Handbook of Plant and Crop Physiology

B. Typical Fruits from Superior Ovaries (Hypogyny)

1. Grape

The grape is the simplest of hypogynous fruits and one that conforms exactly to the botanical definition
of a berry. Remnants of floral parts other than the ovary are absent or vestigial and the developed ovary
tissue is fleshy, succulent, and homogeneous.

2. Hesperidium

The hesperidium is the highly specialized form of berry specific to citrus fruits. (Etymologically, the term
hesperidiumis a misnomer based on the assumption that the “Golden Apples of the Hesperides” in Greek
mythology were oranges. However, citrus, as the etrog. Citrus medica, did not reach the Mediterranean
area until historical times [33].)
The hesperidium (which is derived entirely from the ovary) has several sharply defined tissues (Fig-
ure 1). The usually five-lobed calyx remains attached unless the fruit naturally abscises; then it remains
attached to the bearing branch.
The outer layer, or peel, includes the pigmented flavedoand the white or colorless albedo. The
flavedo (Figure 1A, top left) consists of the epicarp proper, hypodermis, and the outer mesocarp. Em-
bedded in it are the so-called oil glands, containing “essential oils” specific for each citrus species or hy-
brid. These are principally terpenes (mainly d-limonene) and are highly toxic to surrounding tissue if ex-
truded due to rough handling of the fruit. The cells of the single-layered epicarp contain green chloroplasts
that metamorphose into chromoplasts as the fruit degreens. Over the epicarp is the intact cuticle (Figure
1A, lower left), composed largely of cutin, and over it an outer layer of epicuticular wax deposited as eas-

146 GRIERSON

Figure 1 Citrus fruit: (A) transverse section with enlarged views of the flavedo and cuticle on the left and of
the albedo and juice vesicles attached to the outer tangential and radial locule walls on the right; (B) longitudinal
section showing the lunate locules with seeds attached to the inner tangential wall next to the central axis; (C)
separate juice vesicles. (From Ref. 34.)