BIOLOGY OF FRUITS AND SEEDS

The inability of flowering plants to move from place to place is a serious disadvantage, since when fruits and seeds are shed they may alight near to one spot, so that when germination occurs the young plants are overcrowded and may also be overshadowed by the parent plant. Thus there is a competition for light and raw materials. In the great majority of cases this does occur, so that in the event of adverse conditions only those lucky enough to be best fitted to cope with them will survive. This natural ’weeding out ’process is largely counterbalanced by the large numbers of seeds which are usually produced. The chances of survival are increased where there is some special factor operating to spread the fruits and seeds, so that some at least will find suitable spots with suitable conditions for germination.

Before proceeding to deal with these factors it is necessary to distinguish between seeds and fruits and between true and false fruits. A ripe seed consists of an embryo plant enclosed by one set of coverings derived from the ovule wall during its development. If the embryo plant is lying within a fruit, then it will be enclosed by two sets of coverings, the outer set having developed from the ovary wall, the inner being the seed coat.

A seed will show only one scar, the place where it was attached to the ovary, whereas the fruit will usually show two, one where it was attached to the receptacle and the other of the remains of the style or stigma.

FRUITS

After fertilization the gynascium matures to form the fruit, and the sepals, petals and stamens wither and fall. The style and stigma also wither and the ovary wall alters, becoming either hard and dry, or soft and fleshy. Sometimes the receptacle grows after fertilization and becomes swollen and juicy. The fruit can be defined as the structure which develops as a result of fertilization, and contains the seeds. Fruits can be divided into three classes :—

Simple Fruits

These are formed from a gynaxium consisting of a single ovary, which may represent one carpel or several carpels joined together, I.e. syncarpous.

Aggregate Fruits

These are formed from a gynaxium consisting of several free carpels, I.e. an apocarpous gynascium.

Multiple Fruits

These are not formed from a single flower but from several, and other parts beside the ovary may have taken part in the formation of the fruit.

Simple Fruits

Simple fruits are either dry or succulent, according as to whether the pericarp becomes dry or juicy as the fruit ripens.

Dry Fruits.

Dry Indehiscent Fruits

Such fruits are single seeded, and the pericarp does not split to set free the seed.

Achene

A one-seeded dry indehiscent fruit with a tough, leathery pericarp ; e.g. buttercup, composites.

Nut

This has a hard woody pericarp, and is usually formed from two to three carpels ; e.g. hazel, oak, beech, sweet chestnut. The hazel nut is surrounded by a green leafy cup called a cupule. The cupule of the acorn is hard and formed of small scales, and that of the beechnut consists of four woody scales.

Samara

In this fruit the pericarp has become flattened to form a wing ; e.g. elm, ash.

Cypsela

This is similar to an achene, but is formed from two carpels ; e.g. composites.

Dry Dehiscent Fruits

The pericarp of the dry dehiscent fruit splits in order to set the seeds free. The fruits are classified according to the number of carpels.

Follicle

A dry many-seeded fruit formed of one carpel, and which splits down one side only- Single follicles are not common. The fruits of monkshood and marsh mallow consist of a collection of follicles.

Legume

A dry, many-seeded fruit formed of one carpel, which splits down both sides ; e.g. pea, bean.

Capsule

A dry dehiscent fruit formed from two or more carpels, and which may open in one of several ways. Some capsules open by longitudinal slits ; e.g. the willow by two slits, the violet by three and the willow herb by four. The capsule of the campion opens by ten slits, which extend only a short way down the ovary so as to form ten teeth. These teeth open widely on fine days and remain closed when it is wet. The seeds of the poppy capsule are set free by a ring of pores just beneath the top. In some capsules, e.g. plantain and pimpernel, the pericarp splits transversely, so that a lid is cut off. The fruits of crucifers are formed from two carpels, and are divided into two chambers by the growth of a partition or replum between the placenta?. Eventually the pericarp breaks away, leaving the seeds attached to the replum. A long capsule of this type is called a siliqua, e.g. wallflower ; while a short one is called a silicula, e.g. shepherd ’s purse.

Schizocarps

A schizocarp is a many-seeded dry fruit which breaks up into several parts, each called a mericarp, containing one seed. A mericarp may represent a whole carpel, e.g. mallow, nasturtium, or it may be a part of a carpel, e.g. bird ’s-foot, radish. The double samara of the sycamore is a schizocarp formed of two winged mericarps.

