ily dislodged platelets. (No citrus fruit is naturally shiny; the shine demanded by retail customers has to
be applied as some form of approved wax or resin after washing, an operation that dislodges much of the
natural nonshiny wax.) The cuticle is penetrated by numerous stomata, except in a narrow (ca. 3 mm) area
around the calyx.
The albedo, or inner mesocarp (Figure 1A, top right), consists of a loose network of parenchyma-
tous cells with large airspaces formed when small, originally spherical albedo cells retained their orig-
inal points of contact as the fruit expanded. Thickness of the albedo can range from as little as 1 to 2
mm in some limes and tangerine hybrids to 2 cm or more in large shaddocks (pummelos, Citrus
grandis).
The edible flesh of a mature citrus fruit is divided into segments, each derived from an ovary locule.
The number of segments varies widely but is typically between 10 and 15. Each segment is surrounded
by a tough endocarp membrane and filled with tightly packed juice sacs or vesicles (Figure 1A, lower
right, and C). Each of these thin-walled juice sacs has a minute oil gland in its center and is attached by a
fine stalk to vascular bundles in the radial segment walls. Except in parthenocarpic fruit, seeds are within
the segments and attached to axial vascular bundles. Despite various varieties being sold as “seedless,”
few except navel oranges and Persian (Tahiti) limes (Citrus latifolia) are truly seedless. Purists prefer the
term “sparsely seeded,” for which citrus dealers show no enthusiasm at all.
The vascular system is a highly ramified network whereby every cell is connected to, or adjacent to,
a cell in contact with a particular sector of the vascular system. In many types of citrus fruits, particularly
seedless grapefruit and tangerines, the central “core” bundles separate as the fruit matures, leaving a con-
siderable cavity in the center of the fruit (a complication in specific gravity separation of freeze-damaged
fruit). For a more detailed discussion of citrus fruit anatomy, see Soule and Grierson [34].
- Drupe
Drupes start out as though they were going to be berries but then develop their typical hardened “pit.” The
resultant fruit is technically described as a “simple fruit with soft exterior, fleshy, usually indehiscent,
with heterogeneous texture and the center with a hard, bony, or cartilaginous endocarp enclosing the seed
proper” [1].
The most familiar drupe fruits are peach (and its genetic recessive, nectarine), plum, cherry, and apri-
cot, and in the tropics, the mango. In all of these fruits, the edible portion is the fleshy mesocarp. Other,
less obvious drupe fruits are coffee, in which the fleshy mesocarp (though edible) is discarded. It is an
anomalous drupe, having two seeds enclosed in a parchment-like endocarp, the seeds being the “coffee
beans” of commerce. Other drupes grown for their seeds are almond and pistachio. The most atypical of
all drupes is the coconut, in which the dry, fibrous epicarp and mesocarp become the husk (the source of
coir fiber used in brushes, matting, and rope). The large seed has edible white oily flesh and a liquid en-
dosperm (the “coconut milk”).
The epidermal characteristics among drupe species vary widely from the smooth epicuticular wax of
the cherry, mango, or date to the “fuzzy” epidermis of the peach, whose soft “hairs” are extensions of epi-
dermal cells. Particularly striking is the white “bloom” on the surface of some drupes, particularly plums.
Electron microscopy shows this to be formed from an amorphous wax layer adjacent to the cuticle proper,
together with crystalline granules of wax protruding from the surface. It is easily brushed off in routine
handling. This is considered undesirable [35].
Drupe crops can be of purely temperate-zone origin with specific winter-chilling requirements
(peach, plum, cherry, apricot) or purely tropical (mango, date). Intermediate is the pistachio, which has a
brief winter chilling requirement but very limited freeze hardiness [36].
C. Typical Fruits from Inferior Ovaries (Epigyny)
In flowers of epigynous fruits, the other major floral parts, sepals, petals, and stamens, are fused at their
bases and located above the ovary. As such fruits develop, nonovarian tissues become intrinsic parts of
the fruit. It is often very difficult to discern ovarian from nonovarian tissue.
- Pome Fruits
All the pome fruits are members of the Rosaceae family, for example, apple, pear, quince, medlar,
hawthorn, and the tropical loquat. A pome is defined as a fruit in which the papery or cartilaginous en-
FRUIT DEVELOPMENT, MATURATION, AND RIPENING 147