External and internal secretory structures exist in many plants. External secretory structures are of epidermal origin, and are usually glandular. Glandular trichomes are composed of a stalk and a head. The stalk may be unicellular or multicellular. In some species, several rows of cells make up the stalk. The head may, like the stalk, be unicellular or multicellular. Vari-ous secondary plant products are secreted within the gland, which, although usually covered by a cuticle, allows the essence through the apparently pore-less glands. Some trichomes can be regarded as being extra-floral nectaries whilst others are hydathode-like structures.
Secretion within the plant may be carried out either by single cells, small groups of cells or by tissues. Oil-secreting cells may be distributed through-out particular tissues.
In many instances, groups of thin-walled cells form an assemblage gen-erally referred to as secretory cells, which surround a duct that is schizoge-nously formed, as for example in the sunflower (Helianthus) stem. In plants, a combined process of lysigeny and schizogeny may form ducts.
The resin ducts formed in conifers, are thought to be of schizogenous origin, as here, like the sunflower, the resin ducts are surrounded by a ring of clearly demarcated secretory (epithelial) cells.
Laticifers are considered to be an important internal secretory channel. The latex may contain a combination of secondary plant metabolites, in-cluding carbohydrates, organic acids and alkaloids. Laticifers may be clas-sified into non-articulated (originating from single cells) which are capable of potentially unlimited growth, and articulated laticifers, which being compound in origin, consist of longitudinal files of cells, in which the cross walls are hydrolysed, thus forming an extensive anatomising network of multinucleate cells.
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