SEEDS – the ongoing generation

Seeds represent the culmination of the reproductive cycle of flowering plants. Many flowerless plants produce them as well, for they are a very efficient type of dispersal structure. They existed over 250 million years ago and some of the earliest seed bearers still flourish today, such as the conifers and cycads (or ‘sago-palms’).

Not all plants, however, produce seeds, some rely on spores for reproduction and dispersal.

Fertilization

Pollination is an essential preliminary to the union of the male and female gametes in the fertilization process. When the pollen grain containing its male gamete is resting on the stigma surface, still separated from the ovule and its female gamete by the length of the style and the carpel wall, the pollen grain must somehow ‘bridge the gap’.

But before the pollen grain can show any activity, certain conditions have to be met with. Firstly, nothing will happen unless it is on the stigma of its own species, though exceptions occur, resulting in rare hybrids such as the cabbage-radish cross. For normal pollen development, a sugar solution of specific concentration, particular to each species, is necessary. This is somewhere between 5 and 30 percent cane sugar.

In favourable conditions, the pollen grain pushes out a delicate tube, which is in fact a bit of the intine which appears through a hole in the exine. Energy for this is provided by the rich store of protein and fatty substances in the pollen grain. The pollen-tube has a natural tendency to grow away from the light and air, so it disappears into the stigma, and starts to wend its way through the stigmatic tissues, partly by pushing a path between the cells, which are usually loosely arranged, and partly by dissolving the cells by means of enzymes. The method which predominates depends, on the species. The digestion of cells helps provide extra energy for the developing tube, which is guided on its long journey by a chemical substance of a sugary nature given off by the ovule. ‘ As it travels down the style, the tube carries the pollen-tube nucleus like a pilot at its tip with the two male gametes right behind. Eventually the carpel cavity containing the ovules is reached. Now the tube tip penetrates an ovule, usually via the micropyle, opens and discharges its contents into the embryo sac. The tube nucleus disintegrates, its business over, one male nucleus fuses with the female nucleus, or ovum, thus initiating the embryo, and the other fuses with the two polar nuclei, bringing the ‘endosperm’ into being. In conifers, the ovum alone is fertilized.

This fertilization process in plants is thus both a complicated and a remarkable feat. Considering the pollen-tube development alone, it is quite astonishing to compare the small size of the pollen grain with the length of tube it produces; in corn, the pollen tube has to grow through the silk and whole length of the ear in order to reach the embryo sac, a distance of some 30cm (i2in), while the pollen grain itself is little more than one-four-hundredth of a centimetre.

Development of seeds

Once fertilized, an ovule starts to develop into a seed. It enlarges, and the embryo, by repeated cell division, forms a flat structure called a cotyledon. A seed produces either one or two of these cotyledons. In the former case the plants are called monocotyledons, a group which evolved later than the second group, the dicotyledons. This division amongst the flowering plants is rather more apparent later on; monocots most often have grasslike leaves with parallel veins, whilst, typically, dicot leaves are broad with herringbone or fingerlike veins. There is a difference in the number of flower parts also, monocots mostly having them arranged in threes, whilst fours and fives, and infinite numbers, are common in dicots.

Back in the seed, the developing embryo is pushed into the seed centre by a sort of tiny elongating pedestal. Here it becomes surrounded by the endosperm which provides nourishment during its development. Many dicots absorb the whole of the endosperm, and the cotyledons end up thick and fleshy, taking up almost all of the seed, as in the broad bean and the pea. In most monocots, and some dicots, the bulk of the reserve food rests in the endosperm until the seed germinates, as for example in maize, onion and date seeds. The seed now has its embryonic seedling leaves in the form of its cotyledons, an embryonic shoot called the plumule and an embryonic root called the radicle. In most seeds the embryo itself is very small—less than one millimetre long in fact. The seed has become invested in a protective coat or testa, formed from the old integument of the ovule and, with its essential structure formed, the whole seed undergoes drying-out so that its water content may drop to a value as low as 10 percent by weight of the seed. Finally the mature seed passes into a dormant condition. Growth stops, respiration slowing down to a point where it is no longer detectable; the seed in fact passes into a sort of suspended animation.