Succulent Fruits The drupe ; the berry ; false fruits.

Drupe

The pericarp of a drupe is divided into three layers :—

An outer skin, the epicarp.

A middle fleshy layer, the mesocarp.

An inner hard and woody layer, the endocarp, which forms the ’stone ’which encloses the seed.

Plums and cherries are examples of typical drupes. The walnut as sold in shops is part of a drupe, being the endocarp and seed only. The endocarp of the whole fruit is covered by a green fleshy mesocarp and a thin epicarp. The coco-nut, too, is a drupe with the epicarp and mesocarp removed. The epicarp is a thick tough skin which covers the fibrous layer of mesocarp. The shell of the coco-nut is the endocarp. The coco-nut seed is endospermic, the endosperm being partly the hard white tissue lining the shell and partly the liquid contained in the nut.

Berries

These are succulent fruits in which the whole of the pericarp is soft, and which have no hard stone formed of cndocarp as in the drupe. The tomato, orange and grape are examples formed from superior ovaries, while the gooseberry, red currant, banana and vegetable marrow are berries formed from inferior ovaries. The date is a berry with soft pericarp, but containing a hard stone which is the single seed.

False Fruits

Certain fruits are referred to as false fruits because the fruit does not consist of the ripened ovary only but is partly formed by the receptacle. Apples and pears are examples of the type of false fruit called a pome. If an apple is cut across transversely, the true fruit consisting of five membranous carpels is to be seen embedded in fleshy tissue which is formed by the growth of the receptacle. In the strawberry the receptacle is the red fleshy part which bears small achenes scattered over its surface. The receptacle of the rose fruit or hip is a red cup-shaped structure on the inside of which occur the achenes interspersed with hairs. The hawthorn fruit is a pome in which the carpel walls become hard and woody, so that there are one to five ’stones ’embedded in the swollen receptacle.

Aggregate Fruits

An aggregate fruit is formed from a flower which has several carpels, I.e. from an apocarpous gynacium. It is therefore a collection of simple fruits. An aggregate fruit may be either :—

A collection of achenes ; e.g. buttercup.

A collection of drupes ; e.g. blackberry and raspberry.

A collection of follicles ; e.g. aquilegia, marsh marigold.

Multiple Fruits

A multiple fruit is not formed from a single flower but from a whole inflorescence. Such fruits are not common, but examples are the mulberry, fig and pineapple. The mulberry looks like a large blackberry, but each small round portion is a drupe covered by the four perianth leaves, which have joined and become fleshy.

The Fig

The fig is formed from the end of the peduncle of the inflorescence, which is greatly enlarged and contains a pear-shaped cavity, opening by a narrow hole at the top. Within this cavity the staminate and pistillate flowers develop, and later the pistillate flowers ripen into small drupes.

The Pineapple

Each hexagonal marking on the surface of the pineapple represents a flower, every part of which helps to form the fruit. The central part of the pineapple is the much swollen axis of the inflorescence, which is continuous above the fruit as a tuft of leaves.

SEED DISPERSAL

Dispersal by Wind

Seeds which are dispersed by wind are either very small and light or possess hairs or wing-like structures which enable them to be easily carried by the wind.

Small light seeds ; e.g. orchid.

Seed Parachutes

The seeds of willow and willow herb have a tuft of hairs which act as a parachute. The cotton seed is covered all over with long white hairs.

Fruit Parachutes

In this case the fruit and not the seed is hairy and easily blown about by the wind. The achenes of dandelion and groundsel have a tuft of hairs called a pappus, which is formed by the growth of the calyx after fertilization. The achenes of clematis have a persistent hairy style. Winged Seeds

The seeds of the tropical plants belonging to the genus Bignonia have a flattened extension of the testa, which acts as a wing. The seeds of Pinus are also winged, but in this case the wing represents part of the scale on which the seed rests in the cone. Winged Fruits

The wing of the single samaras of the elm and the ash is a flattened extension of the pericarp of the fruit. The sycamore fruit is a double samara which separates into two winged halves before dispersal. The wing of the hornbeam fruit is formed by three persistent bracts which have become joined. In the lime there is a persistent bract attached for half its length to the main stalk of the inflorescence. The Censer Mechanism

This method of dispersal is well shown by the poppy. The small seeds ripen in a capsule at the end of a long stalk. When the wind blows the stalk sways and the seeds are shaken out through the pores of the capsule. Such a method of dispersal is also shown by the foxglove and campion.

