How do seeds develop
The Development of Seeds
Seeds develop from the ovules in the female plant, after they have been fertilised by the pollen from the male parent plant. This is termed ‘sexual reproduction’, as seeds contain the genes of both the male and female parent, and usually both male and female flowers are required to produce seeds. Sometimes, the male and female flowers are from separate plants, sometimes they are from the same plant, and sometimes a flower may be fertilised by its own pollen.
The ovules are the embryos from which the seed will develop. At the time of fertilisation, they are very small compared to the mature seed. Without being fertilised (in most cases), the ovules will not develop into seeds, any more than pollen can. So in order to develop into seeds, the ovules must first be fertilised.
We can often judge how close the seeds are to being ripe from the size or colour of the fruit or seedpod.
This is a Passionflower.
In the photos, the petals are brown and shrivelled at the top, and the dried stigma is at the bottom.
The ovary is a small oval underneath the stigma, which in this species is divided into three. Underneath the ovary are the five stamens.
At this stage, the seeds are too small to be visible.
As the seeds develop, the ovary gets bigger, and at this stage we can recognise it as a seedpod, or, in this case, a fruit.
Inside the developing seedpod, the seeds are still barely visible around the edge of the fruit, and most of the seedpod is empty space.
Each seed is attached to the wall of the ovary by a small stalk, through which it receives the food it needs to grow. It is therefore vital that the seeds remain attached to the ovary while they are developing.
The seeds need to form a tiny root and shoot, and to collect a store of food for the plant to use when it germinates before it has developed sufficiently to gather its own food supply.
The seedpod grows larger, but even when it has almost reached its full size, the seeds are still soft and green, most of the seedpod is still empty space, and the seedpod is still hard and green.
The seeds still need to develop a hard covering that will protect them until they are able to germinate.
The mature seedpod has reached its full size and has begun to change colour. The inside of the seedpod is full of seeds and the juicy flesh that will attract animals to eat the fruit and disperse the seeds.
The seeds are large, fully formed, have a hard protective coat and may no longer be connected to the wall of the fruit.
If we left the fruit on the plant, the flesh and juice would dry up, and the pod would wither and dry, because it is no longer being supplied with food from the parent plant. Eventually the pod would split open to release the seeds.
In the example, the seedpod is a Passionfruit, but the development of all seeds follows the same pattern. Seeds cannot be produced unless the flower has been fertilised, and seeds are not viable unless they are mature and ripe. Unripe seeds are soft, white or green, and enclosed in a hard green fleshy fruit (pod). When seeds are ripe and mature, they change to brown or green and develop a hard covering, and the seedpod dries out and changes to white or brown and eventually splits to release them.
Producing seeds uses a lot of the plant’s energy. If the plant is dead or dying, it will not have enough energy to complete the process. This is one of the reasons you are unlikely to be able to get seeds from cut flowers. The plant needs to be able to gather food while it is developing seeds, and the seedpod needs to remain attached to the parent plant until the seeds are fully developed.
Development of the seeds, from fertilisation to shedding of the ripe seeds, can take several weeks or even months, but seeds gathered before the process has been completed will not be viable.
If you want to collect seeds from the plants in your garden, you can help by ensuring that the plant has enough food and water to give it the energy it needs to produce seeds.
Other entries with information which might be useful
How do seeds develop The Development of Seeds Seeds develop from the ovules in the female plant, after they have been fertilised by the pollen from the male parent plant. This is termed
Chapter 2 SEED AND FRUIT DEVELOPMENT, GERMINATION, DORMANCY
Some knowledge of the biology of seeds is essential to their proper handling. The use of seed for artificial regeneration makes possible a considerable degree of control over the conditions in which it is collected, processed, stored and treated, but the seed’s inherent characteristics have been evolved as a result of millenia of adaptation to natural regeneration under local conditions. Knowledge of flowering phenology enables the collector to select the timing and methods of seed harvesting most appropriate to the species, while handling, storage and pretreatment of seed will benefit from a knowledge of how seeds develop in nature.
