Seeds & Fruits Dispersal

Photo courtesy of The Blue Tree CoOpens in new window

The Seed

PollinationOpens in new window may be followed by fertilization, which is the fusion of the male sex cells (male gametes) in the pollen and the female sex cell or ovum (female gamete) in the ovule to produce a new embryo plant contained in a seed. The seed itself is formed from the ovule of the flower and enclosed in the fruit which is formed from the flower’s ovary.

The seed (Figure X-1), resulting from sexual reproduction, creates a new generation of plants that bear characteristics of both parents.

The plant must survive often through conditions that would be damaging to a growing vegetative organism, so the seed is a means of protecting against extreme conditions of temperature and moisture, and is thus often the overwintering stage. The seed, together with the fruit, may also enable the embryo to be dispersed away from the parent plant and may have dormancyOpens in new window mechanisms which prevent germinationOpens in new window until conditions are favorable.

Figure X-1 Seeds: a range of species. Top: runner bean; left to right: leek, artichoke, tomato, lettuce, Brussels sprout, cucumber, carrot, beetroot | Source: Grovida GardeningOpens in new window

Seed structure

The basic structure of a dicotyledonous seed is shown in Figure X-2. The main features of the seed are discussed below.

Figure X-2 Dicotyledonous seed structure: (a) germinating Phaseolus coccineus (runner bean) seed showing developing radical; (b) long section of a Phaseolus vulgaris (French bean) seed | Source: Grovida GardeningOpens in new window

1.   Embryo

In order to survive, the seed must contain a small immature plant protected by a seed coat. The embryo consists of a radicle, which will develop into the root of the seedling to take up water and nutrients, and a plumule, which develops into the shoot system, bearing leaves for photosynthesizing and flowers for seed and fruit production.

The region between the cotyledons and the radicle is the hypocotyls while the short length of stem between the cotyledons and the shoot is termed the epicotyls. A single cotyledon will be found in monocotyledons, while two are present as part of the embryo of dicotyledons.

The cotyledons may occupy a large part of the seed, such as in Vicia faba (broad bean), and act as the food store for the embryo. In other seeds, such as Zea mays (sweetcorn), the single cotyledon remains small and the food store is provided by another tissue called the endosperm which is not part of the embryo.

2.   Testa

The testa, also known as the seeed coat, is formed from the outer layers of the ovule after fertilization. It is waterproof and airtight and may contain germination inhibitors which enable seeds to stay dormant over winter.

3.   Micropyle

The micropyle is a weakness in the testa where water uptake occurs triggering germination.

4.   Hilum

The hilum is a scar on the test where the seed was attached to the fruit.

Food Storage in Seeds

In some species, such as grasses and Ricinus communis (castor oil plant), the food of the seed is found in a different tissue from the cotyledons, which is called the endosperm.

Plant food in either cotyledons or endosperm is often stored as the carbohydrate starch, formed from sugars as the seed matures — for example, in peas and beans.

Other seeds, such as sunflowers, contain high proportions of fats and oils, and proteins are often present in varying proportions. These substances store energy in a very concentrated form, which is released through the process of respiration when the seed germinates, fuelling rapid growth. The seed is also a rich store of nutrients, such as phosphate, which it requires for seedling growth. This explains why seeds are such a useful foodstuff for humans too.

The Fruit

The development of a fruit involves either the expansion of the ovary into a juicy succulent structure, or the tissues becoming hard and dry. Fruits provide a means of protection and often a means of dispersal for the seeds they contain and may also contribute to delayed germination through dormancy.

Some dry fruits split to release their seeds (described as dehiscent) while others rely on the fruit coat beign broken down to release the seeds (described as indehiscent) (Figure X-3). Some methods of dispersal include:

1.  Explosive (or Self-Dispersed)

The fruit splits open propelling the seeds into the air, e.g. Cytisus (broom), Lupinus (lupin), Lathyrus odoratus (sweet pea), Erysimum (wallflower), Lunaria annua (honesty), Cardamine hirsute (hairy bittercress) and Geranium.

2.  Wind

The seeds of poppy capsules (Papaver) are shaken from small pores in the fruit as the plant sways like a church censer.

Other fruits have tiny feathery parachutes attached as in Epilobium (willow herb), Clematis and many members of the daisy family including Senecio (groundsel), Taraxacum (dandelion) and Cirsium (thistles). Many woody species such as Tilia (lime), Fraxinus (ash), Acer (sycamore and maples) produce winged fruit.

3.  Animals

Mammals and birds can distribute fruits either externally or internally. Hooked fruits, e.g. Galium aparine (goosegrass) and Arctium (burdocks), become attached to animal’s fur. The sticky succulent fruits of Viscum album (mistletoe) attach to birds’ beaks and are rubbed off onto trees where they germinate.

Squirrels may bury fruits such as nuts, e.g. Quercus robur (oak), Fagus sylvatica (beech), Castanea sativa (sweet chestnut), in the ground far from where they were collected (scatter hoarding).

Succulent fruits, e.g. Solanum lycopersicon (tomato), Rubus fruticosus (blackberry), Prunus spinosa (sloe), Viburnum and Sambucus nigra (elderberry), or those that are filled with protein, e.g. Rumex (dock), are eaten by birds and other animals, the seeds passing through their gut before being deposited elsewhere (frugirory).

4.  Water

Many aquatic plants such as Nymphaea (waterlilies) or those growing close to rivers and seashores use water to disperse their fruits. The fruits of coconut palms can travel thousands of kilometers in ocean currents.

The introduced weed Impatiens glandulifera (Himalayan balsam) has an explosive mechanism to disperse its seeds but they are also spread along waterways where they have become a serious threat to biodiversity in Britain and Ireland.

Some examples of fruits types and dispersal methods are illustrated in Figure X-3.