are generally characterized by a large subcuticular space and a rigid cuticle which thins during the maturation of the gland which facilitates

bursting at maturity or after mechanical contact with predators.

logical criterion, the content, as well as the mode of secretion [4–8, 12].

the state of non-maturity that has already been reported.

**94**

The captioned glands are subdivided into several categories according to their forms:


These capitate glands are characterized by a thin cuticle with the synthesized secretory material which is ready to be released just after its production via a porous cuticle and usually consist of a united or bicellular head with lipophilic content.

Similar observations have been found in *Salvia* [15], since according to these authors the capitate glands are morphologically very variable and four types can be distinguished. Type I has a single or double bicellular stem and a united or bicellular head. The cuticle is thin and there is no subcuticular space. The secretory material is released slowly through the cuticle and can be released suddenly if the cuticle is broken. Type II is very small, possessing a unicellular stem and a small subcuticular space. The secretory material is probably secreted by a pore. Type III is a large gland with a long stem made up of several cells (about three or more), a neck cell, and a unicellular head, which probably releases the secretory material often collected as a drop on the head. Type IV is a large gland with a long thin stem, made up by four cells.

According to these authors, each type of gland is responsible for the synthesis of a distinct substance. Venkatachalam et al. [16] found a correlation between the increased number of peltate glands and increased camphor and thujane synthesis.

These observations are in agreement with those of Pedro et al. [17] who deduced the presence of three types of glands, a peltate type and two capitate types. According to these authors, the glands with a unicellular stem represent a distinct capitate gland with a specific content different from that of the peltate structure. According to the same work, these three types of glands ensure the synthesis of three types of different substances, terpenoids, phenols, and flavonoids.

In other works by Ko et al. [18], *Pelargonium*'s peltate and capitate glands all have a single basal cell. These two types of glands differ in the number of stem cells and in the diameter of the head depending on the stage of maturity [3]. According to these authors, the glands defined in our study as glands capitate to a head and a unicellular stem represent only one stage of the formation of the peltate glands. However, observations made in *Salvia* leaves show structures identical to this structure, possessing a small unicellular head and a single-celled stem. This structure has been classified as a capitate gland.

These kinds of peltate and capitate glands are found in several species (*Mentha piperita*, *Origanum dictamnus*, *Monarda fistulosa*, *Pogostemon cablin*, etc.) and have been described in ancient works [16]. In mint, the peltate glands have eight head cells, and the capitate glands have a single-stem cell and a head cell. In *Origanum dictamnus*, the head of the peltate glands has 12 cells, whereas in *Pogostemon cablin*, it has only 4 cells.

According to Werker [8], young basil leaves (*O. basilicum*) that are highly vulnerable to predators are heavily covered by capitate glands that are characterized by rapid secretion to repel herbivores. When the plants become older, the rate of the peltate glands increases; these terpenoid-rich glands will be used for defense against pathogens in case of injury and pollinator attraction.
