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Glandular trichomes are more abundant and produce more resin on the pistillate inflorescence than on the staminate, for the growing embryo is in need of protection while anthers shed their pollen and expire. It is commonly agreed by most investigators that resin secreted by the glandular trichomes contains the psychoactive constituents of Cannabis. However, recent research by Fujita et al. (1967) points to the disc-shaped cap of cells on the trichome as the location of psychoactive THC, and not the actual resin secreted by these cells.
The glandular trichomes are divided into three types—bulbous, capitate-sessile, and capitatestalked. All three are characterized by a secretory disc of 1 to 13 cells supported by a layer of stipe cells above a layer of base cells embedded in the epidermis. The secretory cells of mature glandular trichomes produce a resinous fluid which accumulates beneath a membranous sheath; this entire structure is termed the head of the trichome.
Bulbous glands are small, and consist of a 1- to 4-celled secretory portion, 1 or 2 stipe cells, and 1 or 2 base cells. They measure 25 to 30 microns in height with a 20-micron diameter head.
Capitate-sessile glands have heads that measure from 40 to 60 microns in diameter. A bilaterally symmetric cluster of 8 to 13 head cells rests on a short stipe, and the gland appears sessile (attached flush with the surface). Capitate-sessile glands look much like sessile glands, but often have a larger head (up to 100 microns) and a long stalk derived from epidermal tissue. Also, abscission layers, where the heads break off, are present in stalked glands, both between the head and stipe cells and between the stipe and base cells; these are absent in bulbous and sessile types.
Gland initiation begins with protrusion of an individual epidermal cell and subsequent anticlinal (oriented perpendicular to the surface) division in the plane of the long axis of the calyx or leaflet, which establishes a persistent bilateral symmetry. A periclinal (oriented parallel to the surface) division follows, which delimits the secretory and supportive initials. A second periclinal division separates base and stipe cells. The base cells remain double but the stipe layer divides perpendicular to the first division, forming a persistent 4-cell layer. The secretory disc forms first by a perpendicular division to form a 4-celled stage that enlarges radially and continues to divide anticlinally, forming 8 to 13 cells. Cell division ceases and cellular enlargement doubles head diameter. A bulbous gland would begin secretion at or before the 4-celled head stage. Capitate glands continue to develop a more complex secretory structure. As secretion commences, a membrane detaches from the top surface of the gland head, trapping the exuded resin and giving the head a spherical shape. If the capitate gland is to be stalked it will become elevated on elongated epidermal cells, raising the head up to 500 microns above the epidermal surface. (Hammond and Mahlberg, 1977.)
The changing ultrastructure of developing glandular trichomes reflects the mechanism of resin secretion. Gland initials are distinguished from epidermal cells by a large central nucleus, less-developed vacuolar system and endoplasmic reticulum (ER), fewer dictyosomes, and poorly developed plastids. By the 4-celled disc stage, secretory cells show a well-developed fibrillar system with associated lipid bodies, mitochondria and ER. Ephemeral lipid bodies and fibrillar material disappear well before secretion begins. Plastids at this stage are 0.01 microns in diameter and lack a granular lamellar system. By the 8-to-13-cell stage, the cytoplasm of the secretory cells is very dense, possibly reflecting high ribosome counts. Mitochondria and dense, elongated plastids are abundant. A large central vacuole forms, possibly from the participation of ER. Branched and tubular portions of ER proliferate throughout the cytoplasm. Just prior to secretion, a symplast (continuous cytoplasm) develops between the head cells by removal of cell wall material at the sites of plasmodesmata (pores in the cell wall) allowing free movement of organelles and resin.
Secretion commences with the production and extrusion of a resinous fluid which collects between the head cells and the membrane that covers the secretory structure. At this time plastids in the secretory cells increase in number and size. Simple plastids with dense stroma and few membranes slowly develop a complex paracrystalline membranous inclusion as they mature. This paracrystalline structure grows until it occupies nearly all of the stroma area and the plastid assumes a spherical shape 1.4 to 1.6 microns in diameter. Plastids in stipe and base cells appear as slightly elongated, typical chloroplasts. The resinous fluid appears at the periphery of young plastids, and increases as plastids mature. No membrane surrounds the fluid and it accumulates into globules which may contain other secretory products attracted and trapped as they migrate through the cytoplasm to the cell surface.
The secretory cavity forms by a breakdown of the middle layer of the outer secretory-cell wall, differentiating head cells and a separate membrane of cuticle and epidermal cells. Extrusion occurs directly through the head-cell walls and cell mem branes, and the resin seems to form variously sized (4 to 5 microns and 0.1 to 0.3 microns in diameter) spherical bodies with membrane-like 'coverings after extrusion. (Hammond and Mahlberg, 1978.)
The essential oil of Cannabis, extracted from the resin and associated structures, contains at least 103 monoterpene and sesquiterpene hydrocarbons along with ketones, alcohols, and esters (Turner et al.,1980). Many of these terpenes are aromatic and may influence the taste and aroma of Cannabis resin.
Non-glandular trichomes are single-celled and occur on all plant parts except roots and root hairs. They arise as extensions of epidermal cells and elongate by ten to twenty times their original thickness, up to one millimeter. In appearance they are long, hollow, and clear; they taper to a sharp point, with the cytoplasm and nucleus restricted to the base. Calcium carbonate crystal, up to 75 microns in diameter, occurs within the base of the trichomes, and the outer surface of the trichome is covered with many sharp or warty protrusions. Both long-thin and short-swollen types occur. The trichomes are oriented so that their points are all directed upward toward the apex of the calyx. Besides providing physical protection to the epidermal tissues, non-glandular trichomes may also afford protection from desiccation by reducing the free circulation of the atmosphere against the epidermal tissues. Trichomes also act at a deterrent to attack by insects.
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