A shelled peanut reveals seed structure nicely. Around the outside lies the protective brown testa. Inside, the two halves of the peanut are the cotyledons, fitting snugly together. At one end the cotyledons are hinged together at the point of their attachment to the embryo, which lies tiny and perfectly sculptured between the two halves. Upon germination, this small embryo will start using the food stored in its cotyledons, and begin its development into a whole new peanut plant.

The seed is a complex structure. Even the tiniest seed consists of many cells; larger ones already contain a highly developed embryo before they leave the parent plant. All contain a food store, either in fleshy cotyledons or the endosperm. The supply may be quite substantial, hence the food value of such seeds as peas, beans and nut kernels to man. The testa protects its contents from excessive heat and light and prevents water-loss. Clearly, then, the seed is well prepared for its journey away from the parent plant. A spore, by comparison, is simply a single cell with a relatively flimsy cell wall. It contains only very limited food resources, and its chances of surviving and growing into a new plant are correspondingly very small. Consequently, a spore-producing plant must devote a considerable proportion of its energies into producing sufficient of these particles to compensate for the all-toolikely losses.

Conditions necessary for germination

In its dormant state the seed can withstand extremes of temperature, way below freezing or near the boiling point of water. How long the seed remains in this condition depends on the particular plant species. In the tropical mangroves, there is no dormant period. Here germination takes place whilst the seed is still held within the fruit, and the young seedling falls like a dart with its long pointed radicle straight into the mud of the mangrove swamp. Other seeds actually need a long dormant period before they are able to germinate. Certain other conditions must be fulfilled, too, for germination to take place: water and oxygen must be readily available and the temperature must be within a certain range, depending on the species but usually between 5 and 30°C (4i-86°F). Some seeds require other, special conditions, such as the water plantain Alisma plantago, the testa of which has to decay or become damaged before the embryo can emerge.

Under very dry conditions, seeds can remain viable for quite a time, but there has been a tendency in the past to overestimate their longevity. Under normal conditions, seeds can be stored for only one or two years. After this time the natural humidity of the air makes them dissipate their store of energy and they can no longer germinate. A closer look at the triggers of seed germination reveals why this is so.

The seed begins to ‘stir’ when water penetrates the outer seed coat and the water content of the seed comes to reach around 8 percent of its total bulk. This is sufficient to support respiration but little else. Below this level the seed remains inert. Above 12 percent there is sufficient water for both active life and the initiation of growing. In ordinary air, there is usually sufficient moisture for very slow respiration, but not enough to support growth. Without further water, the seed will slowly burn up its food reserves, until its potential for germination runs out, and the seed dies. Seeds with firm hard coats tend to retain their viability for longer. The best conditions for the storage of most seeds are low temperatures and a dry atmosphere; in this way, seeds of common farm and garden plants have kept for 10 to 25 years.

A few years ago, stories were circulating concerning wheat seeds taken from the desert tombs of ancient Egyptian pharaohs which grew upon being sown, 4,000 years after their formation. Sadly, the stories were untrue. Even in the ancient sarcophagi some moisture exists, and wheat seeds last only a short time before internal changes render them incapable of germination. A well-authenticated case of seed longevity concerns some seeds of the Indian lotus (Nelumbo nudfera) which had been kept in the herbarium of the British Museum. In 1940, the museum was bombed, and the seeds were soaked with water which resulted in their germination. The date on the herbarium sheet showed that the seeds were 237 years old. Since then, tough-coated seeds of the same plant have been found in Far Eastern peat bogs where carbon-14 dating has shown that they have successfully survived centuries of dormancy. Of these seeds, the three oldest ones were recovered from a Neolithic canoe buried under 5.5111 (18ft) of peat bog near Tokyo. Viability tests under vacuum conditions have shown that even many years’ storage causes no change of viability in at least 50 different plant species, though even so certain species do tend to deteriorate. The final results of these tests, of course, will not be available for many generations to come.

