## 2 Trichome Morphology in Mint, Basil and Tomato  
Glandular trichomes can be subdivided in capitate and peltate trichomes. Both types are frequently present in, for example, the Asteraceae, Lamiaceae and Solanaceae. Capitate trichomes typically consist of one basal cell, one to several stalk cells, and one or a few secretory cells at the tip of the stalk [17]. They predominantly produce non-volatile or poorly volatile compounds that are directly exuded onto the surface of the trichome [16]. Peltate trichomes, of which typical examples can be found in mint and basil, consist of a basal cell, one (short) stalk cell, and a head consisting of several secretory cells, which is surmounted by a large sub-cuiticular storage cavity. This cavity is formed by separation of the cuticle from the cell wall of the secretory cells [18] and it is filled with the products of the secretory cells, thereby giving these trichomes a characteristic \"bulb-like\" shape [18]. Cell walls of stalk cells are usually cutinized, presumably to prevent contact of trichome-produced compounds, which can be autotoxic, with other parts of the plant [19]. The trichomes of the Solanaceae have been studied in detail, especially those of _Solanum_ species, because of their role in plant resistance. The morphology of the _Solanum_ spp. trichomes was originally described by [PERSON] [20], but later revised by [PERSON] _et al._[21]. Typically, eight different types are distinguished of which four (_i.e.,_ type I, IV, VI and VII) are glandular capitate trichomes and four (_i.e.,_ type II, III, V and VIII) are non-glandular (Figure 1). Of the glandular trichomes, type I and IV are capitate, whereas type VI and VII are globular. The glandular trichome types differ in number of stalk and secretory cells (see Table 1 for a description of trichome morphology), as well as in their chemical contents. Figure 1: _Glandular trichomes in section Lycopersicon_. Wild accessions have high densities of glandular trichomes that confer resistance to several pests. Panel (**A**) shows the leaflet surface of _Solanum habrochaites_ acc. LA 1777 with high densities of glandular trichome types IV and VI (**B**), and type I (**C**). Surface of _Solanum pennelli_ acc. LA 716 is also covered by type IV trichomes (**D**, **E**) producing and secreting acyl sugars. This accession also has type VI trichomes, but in low density (**F**). Panel (**G**) shows the surface of _Solanum lycopersicum_ cv. Moneymaker. Cultivated tomato has low density of type VI trichomes (**H**) and type I trichomes. Sometimes, type IV-like trichomes (**I**) are observed on stems, veins, and on the leaflet edges. White bars represent 500 \(\mu\)m in panel A, C, D, and G. In panels B, E, F, H, and I, bars represent 50 \(\mu\)m.  
Figure 1: _Cont._ \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Type**} & \multicolumn{2}{c}{**Description**} \\ \hline I & & Thin glandular trichomes consisting of 6–10 cells and 2–3 mm long. Globular and multicellular base with a small and round glandular cell in the trichome tip. \\ \hline II & & Similar to trichome I but non-glandular and shorter (0.2–1.0 mm). Globular and multicellular base. \\ \hline III & & Thin non-glandular trichome consisting of 4–8 cells and 0.4–1.0 mm long with a unicellular and flat base. External walls lack intercellular sections. \\ \hline IV & & Similar to trichome I but shorter (0.2–0.4 mm) and with a glandular cell in the tip. Trichome base is unicellular and flat. \\ \hline V & & Very similar to type IV with respect to height and thickness but non-glandular. \\ \hline VI & & Thick and short glandular trichomes composed of two stalk cells and a head made up of 4 secretory cells. \\ \hline VII & & Very small glandular trichomes (0.05 mm) with a head consisting of 4–8 cells. \\ \hline VIII & & Non-glandular trichome composed of one basal and thick cell with a leaning cell in the tip. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Trichome description according to [PERSON] [20] and revised by [PERSON] _et al._[21].