11.1.2 Origin of fruit tissues

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The generic term ‘fruit’ covers a wide range of structures, all supporting and protecting seeds, but where the various parts have developed from the original fertilised flower in various distinctive ways. In the simplest form, ovary walls grow along with seeds, and as they develop, the ovary walls dry out to become a pod (legume) or capsule (poppy). In others (particularly fleshy fruits), the main structure can arise by exaggerated development of a particular part of the original floral unit. These include ovary wall or central axis, the receptacle that supports anthers and ovary, or even petals and sepals. In morphological terms, fruits are structures that develop from fertilised or stimulated ovules, plus associated floral parts that originate from the parent plant.

Mechanistically, a fruit is a single dispersal unit which includes seeds and associated tissues, developed as a single body. This broad description includes structures derived from a single ovary (as in simple fruits such as apple, avocado and mango) as well as compound fruits where separate ovaries are joined (an aggregate fruit such as blackberry and cherimoya) or where separate flowers are collected into a single structure (pineapple and breadfruit). A detailed discussion on fruit structure and classification is given in Spjut (1994).

During fruit development, an ovary wall becomes a pericarp: either dry as in a dehiscent pea pod and the indehiscent caryopsis of barley, or fleshy as in berries (grapes). Three morphologically distinct strata are present and developed to varied degrees: exocarp (fruit skin), mesocarp (fruit flesh) and endocarp (inner cell layers).

An exocarp will develop a cuticle and may exhibit a variety of morphological features such as coarse hairs (kiwifruit) or fine hairs (peach). Exocarp plus cuticle restrict gas exchange, and determine general appearance of ripening fruit. Most cuticles are highly impermeable to gases, so that water vapour, O2 and CO2 diffuse mainly via either stomata or lenticels or by mass flow through cavities at the calyx and stem ends of fruit.

Mesocarp tissues usually represent the fleshy part of a fruit, and commonly hold chloroplasts and starch grains. In fleshy fruits such as berries (e.g. tomato, kiwifruit and grapes) this tissue typically comprises large parenchyma cells and contains the main vascular network.

Endocarps are less common, but typically develop as a dense hard case around a seed, as in peach, apricot or macadamia.