Germination

For many plants, springtime brings with it the correct combination of triggers needed for germination. Some very short-lived plants, called cphcmerals, can only exist in their very dry habitats (such as the tops of walls and sand-dunes) by virtue of their very early spring germination, enabling seeds to be set before the summer drought sets in. With the right conditions a seed will begin to germinate. Water uptake continues for a week or so, but its effects are no longer generalized. Enzymes are wakened into action, digesting starch to sugar and releasing growth factors, respiration is stepped up with a massive energy release, and all this is aimed at the tiny plumule and radicle, the young stem and root. The cells of these two organs now undergo a drastic change in form; they elongate by as much as 10 to 100 times their previous length, a feat made possible by the supply to each root and stem cell of vast amounts of sugar sent up from the cotyledons. Now the cells are able to take in great volumes of water, relative to their size, and this stretches them ‘beyond the point of no return’. The walls of these cells are so constructed that they can only get longer.

Suddenly the seed can retain its mass of frenziedly active cells no longer. The testa bursts, and out pokes the root which stretches down and anchors the seed-sprout firmly in the soil. Next the stem emerges, the manner varying from species to species. Sometimes, as in the sunflower, the stem carries the cotyledons above the ground. Here the seedling leaves turn green and start to help with food production. If the cotyledons are thick with food, as in the pea, they often stay underground. Unlike the root and stem, they cannot grow very much, their cell walls being thick and inflexible, and once they have enabled the plant to gain a foothold in its new environment, the cotyledons become redundant.

The period of germination ends with the development of the first foliage leaves; the little plant is now completely self-supporting and has reached the seedling stage.

Seed dispersal

An aspect of the life of a seed so far overlooked, is that of its dispersal—a matter of vital importance to the plant-to-be since it offers escape from the rather unsatisfactory fate of germinating en masse immediately beneath the parent plant. Few seedlings can survive such crowding by members of their own species, all struggling against each other for identical requirements in food, water and light. Since plants are sedentary, the reproductive phase offers the only opportunity in the life cycle of the plant to widen its range, to seek and exploit the potentialities of other habitats, with all the attendant possibilities for further evolution that these can bring.

Flowering plants usually produce large numbers of seeds as an insurance against failure of many to reach agreeable destinations. In this way, some at least will find suitable conditions provided that the area over which they are scattered is wide. Since, like pollen grains, seeds are not self-motile, plants have adopted a variety of means in order to effect this wide dispersal of their seeds, means both mechanistic and involving outside agencies—chiefly wind, animals and water. In many cases the dispersal of the seeds is closely connected with’ that of the fruit. Wind dispersal

Plants with very small seeds may have these rather than their fruits blown away by the wind. Small size combined with lightness, often achieved by having a loose dry testa, can make such seeds as easily air-borne as spores. Many orchids rely on this method for the dispersal of their seeds which are smaller than those of any other plant and take to the air with great readiness. Other seeds develop specific structures to aid their wind dispersal. The milk-weed has seeds with tufts of silky outgrowths of the otherwise hard testa and these catch in passing breezes, carrying the seed on its way. In the rosebay willowherb (Chamae-nerion angustifolium), the fruit pods split open whilst still attached to the parent plant, releasing seeds with long silky hairs which are blown out and scattered to considerable distances on account of these hairs. Sometimes winged seeds occur too; wings enable the seed to coast along in the breeze and are to be found for example in honesty (Lunaria biennis) and the field spurrcy (Spergula arvensis). The conifers and the yellow rattle have winged seeds too, while willows and poplars have woolly seeds.

Of all the wind-dispersed seeds, the smalland-light ones tend to be by far the most widely dispersed, while winged seeds cover the least distances, though seeds of the Scots pine have been recorded as travelling 792m (880yd). Wind-dispersed species are often the first to colonize open patches of ground because their seeds are generally so readily transported and randomly deposited. A good example of this is the rosebay willowherb, also popularly known as fireweed on account of its rapid appearance on fire-devastated areas. Animal dispersal

Generally speaking, animals are more important in the dispersal of fruits rather than seeds alone. This is because seeds lack the immediate food appeal which fruits hold for many animals. Sometimes birds feed on seeds. Of course, should a seed become broken up in the bird’s beak or gizzard, it will no longer be capable of germination, but it is an interesting fact that the intestines of a seed-eating bird such as a finch will usually be found to contain many intact seeds which will be able to germinate upon their exit from the bird’s gut.

Quite a few plants, especially woodland species, have their seeds dispersed by ants. Such seeds typically possess little structures impregnated with an oily food-substance which ants find attractive. This structure, often brightly-coloured to increase its appeal to the ants, is called an claiosome, and is found for example in the gorse. Ants carry these seeds off to their nests, dropping some on their way, and thus effect dispersal.

