Most fruits produced by flowering plants (angiosperms) protect their seeds by encasing them within a fleshy or hard exterior. This protective structure develops from the ripened ovary of a flower, safeguarding the next generation and facilitating dispersal. This standard model, however, is not universal across the plant kingdom. A small number of plants exhibit a botanical deviation where the seed or the true fruit appears to reside on the outside of the edible structure. These exceptions challenge the common perception of what a fruit is.
The Strawberry: An Accessory Fruit
The garden strawberry is arguably the most familiar example of this phenomenon. The tiny, speck-like objects on its red surface are often mistaken for seeds, but they are actually the plant’s true fruits, known botanically as achenes. Each achene is a small, dry fruit containing a single seed encased within a hard outer wall. A single medium-sized strawberry can bear approximately 200 achenes on its exterior.
The soft, juicy, red portion that people commonly eat is not derived from the flower’s ovary, which defines a true fruit. Instead, this fleshy part develops from the receptacle, the enlarged end of the flower stalk. As the flower matures after fertilization, this receptacle tissue expands significantly, becoming the bulk of the strawberry.
Because the edible part originates from tissue other than the ovary, the strawberry is classified as an accessory fruit. It is specifically an aggregate accessory fruit because it forms from a single flower with multiple separate ovaries, each maturing into an achene. The achenes are superficially embedded in the enlarged receptacle, creating the illusion that the seeds are external. The receptacle’s red color and sweetness attract animals to consume the structure and disperse the true fruits (achenes).
Other Examples of External Seeds
Another prominent example is the cashew. The fleshy, pear-shaped structure known as the cashew apple is the swollen pedicel, the stalk connecting the flower to the main stem. This juicy, brightly colored structure is an accessory fruit, similar to the strawberry’s receptacle.
The true fruit of the cashew tree is a hard, kidney-shaped structure that hangs from the bottom tip of the cashew apple. This appendage is a single-seeded drupe, and the seed inside is commercially known as the cashew nut. The nut is the true fruit, growing entirely outside the fleshy cashew apple stalk.
A distinct botanical example involves the Yew tree. Yews are conifers and do not produce true fruits, but their seeds are presented externally. The toxic seed is partially enclosed by a fleshy, cup-shaped structure called an aril, which is a modification of the seed coat, not a fruit. The bright red aril is open at the tip, leaving the hard, dark seed clearly visible and exposed.
Understanding the Botanical Distinction
The classification of fruits depends entirely on the floral tissue from which the mature edible portion develops. A “true fruit” is strictly defined as a structure resulting solely from the maturation of the flower’s ovary following fertilization. This process involves the ovary wall developing into the fruit wall, which encloses the seeds.
In contrast, an “accessory fruit,” sometimes referred to as a false fruit or pseudocarp, incorporates tissue from other parts of the flower outside of the ovary. This non-ovarian tissue, such as the receptacle or pedicel, swells and becomes the primary fleshy, edible part. The seeds or the true fruits remain technically separate or embedded within this accessory tissue.
The structures seen on the surface of a strawberry are achenes, a type of simple, dry true fruit derived from the individual ovaries. The key developmental difference is that the flower’s receptacle grows dramatically beneath these ovaries, pushing them to the surface. This mechanical distinction results in the seeds appearing external to the part that attracts consumers.
The cashew demonstrates a similar principle but with a different floral structure: the pedicel swells to form the cashew apple, while the actual fruit develops separately at its base. Understanding these botanical distinctions clarifies why these examples deviate from the typical fruit structure. The external appearance of the seeds is a consequence of the plant’s evolutionary strategy to utilize non-ovarian tissues for seed dispersal.