Dispersal by Water

This is not a very common method of dispersal, for the seeds of most plants lose their power of germination after immersion in water. The white water lily is an example of a plant which spreads by the water dispersal of its seeds. The seeds of this plant have air spaces which enable them to float for some time before they become water-logged and eventually sink. The coco-nut of the Pacific Islands is a water-dispersed fruit. Protected by its hard shell and thick fibrous coat, it can drift in the sea and be carried by ocean currents to neighbouring islands where, after being cast up on shore, the seed can germinate.

Dispersal by Animals

Animals may carry fruits and seeds externally, fixed to their bodies by hooks, or internally, having swallowed them. Hooked Fruits

The fruits of goose-grass and enchanter ’s nightshade are schizocarps which are covered with small hooks. The achenes of geum each have a single hook, formed by the tip of the style. The hooks of the burdock burr are not formed on the fruit, but belong to the numerous bracts of the involucre. Each bract is hooked at the tip. The false fruit of agrimony has hooks on the calyx-tube. Succulent Fruits

These fruits are most frequently eaten by birds, which swallow the seeds as well as the juicy part of the fruit. The seeds are protected from the digestive juices of the animal by a hard testa or a woody endocarp, and may be carried some distance before being egested by the animal. The surface of a ripe succulent is usually brightly coloured so that the attention of animals may be attracted. Some berries have sticky seeds which adhere to the bird ’s beak until it cleans it, e.g. seeds of mistletoe. Other Methods

Seeds may become stuck to the feet of a bird or other animal by mud and be carried a long way from the parent plant before dropping off. Ants collect stores of certain seeds which have oily caruncles, e.g. violet and gorse. The ants do not eat the whole seed, but the caruncle only, and thus may be the cause of the dispersal of such seeds. Similarly squirrels occasionally forget the stores of nuts which they have made, so that some seeds may germinate after they have been carried some distance from the parent plant.

Dispersal by Explosive Mechanism

As the carpels of certain plants are ripening, strains are set up in the carpel walls. The sudden release of the strain leads to the forcible expulsion of the seeds from the carpels. The ripe capsule of the violet, for example, splits into three boat-shaped parts, each of which contains a row of seeds. As the fruit dries, the sides of each boat-shaped portion gradually close together, thus pinching the seeds, which are suddenly shot out one by one in order, from the top downwards. The plants of the Leguminosa; show a somewhat similar method of seed expulsion. The legume of the pea, for instance, first splits into two parts, to each of which several seeds are attached. Then as each part dries it becomes twisted, so that the seeds are eventually nipped and shot out.

In the Geraniums the fruit consists of five carpels with a long central style. When ripe the five carpels separate and the style splits from below upwards into five parts, which remain attached to the carpels below. As the parts of the style separate from one another, they curl up suddenly and so sling the seed out from each carpel.

Dispersal by Man

The seeds of many plants are unintention ally dispersed by man. For example, an important means of dis tribution of weed seeds is through the shipment of wheat and other commercial seeds from distant countries. The weeds growing amongst the plants of a crop are harvested with the crop, and if the weed seeds are about the same size as the crop seeds they are very difficult to remove and are therefore distributed with the crop. It is no doubt largely in this way that such European weeds as dandelion, sheep ’s sorrel, sow-thistle, plantains and wild carrots have reached South

Africa, New Zealand and the temperate parts of Australia and America.

Seeds may also be carried- from country to country in the ballast of ships, which often consists of stone or gravel. The plants of such seeds are to be found growing on the stone banks which are piled up at the sides of rivers and estuaries where ships unload their ballast. The packing of straw or hay round merchandise frequently contains seeds of various plants, and such plants spring up near railway sidings where the goods are unloaded. Another way in which seeds may be unintentionally dispersed by man is through the shipment of raw wool, which always contains the burrs and seeds of many weeds which have become caught in the sheep ’s coat.

SEEDS

Seeds are developed from fertilized ovules. The coats of the ovule become the seed-coat in which the micropyle persists. Inside the testa is a dormant embryo plant developed from the fertilized ovum together with a supply of food. The embryo consists of a small root, a rudimentary shoot and one or two seed-leaves. The cotyledons in some seeds are enlarged to store the food ; a seed having cotyledons of this type is said to be ex-endospermic. In other seeds the food is stored outside the cotyledon in a tissue developed by the division of a central nucleus of the embryo sac after its fusion with the second male nucleus of the pollen tube. This tissue is called the endosperm and seeds possessing it are called endospermic.