This chapter gives a very brief and simplified account of seed biology in angiosperms and gymnosperms, but there is wide variation in the details of seed development between different genera. Readers wishing to obtain fuller information should consult standard texts on seed biology such as Corner (1976), Bhatnagar and Johri (1972), Singh and Johri (1972), Puritch (1972), Allen and Owens (1972), Boland et al. (1980). There are few detailed descriptions of seed development in tropical forest trees.
Pollination and Fertilization
A seed is a reproductive unit which develops from an ovule, usually after fertilization. Ovules are borne by both the angiosperms (true flowering plants) and the gymnosperms (which include the conifers). In the angiosperms the ovules are totally enclosed within the ovary, while in the gymnosperms the ovules are “naked”, typically borne in pairs on the upper surface and near the base of each scale in a female cone. Since the cone scales remain tightly closed except at the time of pollination and later at seed shed, the term “naked” is a relative one.
Seed development is initiated by fertilization, the union of a haploid male nucleus from the pollen grain with a haploid female nucleus within the ovule to form a new diploid organism. Fertilization must be preceded by pollination, the arrival of a pollen grain on the stigma of the female flower in angiosperms or close to the micropyle of the gymnosperm ovule. It is important to distinguish the two separate processes of pollination and fertilization (Fritsch & Salisbury 1947). In most angiosperms the elongation of the pollen tube is rapid and the interval between pollination and fertilization is only a few days or even hours. In a few angiosperms (e.g. Liquidambar, some species of Quercus) and many gymnosperms (e.g. Pseudotsuga, Larix, Picea) the interval is several weeks or months, while in other species of Quercus and in many Pinus it is a year to 14 months (Krugman et al. 1974, Kozlowski 1971).
Angiosperm Seed Development
At the time of fertilization a typical angiosperm ovule consists of one or two protective coats – the integuments – and a central tissue – the nucellus. Often the integuments and the nucellus are clearly differentiated only in the region of the micropyle – the minute pore in the integuments through which, in many species, the pollen tube enters the nucellus. The ovule is attached to the wall of the ovary by a stalk – the funicle.
Meiosis of a mother cell within the nucellus, followed by several mitotic cell divisions, leads to the formation of the embryo sac, a haploid eight-nucleate, seven-celled structure which occupies the central space within the nucellus (Chuntanaparb 1975). When the pollen tube reaches the embryo sac it releases two male gametes. One male gamete unites with one of the nuclei in the embryo sac -the egg cell – to form a zygote which later develops into the diploid embryo plant. The second male gamete unites with two other female nuclei – the polar nuclei – to form a triploid cell which later develops into the endosperm, a tissue which acts as a food reserve for the growing embryo. The remaining five nuclei of the embryo sac (2 synergids and 3 antipodal cells) play no further role in seed development. Successful fertilization of the egg cell and successful triple fusion with the polar nuclei are both necessary for development of a viable seed.
Development of the fertilized ovule into the mature seed involves several different parts. From the outside inwards these are as follows:
- The integuments of the ovule become the seedcoat of the mature seed. This sometimes consists of two distinct coverings, a typically firm outer seedcoat, the testa, and a generally thin, membranous inner coat, the tegmen. The testa protects the seed contents from drying out, mechanical injury, or attacks by fungi, bacteria and insects, until it is split at germination (Krugman et al. 1974). But there is great variation in seedcoat among angiosperms.
2.1 Longitudinal section through a typical pistil just before fertilization. (USDA Forest Service)
2.2 Longitudinal sections through ripe seeds of:- (A) Paulownia tomentosa showing conspicuous endosperm. (B) Tectona grandis. Endosperm has disappeared and cotyledons occupy almost the entire seed cavity. (USDA Forest Service)
|2.3 Examples of different types of fruits:|
|(A) Cross-section of capsule of Eucalyptus preissiana showing loculi, axis, placentae and ovules. (Division of Forest Research, CSIRO, Australia).||(B) Open pod with seeds of Acacia aneura. (FAO/Division of Forest Research, CSIRO, Australia).|
|(C) Samara of Triplochiton soleroxylon. (Forest Research Institute of Nigeria).||(D) Nut (acorn) of Quercus rubra. (USDA Forest Service).|
|(E) Drupe of Tectona grandis. (S.K. Kamra).||(F) Cone of Pinus oocarpa. (A.M.J. Robbins)|
The nucellus may persist in some genera as a thin layer – the perisperm -lying inside the seedcoat and supplying food reserves to the embryo. In most angiosperms, however, it soon disappears and its function is taken over by the endosperm.
