An example of a capitate trichome from Ocimum ssp. (Lemon Basil) is shown here:
Note the obvious globular head, which can be made up of one or more cells. The head is attached to the epidermal surface by several elongated stalk cells. Essential oils build up in the head cells. Before secretions reach the outside, the head cells may release materials into a subcuticular space, which forms when the cuticle detaches from the top (apical) walls of the secretory head (Handilou et al., 1991).
An example of a peltate trichome, from the same Ocimum plant is shown here:
The stalk on the peltate trichome is very short and is actually sunk down into the epidermis. Peltate trichomes can have heads made up of dozens of cells.
All trichomes develop very early in the life of the leaf. First, a protodermal cell, prior to division, undergoes reorganization, with the nucleus migrating towards the upper (apical) end of the cell, and the bottom of the cell filling with large vacuoles. The cell then divides asymmetrically and periclinally. The lower, vacuolated cell becomes the foot cell (or cells), while the top cell undergoes another periclinal division. The bottom cell of the second division is committed to stalk cell formation, while the top cell differentiates to form the head cell or cells (Karousou et al., 1992). The head undergoes additional anticlinal divisions as necessary, depending on whether the trichome is capitate or peltate and contains numerous cells (Ascensão et al., 1995).
Since trichomes develop so early, the number of trichomes per square unit of leaf is dependent on the expansion of the leaf's epidermis, i.e. the growth of the leaf. The following photo shows the close proximity of glandular trichomes on a very young leaf of Rosmarinus officinalis (Rosemary):
Here is a photo of an epidermal peel from the abaxial surface (underside) of a leaf from Coleus:
Note that the distribution of glandular trichomes is more widely spaced and appears to be even. The pointed structures in the bottom half of the photo are non-glandular trichomes.
Handilou, E., S. Kokkini, A.M. Bosabalidis and J.-M. Bessière. 1991. Glandular trichomes and essential oil constituents of Calaminta menthifolia (Lamiaceae). Plant Systematics and Evolution 177: 17-26.
Karousou, R., A.M. Bosabalidis and S. Kokkini. 1992. Sideritis syriaca
ssp. syriaca: glandular trichome structure and development in relation
to systematics. Nordic Journal of Botany 12(1): 31-37.
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