THE BROAD AND RUNNER BEAN SEEDS

Since the seed of the wallflower is small, it is convenient to study the structure of a larger seed, such as that of the broad bean. This seed is kidney-shaped, and consists of the embryo or young plant, which is covered by a tough protective coat called a testa. At the side of the seed is a narrow scar, the hilum, which marks the place where the seed has broken away from the stalk or funicle which attached it to the inside of the bean pod. Near one end of the hilum is a small bump caused by the presence of the radicle beneath the testa. Between this projection and the hilum is a small hole called the micropyle. It is difficult to see the micropyle in a dry seed, but if a seed which has been soaked for a day or two is dried and squeezed the position of the micropyle is shown by a drop of water which is exuded from the hole.

In order to study the structure of the embryo the testa must be removed from a soaked seed, when it is seen to consist of:—

Two cotyledons or seed-leaves, which are flat, oval bodies closely pressed against each other and joined at one point. The cotyledons contain a store of food for the developing embryo, which is mainly in the form of starch granules. The seed is therefore ex-endospermic.

The radicle, which is a short conical peg projecting from the cotyledons. It lies in a small pocket of the testa.

The plumule, which develops into the shoot of the plant. It is a small curved structure continuous with the radicle and lying between the cotyledons. Each cotyledon is joined by a very short stalk to the region in between the radicle and plumule.

Germination of the Broad Bean

The bean first absorbs water and swells considerably. The radicle then lengthens, and after bursting through the testa grows into a tap root on which four vertical rows of lateral roots develop. Meanwhile the short stalks of the cotyledons grow longer, so that the cotyledons are pushed slightly apart and the plumule is able to grow out. As the plumule grows its tip is bent over in the form of a loop, so that the delicate growing point is not damaged by the movement through the soil. Once above ground the shoot straightens out and the first green leaves commence to develop. The cotyledons remain below the ground, and as the food in them is gradually used by the growth of the root and shoot they slowly become smaller and shrivel. When the cotyledons remain below ground, as in the broad bean, the germination is said to be hypogeal. The structure of the runner bean seed and its germination are similar to those of the broad bean.

Germination of the Mustard Seed

The mustard seed has essentially the same structure as the broad bean, consisting of testa, cotyledons, radicle and plumule, but it is much smaller and more or less spherical in shape. The cotyledons are bilobed flat plates, which, when inside the testa, are folded in the middle.

As in the bean, the seed first swells owing to the absorption of water, and the radicle breaks through the testa. It lengthens and develops root hairs. Then the part of the stem termed the hypocotyl, which is just below the point at which the cotyledons are attached, grows rapidly in length so that the cotyledons, which are still enclosed within the testa, are carried up above the soil. The cotyledons expand so as to become the first green leaves of the young plant, casting off the testa as they do so. Finally the plumule begins to grow from between the two cotyledons to form the shoot. The cotyledons are simpler in shape than the leaves which are formed later.

When the cotyledons are pushed above ground, as in the case of the mustard, cress, sunflower and wallflower, the germination is said to be epigeal.

OTHER TYPES OF SEED

The Castor Oil Seed

The castor oil seed is an example of an endospermic seed. The testa is mottled with brown and black, and at one end of the seed is a knob called the caruncle. If the testa is removed a white mass of endosperm covered by a thin skin is seen. That the endosperm contains oil is easily shown by squashing it upon a piece of paper, when an oily stain is left. If the endosperm is carefully pared away the embryo will be found embedded in it. The cotyledons of the embryo are two flat, oval plates lying face to face and marked with delicate veins. The radicle is a short peg projecting from the base of the cotyledons and continuous with the short pointed plumule which lies between the cotyledons. The cotyledons are attached to the short region between the radicle and plumule. On germination, the cotyledons remain below ground until the endosperm has been absorbed and then appear above ground to become the first green leaves.

Monocotyledons and Dicotyledons

Plants such as the runner bean, mustard and sunflower, which have seeds with two cotyledons, are placed in a division of the flowering plants called the Dicotyledons. Those flowering plants with seeds having only one cotyledon are placed in the division called Monocotyledons. Such plants are the grasses, maize, wheat, oats and other cereals.