The endosperm commonly grows more rapidly than the embryo during the period immediately after fertilization. It accumulates reserves of food and its fullest development is rich in carbohydrates, fats, proteins and growth hormones (Kozlowski 1971). In some species the endosperm remains conspicuous and still fills a greater part of the seed than the embryo even when the seed is ripe. In others, such as Tectona, the embryo absorbs food reserves from the endosperm during its later stages of development, until the endosperm disappears by the time the seed is mature.
The embryo occupies the central part of the seed. Its degree of development at the time the seed is ripe varies greatly according to species. In some it is possible to distinguish all parts of the rudimentary plant – the radicle which at germination will give rise to the primary root, the seed leaves or cotyledons, the plumule from which will develop the primary shoot, and the hypocotyl which connects the cotyledons with the radicle. If the embryo absorbs all the food reserves from the endosperm, the thick fleshy cotyledons commonly become the main organs for food storage and occupy almost the whole of the seed cavity.
Although the storage function within the embryo is normally performed by the cotyledons, in Anisophyllea, Barringtonia and Garcinia it is completely taken over by the swollen hypocotyl which fills the seed cavity; the cotyledons are vestigial or absent (Ng 1978). This is also the case in Lecythis and Bertholletia (Lubbock 1892), hence the edible content of a Brazil nut (Bertholletia excelsa) is neither endosperm nor cotyledon but hypocotyl.
In some species the embryo is still small and undeveloped when the seed is ready for dispersal, and an additional period under suitable environmental conditions is needed for maturation of the embryo after seed shed, before the seed can become capable of germination, e.g. Fraxinus excelsior.
At its most complex the ripe seed may thus consist of diploid tissue from the mother tree (the seedcoat, including testa and tegmen, and the perisperm), triploid tissue in the endosperm, and diploid tissue of the new genetic combination in the embryo offspring. But both perisperm (nearly always) and endosperm (not infrequently) may be missing. The essential constituents of all seeds are the embryo, the protective covering of the seedcoat and a reserve of food substances which may be stored in the cotyledons, hypocotyl, endosperm or perisperm.
Occasionally more than one embryo may develop in a single seed and such polyembryony has been reported from several genera (Kozlowski 1971). It is, however, the exception.
Angiosperm Fruit Development
Development of the fertilized seed is normally accompanied by development of the fruit. In the simplest case the ovary wall becomes thickened to form the pericarp. This may be:
Dehiscent, splitting open when ripe to release the enclosed seeds; examples are the capsule (e.g. Eucalyptus), a multilocular fruit derived from a syncarpous ovary, and the leguminous pod (e.g. Cassia), which is derived from a single carpel and spilts along two sutures. The pericarp may be dry, semi-fleshy or fleshy at the time of dehiscence. Semi-fleshy to fleshy capsules are common in the humid tropics (e.g. Baccaurea, Durio, Dysoxylum, Myristica) and are often associated with the development of variously coloured, tasty or smelly pulp (aril or sarcotesta) around the seed.
Indehiscent or dry, closely fused with the seed; examples are the achene, a small hard one-seeded fruit with membranous pericarp, the samara, similar to the achene but with pericarp extended to form a wing (e.g. Triplochiton) and the nut, a rather large one-seeded fruit with woody or leathery pericarp (e.g. Shorea, Quercus).
Indehiscent and fleshy, often distinguished by colour, smell and taste to attract fruit-eating birds and animals. Two types are distinguished. The berry has an outer skin and inner fleshy mass, containing seeds that have a hardened seedcoat (e.g. Diospyros, Pouteria). The drupe has the inner layer of the pericarp hardened to protect the seeds (e.g. Prunus, Gmelina, Azadirachta, Mangifera); the seedcoat, having no protective function in a drupe, is usually papery or membraneous. The different pericarp layers in a typical drupe are known as exocarp (the skin), mesocarp (the flesh) and endocarp (the stone). The stone may be actually stony as in Gmelina or leathery as in Mangifera.