As far as dispersal by carriage on animals’ bodies goes, fruits generally show more widespread structural adaptations to this method than seeds. Beggar’s-tick, cocklebur and stick-seed all possess hooks and spines for clinging to fur and feathers. Water dispersal

To be dispersed by water, seeds must be so adapted that they combine buoyancy with an ability to retain their vitality upon submersion. Each seed of the white water lily (Nymphaea alba) possesses an air-trapping spongy aril in addition to a testa with many air pockets. The seeds are liberated underwater, and they float away in the water currents. After a while, the seeds become waterlogged and sink to the bottom of the stream where they germinate. Mechanical devices

The seeds of many plants are dispersed by forcible ejection by the fruit. Touch-me-not and jewel-weed have seed pods which swell as they mature so that when the pods finally burst, the ripe seeds enclosed within are hurled out to some distance. The witch-hazel fruit splits open slowly, squeezing its slippery seed between its moist halves, and finally sending it skidding on its way. Ecballium elaterium, better known as the squirting cucumber, has a very turgid fruit which, upon dropping from its stalk, leaves a hole in its lower end through which contraction of the fruit ‘skin’ releases pent-up pressure forcing the seeds along with the excess fluid out through the hole to a distance of several metres.

The poppy, Papauer rlweas, disperses its seeds by the ‘censer mechanism’. Here mechanical dispersal is assisted by the wind. The ripe fruit develops a ring of holes around its upper side; through these holes, the seeds are jerked out like pepper from a pepper-pot as the wind whips the fruits about on their long tough stalks. Many other plants, amongst them the campion, primrose and monkshood, also use the censer mechanism.

Gorsc combines another method with mechanical dispersal. As its fruits dry out, tensions are set up which eventually burst open the fruit in a twisting action accompanied by a loud pop, causing the seeds to be shot out. These seeds, lying on the ground, are then picked up and dragged off by ants, as mentioned earlier.

Generally speaking, mechanical dispersal does not usually succeed in spreading the seeds to any great distance, and is therefore not really a very efficient dispersal method. Perhaps this explains why so many plants do in fact combine other agencies with this method, as in gorse and

Poppy.

Accidental dispersal

Quite often seeds are dispersed accidentally, bypassing the special method for which they have become adapted. Any seed which will float can be carried off by water and, provided it does not remain wetted for too long, it may survive the journey. Seeds of water plants specially adapted to floating may become stranded on mud and then stuck to the feet of ducks and other water birds and transported in this way from one patch of water to another. Any kind of seed can be blown immense distances by such things as freak hurricanes. Grasses have light seeds which are normally wind-dispersed but one species, called Cyuodou, is regularly dispersed in rather a strange fashion in the Belgian Congo. Here termites store large quantities of the grass in their nests and, when these are later abandoned, the grass seeds have germinated and established themselves there.

Perhaps we can go even further and say that some species are actually adapted to be non-specialized, and have come to rely on ‘accidents’ for their further existence. For example, the raylcss mayweed (Matricaria matrkarioides) appears to have no obvious adaptations to dispersal, and yet is widespread and extremely common on roadside verges. Other members of the family, the Compositae, have fruits specially adapted to wind dispersal. It appears that this plant relies in fact on carriage by motor car tyres—a kind of plant hitch-hiker. In days before the internal combustion engine it presumably spread by way of cart-wheels and boots.

Man ranks high as a chance dispersal agent. His influences in this respect can be seen everywhere, both purposeful and accidental; seeds of many kinds have been transported into practically every corner of the earth, resulting in the establishment of species in places which could never otherwise have been reached. The red-top grass was introduced into New Zealand quite unwittingly by man: emigrants travelled to New Zealand from Nova Scotia with mattresses filled with hay which included red-top. Upon settling in the new country, the mattresses were abandoned and the seeds of this American grass germinated and became established in this their newly adopted country. Protected motorway and railway verges, and canal systems, provide plants with a continuity of habitat over vast areas, acting as an uninterupted network of corridors along which plants can spread. By such indirect means, man unknowingly aids the dispersal of many plants.

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