The Maize Grain

The maize grain is not a seed in the true sense of the word but a fruit, for the outer coat covering the grain represents the pericarp and testa which have become fused together. The grain is endospermic, and at the narrow end of the grain is a small tuft which is the remains of the stalk by which the grain was attached to the central stalk of the maize cob. The grain is yellow except for a whitish patch on one of the flat sides. This patch is grooved longitudinally and marks the position of the embryo, which consists of radicle, plumule and a single cotyledon. If the skin covering the embryo is removed from a grain which has been soaked in water, the surface of the cotyledon will be laid bare. The cotyledon is folded round so as to enclose the radicle and plumule, and the groove running down the white patch on the grain marks the place where the edges of the cotyledon meet. If the cotyledon is carefully cut into at the groove a small cylindrical structure, consisting of radicle and plumule, will be seen. The inner end is the plumule, which consists of several sheathing leaves surrounding the growing point of the stem. The plumule is continuous with the short blunt radicle which is directed towards the narrow end of the grain. The tips of both radicle and plumule are protected by a sheath. The cotyledon is attached to the region where plumule and radicle meet, and owing to its shield-like shape is often called the scutellum. The rest of the grain outside the embryo is the endosperm, which lies next to the cotyledon, and is coloured yellow on the outside and white within.

Germination of the Maize Grain

The part of the cotyledon that forms a sheath round the radicle lengthens a little so that it bursts through the coat covering the grain. The radicle then pushes its way through the sheath, the torn edges of which form a ragged collar termed the coleorhiza. Meanwhile the part of the cotyledon which surrounds the tip of the plumule grows out from the grain and passes up through the soil. When the pointed tip of the coleoptile reaches the surface, the plumule breaks through the sheath and the first foliage leaf expands. The scutellum remains within the grain and acts as a digestive and absorptive organ, converting the starch of the endosperm into sugar and handing this on to the growing embryo. The radicle soon gives rise to numerous side roots, so that a prominent tap-root is not formed, and a fibrous root system, characteristic of monocotyledons, is developed.

CONDITIONS FOR GERMINATION OF SEEDS

Certain conditions are necessary for the germination of the seed, of which the most important are :—

A supply of water.

A suitable temperature.

A supply of oxygen.

Water

Dry seeds contain only 10 to 20 per cent, of water, and as long as they are kept dry they will not germinate. If placed in water a seed will rapidly increase in weight owing to the absorption of water through the testa and micropyle.

Temperature

A certain niinimum temperature is required before the seed will germinate, and this varies according to the species of plant. Those growing in tropical countries require a higher temperature than those found in temperate regions. For each kind of plant there is a temperature at which growth of the seed takes place most rapidly. This is termed the optimum temperature. Mustard and cress will germinate at a temperature near the freezing-point, while those of cereals will not germinate below 5 C. Maize grains will not germinate below io° C, and the minimum temperature for germination for the cucumber and melon is 20 C.

Oxygen

All seeds need oxygen for germination in order to cause the release of energy by the process of respiration.

Experiment a—To show the Conditions required for Germination

A wad of cotton wool is placed in the bottom of each of four test-tubes, which are set up as follows :—

Test-tube 1

Several mustard seeds are sprinkled on the cotton wool and the test-tube is placed in a warm place. The seeds are thus provided with warmth and air but no water.

Test-tube 2

Mustard seeds are placed on the wool and the test-tube is filled with previously boiled and cooled water. A layer of oil is poured on top of the water in order to prevent air dissolving in it. The test-tube is placed in a warm place, and the seeds are thus supplied with warmth and water but not air.

Test-tube 3

The cotton wool is moistened with water and the seeds are placed on the top of it. The test-tube is placed in a warm place, so that the seeds have warmth, water and air.

Test-tube 4

This is set up in a similar way to test-tube 3, but is placed in a cold place, such as outside the window.

By observing the seeds from day to day the necessary conditions for germination can be found out.

Experiment 3—To show that Oxygen is necessary for Germination

Well-moistened cotton wool is placed at the bottom of two gas jars, and mustard seed is sprinkled on the top of the wool. A test-tube containing pyrogaliic acid dissolved in sodium hydroxide solution is put into one gas jar, which is quickly corked and sealed with grease or wax. The other jar is also corked and sealed, but is not provided with pyrogallic acid solution. This solution has the power of absorbing oxygen from the air so that the seeds in the jar containing it cannot obtain any oxygen.

The jars are placed in a warm place and observed from day to day for germinating seeds. No germination will occur in the jar devoid of oxygen.

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