In some species other parts of the flower, as well as the ovary wall, take part in fruit formation. An example is the pome, found in apples and pears, in which the enlarged fleshy receptacle forms the greater part of the fruit, while the pericarp forms the core. An additional partial or entire protective covering may be provided by fused bracts arising below the flower – the involucre. This may be papery, as in Tectona, or thicker and leathery as in the “acorn cup” of Quercus. Some fruits are formed by the coalescing of an entire inflorescence e.g. Morus, Chlorophora, Anthocephalus, Artocarpus.
At the opposite extreme, in several genera of the Sterculiaceae (e.g. Fimiana, Pterocymbium and Scaphium), fruit formation does not occur at all in the normal angiospermous manner. Soon after fertilization, the carpel (follicle) splits on one side and develops into a large membraneous scale-like or boat-shaped wing; the fertilized ovule develops in a naked position at or near the base of the open carpel, in a gymnospermous manner. Such fruits must be the most primitive of all angiosperm fruits (Corner 1976). At maturity the seeds are dispersed, attached to their carpels which now behave as wings.
The interval between flowering and maturation of seeds and fruits varies greatly with species, even within the same genus. In Eucalyptus it varies from one month in E. brachyandra to 10 – 16 months in E. diversicolor (Boland et al. 1980). In most Malaysian Dipterocarps it is between two and five months (Tamari 1976). In Tectona grandis it takes 50 days from flowering for the green fruits to develop to full size but 120 – 200 days before they are fully ripe (Hedegart 1975). In a study of rooted cuttings of Gmelina arborea in pots in Nigeria, individual flowers took 11 days from flower bud to opening and 45 days from flower bud to ripe fruits (Okoro 1978). In Pterocarpus angolensis the interval between flowering and fruit maturation is 8 months (Boaler 1966). The shortest interval on record between flowering and seed maturation for a tropical timber species is apparently 3 weeks, for Pterocymbium javanicum (Ng and Loh 1974). In contrast, some species of temperate Quercus take about 18 months from flowering to production of mature seeds.
In most species fertilization of one or more ovules must precede fruit formation. In a few species, however, fruits are set and mature without seed development and without fertilization of an egg. Such fruits, called parthenocarpic fruits, occur in several genera of forest trees including Acer, Ulmus, Fraxinus, Betula, Diospyros and Liriodendron (Kozlowski 1971). Mature fruits do not invariably indicate mature seed, still less can the number of sound seeds be predicted from the number of fruits. In Tectona the number of sound seeds per fruit can vary between 0 and 4 (Kamra 1973) and still greater variation is possible in other genera.
Seed Dispersal in Angiosperms
There is thus an immense variety among angiosperm fruits. Much of it is related to the need for seed dispersal. Survival and growth of young seedlings under the parent tree is often difficult, because of lack of light and intense root competition. Dispersal over a wide area can ensure that some seeds find conditions suitable for germination and survival, even though the vast majority will perish from the effects of harsh site conditions, competition or destruction by animals or disease.
Dispersal by wind is assisted when the seeds are very light and small e.g. Eucalyptus, or when either the seedcoat (Salix, Ceiba, Dyera) or the pericarp (Triplochiton, Pterocarpus, Koompassia, Casuarina, Fraxinus) possesses wings or hairs which serve to prolong flight. Fruits may also be winged by the enlargement of persistent sepals (most Dipterocarps) or persistent petals (e.g. Gluta, Swintonia) (Krugman et al. 1974, Ng 1981).
The distance of seed or fruit dispersal by wind depends not only on the weight and type of dispersal unit but also on the local wind conditions and the exposure and isolation of the mother trees. Studies of the winged fruits of Shorea contorta in the Philippines indicated that 90 % of the fruits travelled 20 m or less from the stem of the mother tree (Tamari and Jacalne 1984) and a summary of other dipterocarp studies compiled by the same authors shows that most fruits landed within 30 m or, at most, 40 m. This compares with a dispersal distance within 2–3 m of the crown perimeter for a heavy, wingless seed such as Quercus crispula in Japan and a distance of over 60–90 m for 5 % of the light, winged seeds of Betula ermannii downwind from a belt of mother trees left in a logged over area (Konda 1969 and Nakano et al. 1968, cited in Tamari and Jacalne 1984).
