A locule is a term used in biology to describe a small chamber or compartment within an organ or structure. The word itself comes from the Latin loculus, meaning “little place.” While broadly applicable to any small, enclosed space, its most common usage is within botany. Specifically, a locule refers to the internal chamber found within the reproductive structures of flowering plants, where seeds or pollen develop.
Locules in Plant Reproductive Anatomy
The most frequent application of the term locule in botany is to describe the chambers within a flower’s ovary, which is the enlarged basal part of the female reproductive organ, the pistil. After fertilization, the ovary matures into the fruit, and its locules become the compartments that house the developing ovules, which ultimately become seeds. The number of locules within the ovary is a defining characteristic used to classify fruits.
Ovaries and their resulting fruits can be categorized based on the number of these compartments. A fruit with a single chamber, such as an avocado or a cherry, is called unilocular. Fruits with multiple divisions are termed multilocular. Tomatoes, for example, are typically multilocular, showcasing several seed-filled chambers visible when sliced open.
Citrus fruits, like oranges, also provide a clear example of a multilocular structure, where each easily separated segment represents a single locule. The locules protect the ovules and seeds as they mature. The structural organization of these chambers influences the final size, shape, and internal arrangement of the mature fruit.
Structural Features That Define a Locule
The formation of distinct locules within a single ovary is determined by internal partitions that create the separate chambers. These dividing walls are known as septa, with the singular form being septum. When multiple carpels—the basic units of the pistil—are fused together to form a compound ovary, the septa are the resulting walls separating the locules.
The ovules within the locule are attached to specialized regions of tissue called the placenta. The way the placentae are positioned relative to the locules is termed placentation, which is another significant feature in plant classification.
In ovaries with multiple locules, such as those found in lilies or daffodils, the placentae are often located at the center where the septa converge, a pattern known as axile placentation. In contrast, some ovaries may have parietal placentation, where the ovules are attached to the inner walls of the ovary, often resulting in a single, large locule. The precise architecture of the septa and placenta dictates the final spatial arrangement of the seeds within the fruit.
Additional Contexts for the Term Locule
While the ovary is the most common context, the term locule is also used to describe other distinct chambers in plant anatomy, particularly in the male reproductive organs. The anther, which is the part of the stamen that produces pollen, contains internal compartments that are also called locules. These chambers, often referred to as pollen sacs, are where the pollen grains develop and are stored before being released for pollination.
Beyond flowering plants, the concept of a locule extends to the study of fungi, specifically in a group called Ascomycetes. In mycology, a locule is a chamber found within a fungal fruiting body, such as an ascostroma, where the spore-bearing structures known as asci are formed.
The term also appears infrequently in human and animal health contexts, where it may be used to describe a small, pathologically-formed cavity. For instance, a multiloculated cyst is a medical term for a fluid-filled sac that is divided into many smaller compartments by internal septa.