Secretory tissues and particular cells.
They may be localized in the different organs, in all tissues and are of different types. Based on their localization, we distinguish:
– External secretory structures:
Secretory trichomes & glands
– Internal secretory structures:
Isolated secretory cells example in a stem of rosa
Secretory cavities :examples in a celery petiole and in a stem of tarragon.
The substances produced are numerous, such as
Tannins, alkaloids, calcium oxalate, nectar, …
Particular cells accumulating crystals of mineral salts.
Some cells accumulate in their vacuole secretory substances that are not used by the plant. These specialized cells in which crystals accumulate are called “idioblasts”.
One of the best known example is that of cells containing calcium oxalate crystals. Oxalic acid is a cellular poison rendered harmless by its immobilization in the vacuole, so that it is switched out of the cytoplasm. In the vacuole it exists under K or Na oxalate or under insoluble precipitate of calcium oxalate. Being insoluble it cannot cross the membrane of the vacuole.
Calcium oxalate crystals occur in many species and can be found in any tissue or organ in plants (roots, leaves, stems, floral organs, anther). In most cases they are contained in cells scattered in parenchyma. They take several shapes (crystal sand, tetragonal crystal, prisms, styloid) and may form aggregates such as sea urchin, star-shaped druses, needle-shaped crystals called raphides. These are frequent in monocotyledons and less frequent and smaller in dicotyledons.
Other cells accumulate calcium carbonate crystals and they are called “cystoliths”. They are outgrowths of the epidermal cell wall formed in the cellulose matrix and they may be found also in mesophyll. The cells containing cystoliths are called lithocysts. The crystals are arranged as grapes on an axis coming from internal cell wall. In leaves cystoliths may be found in epidermis but also in parenchyma cells. A few examples of plants in which they have been observed: leaves of Ficus sp. (Moraceae), Humulus lupulus (Cannabaceae), Urtica dioica (Urticaceae). See also section through a leaf of Ficus macrophylla.