Lecture 24

Fruits

I.  Concepts and Classification

A. Definition of a fruit: a mature gynoecium plus any extracarpellary parts united with it during the fruiting phase. Major part of most fruits is the ovary which expands greatly during fruit development. In some fruits, extracarpellary tissues are also important (e.g. accessory fruits) and they include the receptacle, calyx, bracts, and hypanthium.

B. Parthenocarpy – the development of fruits without fertilization. Happens naturally and induced by humans. Examples: banana, fig, melon, pineapple, tomato, citrus, and pepper. Pollination (but not fertilization) may be enough to stimulate fruit production.

C.  Seed vs. fruit.  Functionally, what is the dispersal unit (diaspore)? In some cases a seed, in others a fruit. The morphological distinction between these may be small.  For example the sunflower “seed” is a fruit (cypsella) and the strawberry “seed” is a fruit (achene).

D.  Fruit classification.  To fully understand fruit structure, one must understand the flower that produced it.  See the PLB 304 web page for fruit types linked to photos of examples.  The following floral characters are important in determining the morphology of fruits (modified a bit from Esau).
1.  carpels free (apocarpous) or fused (syncarpous)
2.  ovary superior (hypogynous) or inferior (epigynous)
3.  floral cup (hypanthium) present or absent
4.  Combining these in various ways generates some of the floral variants seen in Figure 22.1.  Note that some combinations do not occur, such as the apocarpous condition in an epigynous flower.
5.  The mature fruit has general descriptors of moisture level (dry vs. fleshy) and whether it opens at maturity or not (dehiscent vs. indehiscent).

E.  The fruit wall consists of a pericarp = the wall of the mature ovary.  The fruit wall may include extracarpellary tissues as well. Pericarp consists of:
1.  Exocarp (epicarp) – outermost cell layer or layers.
2.  Mesocarp – the inner, often fleshy layers
3.  Endocarp – the innermost cell layer or layers

II.  Fruit Types

A. Dry fruits
1.  Dehiscent.  May derive from gynoecium composed of one to many carpels.
a.  Follicle.  Not discussed specifically in Esau, but an important fruit type.  Seen in a number of families such as Magnoliaceae, Ranunculaceae and Crassulaceae.  A dry fruit derived from an apocarpous gynoecium that opens (dehisces) upon maturity.  Examples: Aquilegia, Eranthis, Magnolia.

b. Legume. Specific fruit for Fabaceae.  Derived from one carpel. Commonly called a “pod” as in this pea (Pisum).  Dehisces along both the suture and median vein sides (Figure 22.2 A-C, Figure 22.4 B). Wall of the soybean legume shown in Figure 22.3.  Cells oriented in different directions, thus putting stesses on the walls that promote opening on drying.  In Phaseolus (green bean) there is an outer parenchyma layer and an inner layer from the inner epidermis. These are separated by a sclerenchyma layer.  On opposites sides are the median and lateral carpellary bundles with smaller vascular bundles in between.

c.  Siliques and silicles (photo comparing two).  Specific fruit types of Brassicaceae. Two carpels with replum (false septum or partition) in between.  Replum is derived from the placenta.  Figure 22.4 C. The silique in Cardamine hirsuta is explosively dehiscent (images).

d. Capsule = a dry, dehiscent, multicarpellate fruit. As shown in Figure 22.2 G-I, if it opens along the sutures it is called a septicidal capsule (example: Yucca). If it opens at the locules, a loculicidal capsule (example: Populus, Lagerstroemia).  Capsules can also be poricidal (example: Papaver) and circumsissile (example: Amaranthus, also called a pyxis).

2.  Indehiscent. Also derived from gynoecium composed of one or many carpels, but does not open at maturity to release the seeds.  Often one carpel with only one ovule, although originally there may be more than one ovule and others are lost via abortion such that the fruit has one seed. A good example of this is the acorn from oak (Quercus).  It has a 3-carpellate, sycarpous gynoecium, but only one ovule develops inside each nut.
a. Achene.  Not discussed specifically in Esau, but a very common dry, indehiscent fruit type.  Seen in families  Ranunculaceae and Rosaceae.  Derived from an apocarpous (hypogynous) carpel. Examples: Ranunculus, Geum, Anemone, Rosa.

b.  Cypsella.  Specific fruit for Asteraceae – like an achene but from an inferior ovary. Examples: Liatris, Bidens, Verbesina. Two carpellate, but just one ovule shared between the two carpels. The ovule had just one integument, which disorganized and compressed when present as the seed coat in the fruit.  Figure 22.5.

