Instant Notes: Plant Biology

differential drying in the fruit wall leads to an explosive break and twisting of

the two halves of the fruit, dispersing the seeds.

Indehiscent fruits Many fruits do not dehisce and these may be dry, as in grasses, or fleshy. In dry
indehiscent fruits the fruit normally resembles the seed testa and the fruit is
dispersed with the seed. The fruit consists solely of one or more layers of
sclerenchyma and parenchyma cells, this type being characteristic of grasses,
composites (daisy family) and some other families. It may include the ovary
wall and part of the receptacle or base of the flower which has become indistin-
guishable from it. In wheat this wall, along with what remains of the seed
integuments, is the bran.
Fleshy fruits have a more complex structure. Some have a rindquite separate
from the flesh (e.g. citrus fruits, banana) and most change color, texture and
chemical structure as they mature. As one example, citrus fruits (Fig. 2) have an
outer cuticle and epidermis over compact parenchyma with oil glands. Inside
the epidermis, the white tissue consists of parenchyma with large air spaces and
a vascular network, and another epidermal layer lies inside that. The edible
segments consist mainly of juice-filled sacs derived from the inner epidermal
cells. Other fleshy fruits have no separate rind, such as cherries and tomatoes,
and here there is an epidermis and elongated fleshy cells underneath and, in
some, an inner lignified layer around the seed. In apples and pears the flesh is
derived from the receptacle, while other fruits are derived from the base of the
sepals and petals. These grow around the ovary, the ovary itself becoming the
core. In the fruits deriving from inflorescences, petals, sepals and flower stalks
all swell to aggregate together as the dispersal unit with fleshy cells mixed in
with harder epidermal cells and other types.
Many fleshy fruits are green and photosynthetic, either when immature or
at maturity (e.g. cucumbers), from chloroplasts in the outer parenchyma or
epidermis. These may change to become colored chromoplasts(Topic B3) and
produce anthocyanins or carotenoids (Topic D1) as the fruit matures. Other
chemical changes involve the production of sugars from more complex poly-
saccharides. In many plants, the maturation of the fruit involves a fusion of the
floral parts that have given rise to them.

Dispersal Most dehiscent fruits open with a slitorporesand the seeds simply drop out.
Such a fruit, without any specialized structures for dispersal, is the commonest
type. Many dehiscent fruits of all sizes are like this, but some have a nutritious

D4 – Fruits 53

Ovules

Line of weakness

Remains of style

(a) (b)

Fig. 1. Dehiscent fruits: (a) Bean derived from one carpel; (b) transverse section of fruit of

Arabidopsisderived from two fused carpels.