The functions of fruits in the angiosperms are mainly in relation to reproduction by which plants perpetuate their species. The fruit is in fact described as a reproductive organ of plants just like the flower and seed. However, the functions that fruits perform in reproduction are indirect. (Click on to read What is a Fruit?)
1. Fruits protect the seeds. The fruit serves as a physical barrier between the seed or seeds and the external environment during seed development. The developing fruits (ovary) promote ovule or seed development by preventing dessication and ensuring moist environment for the embryo.
In some species, fruits cause seed dormancy or prevent premature germination. It was shown that fruit tissues prevent the germination of developing seeds (termed precocious germination) in tomato (Berry and Bewley 1992). This further ensures seed development inside the fruit.
But it appears that fruit and seed development must be well synchronized. In sweet cherry and peach, early fruit ripening results to abortion of the embryo. It was suggested that during ripening the fruit extracts large amounts of photosynthates from the seed endosperm. By the time that the young embryo commences development, its source, the endosperm, is already depleted of its stock of nutrients and energy (Ryugo 1988).
In addition, the pericarp or fruit wall hinders easy access to seeds by some insects and other herbivores. Some fruits also contain toxic substances or the outer exocarp is supplied with thorny projections which serve as anti-herbivory defenses.
2. Fruits aid in the dispersal of mature seeds. For example, coconut nuts float in water and are thus transported to distant places; some fruits are part of the diet of certain animals like the civet cat (Paradoxurus hermaphroditus) which eats ripe coffee berries and birds which feast on colorful fruits and then defecate with the seeds still intact; some fruits explode to catapult the seeds or are supplied with appendages like parachute, wings and hooks that aid in seed dispersal.
Fruits, as well as the seeds inside (in the angiosperms), are formed through a process called double fertilization. Some of these fruits have been used as distinct organ in producing new plants either naturally or deliberately. With the use of such plant organ, the method of reproduction is still considered sexual. This takes exception to the established rule that in the flowering plants or angiosperms, sexual method of conventional propagation is accomplished with the use of seeds only, with rare exceptions as in apomictic seeds.
I admit that until recently, I held to the rule that sexual method of conventional plant propagation consists of using a seed or spore as planting material. The seed applies to the gymnosperms and angiosperms while the spore is in ferns and allies. There was simply no reason to think otherwise or to make any rethinking.
But read the following narration leading to the realization that plant propagation is also one of the functions of fruits in some species :
One day, I was conducting a review-demo on the basic concepts and techniques in plant propagation, specifically sexual vs. asexual propagation, seed vs. spore propagation, orthodox vs. recalcitrant seeds, and natural vs. artificial asexual propagation. Occasionally, I gave specific crop examples including their scientific names. For sexual propagation by seed, I mentioned some vegetable crops, grain legumes, fruit crops such as mango (Mangifera spp.), durian (Durio zibethinus), jackfruit (Artocarpus heterophyllus), soursop (Annona muricata)...
... cereal crops such as corn or maize (Zea mays) and rice (Oryza sativa), plants under family Palmae or Arecaceae such as coconut (Cocos nucifera), buri (Corypha elata), fan palm or anahaw (Livistona rotundifolia), fishtail palm (Caryota spp.) ...
Then, suddenly and from out of nowhere, that big question mark. Is plant propagation not a function of fruits? And... Wait a minute! Are those propagation materials called kernels in corn and grains of rice as well as the coconut “nut” seeds? ... No, botanically they are not!
The kernels and grains in cereals are fruits or contains fruits of the type caryopsis while the "nut" of the coconut palm complete with the husk covering is a drupe (click here to read relevant page). These kernels and grains and the coconut nuts, as well as true nuts and the nut-like, kidney shaped fruit of cashew (Anacardium occidentale), are used as planting materials intact, without removing the outer pericarp. In caryopsis, the parts of the fruit wall or pericarp (consisting of the exocarp, mesocarp and endocarp) are thin. They are fused with and are inseparable from the inner seed coat.
Therefore, the use of these plant organs as planting materials intact means that the propagules used are botanical fruits rather than seeds. Consequently, it is clear that one of the functions of fruits is to serve as a natural propagule or planting material in some crop plants like those belonging to cereals and some palms.
However, the new plant does not actually form from the fruit but from the seed, specifically from the embryo within the seed. This means that although the plant organ used as propagule is the fruit, the method of propagation is still considered as sexual propagation.
Now on the question What plant organs are used in sexual propagation of plants? It appears that the answer should be: seed, spore, and fruit.
Additionally, some “seeds” used as planting materials are not entirely seeds. Examples are some palm seeds with the mesocarp (husk) removed but with the endocarp (shell) that sorrounds the seed retained. The planting material for pili nut (Canarium ovatum), after removing the fibrous mesocarp, consists of a botanical seed and a hard shell which is an endocarp.
Addendum (more on the functions of fruits): The seedless fruits of the cactus plant teddy-bear cholla (Opuntia bigelovii), after falling to the ground, can grow into new plants having identical genotypes (clones) of the parent plant (Moore et al. 2003).
(Ben G. Bareja 2012, edited Nov. 27, 2013)
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Introductory account leading to the making of a rice paddy on land unfavorable for lowland farming.