of plants (plant glandules), formations that perform the function of secreting drops of water or other substances.
There are several groups of glandules, including hydathodes, nectaries, and digestive, oily, resinous, mucous, and gum glandules. In insect-eating plants, the digestive glandules secrete digestive juices. Oily, resinous, mucous, and gum glandules secrete resins, essential oils, and other substances. They may be external or internal. External glandules are common in the bud scales (for example, of poplars or birches), leaves, and stems of many plants (of the family Labiatae and others); these glandules are a type of hair (sometimes called glandular hairs, for example in primroses and dead nettles). Internal glandules most often consist of tightly joined, mainly squamous, epithelial cells surrounding a cavity (intercellular space), into which they discharge the resins, essential oils, and other substances they have secreted. They are often visible (for example, in the leaves of citrus or St. John’s wort) as light, translucent spots. In a number of cases they are long (most often in the stems and roots) and tubular, and for the most part they are connected to one another. Sometimes internal glandules are single cells or groups of cells in which the substances they have elaborated accumulate (most often essential oils). The secretions of glandular tissues may have practical application (for example, oleoresin and essential oils). Some of them are poisonous.