c.  Caryopsis.  Specific fruit for Poaceae. Image of "the big three".  Here the seed coat is fused with the pericarp of the ovary. Figure 22.6, for wheat (Triticum). Image1 showing cross section of entire caryopsis, image2 of pericarp and endosperm. The pericarp has an outer epidermis with cuticle, parenchymatous inner layers, and an inner epidermis modified into tube cells.  The seed is composed of a seed coat whose outer integument disintegrates and the inner one becomes very compressed. Just inside the seed coat is the aleurone layer which is the outermose layer of the endosperm.  It is rich in lipids, proteins and enzymes required for germination (e.g. a-amylase to break down starch into sugar). Processed (white) wheat flour is mostly starch, having removed the pericarp (bran), nucellus, aleurone and embryo (wheat germ).

d. Schizocarp. A multicarpellate fruit that breaks apart into multiple mericarps, but the seeds are not dehisced from these. Because it is the carpels that are the dispersal units, not the seeds, this fruit is technically indehiscent. Examples include the fruit from Apiaceae. Image of fruits from several species. Figure 22.8 of Carum, image of fruit X.S. and of Oxypolis. The central carpophore branches at the top where it connects to the individual mericarps.

B.  Fleshy fruits.  Entire pericarp, parts of the pericarp, as well as extracarpellary tissue may become fleshy.

1.  Fruits with a rind.  Examples banana, Cucurbitaceae, etc.

a.  Hesperidium. Fruit (image) specific to the genus Citrus (Rutaceae).  Pericarp (Figure 22.9, image1, image2) composed of:
• exocarp (flavedo) – yellow/ orange in color with oil glands (schizogenous cavities), crystal-containing cells.
• mesocarp (albedo) – white spongy tissue with aerenchyma
• endocarp – produces the juice sacs that grow into the carpellary locules (image).

b. Pepo.  Fruit specific to family Cucurbitaceae.  Derived from an inferior ovary (L.S. female flower of Cucurbita pepo). Cannot distinguish carpellary and extracarpellary tissues – completely fused (congenitally).  Figure 22.10 and image of Cucumis sativa (cucumber) fruit in cross section. Hand cross section of Cucurbita pepo, Placentation appears superficially to be parietal, but the presence of inverted lateral bundles refutes this.  Figure 22.11C gives an interpretation of how this carpel structure originated. The margins of the carpels are inverted twice.

c.  Banana (Musa acuminata).  Derived from a flower with a 3-carpellate, inferior ovary with axile placentation (image of flower in L.S.). Note error in Esau – she says ovary is superior.  (Figure 22.12 A, B).  Cultivated dessert banana has parthenocarpic fruits – the ovules disintegrate. But of course, wild bananas produce fruits with seeds (image). The locular space is filled with pulp derived from the pericarp.

2. Fruits without a rind
a.  Berry. A berry is a fleshy fruit derived from a syncarpous gynoecium. For tomato (Figure 22.12C, Solanum lycopersicum) most of the outer fruit wall is pericarp. The placenta fills the locule in the young fruit (image), but as the fruit matures, the placenta degrades into a gelatinous material via cellulases (image).  Upon ripening, chloroplasts converted to chromoplasts (image).

b.  Drupe.  A fleshy fruit derived from a single carpel.  In Prunus (cherry, Rosaceae, subfamily Prunoideae), the “skin” is the exocarp, the mesocarp and endocarp are fleshy and the inner epidermis differentiates into a stony layer (Figure 22.13, image of "pits").  The stony layer has sclereids with different orientations. Image of L.S. of drupe. Closer view of the pericarp.

c.  Drupelets. Fleshy fruits derived from the apocarpous carpels of one flower. The fruit is called an aggregate.  Example: Rubus (blackberry, raspberry; Rosaceae, subfamily Rosoideae). Like Prunus, with a fleshy endocarp, except here the stony part is the endocarp. Figure 22.14, image of druplet, cross section through entire aggregate of druplets, section of one druplet.

d. Pome.  Fruit type specific to Rosaceae, subfamily Maloideae (e.g. apples, Malus and pears, Pyrus). From flower with inferior ovary – hence carpellary and extracarpellary tissue involved in making up the bulk of the fleshy fruit (to a small degree, hypanthium too).  This is the appendicular origin interpretation. Receives support from plant anatomy where vascular bundles of calyx and corolla are seen within a common tissue with carpellary vascular bundles
• Pyrus. Pear flower in L.S. and another stained.  Image of P. calleryana pomes in X.S. and L.S. Image of Pyrus communis pome X.S.  Figure 22.15. Brachysclereids abundant in Pyrus (pear, Figure 22.17).
• Malus. Image of apple pomes in X.S. and L.S. Figure 22.16. Note the median and lateral carpellary bundles can be seen, the later converging at the center of the fruit. Most of the fruit is parenchymatous, except for the cartilaginous endocarp tissue lining the locules (composed of sclereids). 
Last updated: 14-Oct-22 / dln