Fleshy edible fruits and arillate seeds, on the other hand, encourage dispersal by birds or mammals. When such fruits or seeds are eaten by animals, the seeds, protected by the hard seedcoat or endocarp, often pass unharmed through the digestive tract and are deposited in the faeces at a considerable distance from the place where they were consumed. In many cases the digestive juices actually assist subsequent germination through softening of the hard seedcoat. In Africa the hornbill is a highly efficient dispersal agent for seeds of Maesopsis eminii. Sometimes the process is so effective as to be an embarrassment. Free-ranging goats eat the pods of Prosopis in some countries and spread the seeds indiscriminately over large areas; excellent germination and the aggressive pioneering qualities of the young plants may then render this genus a dangerous weed-tree. Coralling of the goats and collection of the seeds for use under strictly controlled management can overcome the problem. In other cases the fruit is eaten while the stones or seeds are rejected, but the animal may carry the fruit some distance from the parent before dropping the seeds. Rodents remove and store nuts or seeds; many are subsequently eaten but some may escape to germinate in the new situation.
Wind and animals are the most important agents of dispersal, but dispersal by water is common in some riverine species and large and heavy fruits are distributed to some extent by gravity on steep slopes (Krugman et al. 1974).
Gymnosperm Seed Development
Gymnosperm ovules have certain characteristics in common with angiosperm ovules, but there are a number of differences. There is normally a single protective integument which in a typical female cone is partially fused to the ovuliferous scale carrying the paired ovules. Within the integument is the nucellus which at fertilization, as in angiosperms, is clearly separated from the integument only in the region of the micropyle (Fritsch and Salisbury 1947). Meiosis within the nucellus, followed by mitotic cell divisions, leads to the formation of a multicellular haploid tissue – the female gametophyte. By the time of fertilization it has developed much further than the 8-nucleate embryo sac in the angiosperms and has largely displaced the nucellus. At its micropylar end it is differentiated into one to many archegonia, each of which contains a large egg cell (Chuntanaparb 1975).
At fertilization the pollen tube releases two male nuclei into an archegonium, one of which unites with the egg nucleus. The resulting zygote later develops into the new diploid embryo. The second male nucleus aborts in Pinus but may fertilize a second archegonium in other genera e.g. Cupressus (Fritsch & Salisbury 1947). It never unites with female polar nuclei to form a triploid tissue analogous to the endosperm of angiosperms; this type of tissue is unknown in gymnosperm seeds. For detailed descriptions of gymnosperm embryogeny, readers are referred to the specialized literature (e.g. Singh and Johri 1972).
The mature seed consists of some or all of the following: (1) The seedcoat or testa developed from the integument, diploid from the female parent. (2) The diploid perisperm, developed from the nucellus. In most species this is absorbed by the female gametophyte and has disappeared by the time the seed is ripe, but it is still recognisable as a distinct tissue in e.g. Pinus pinea. (3) The haploid female gametophytic tissue which serves as a food storage organ to nourish the embryo. Its function is the same as the endosperm in angiosperms and it is frequently called by that name, though this usage has been deprecated (Bonner 1984a). (4) The embryo, with the same parts of radicle, cotyledons, plumule and hypocotyl as in angiosperms. The number of cotyledons varies between and within genera, being up to 18 in Pinus, compared with the constant two in the dicotyledons which comprise the great majority of angiosperm trees. The essential constituents of embryo, protective covering and food storage tissue are present in all gymnosperm, as in all angiosperm, seeds.
More than one archegonium may be fertilized within a single ovule, but in the great majority of cases only one embryo per seed develops to maturity. Polyembryony does occur but is uncommon in most genera.
Gymnosperm Fruit Development
After fertilization the female cone which is typical of several important gymnosperm genera e.g. Pinus, Picea, Pseudotsuga, Araucaria increases in size and weight, in moisture content and accumulated food reserves. As the cones approach maturity, the moisture content decreases again, accumulated food reserves move from cone to seed and the cone becomes more or less woody.
In Pinus a thin membranous flake becomes detached from the ovuliferous scale and adheres to the ripe seed, forming a wing (Fritsch & Salisbury 1947). In Juniperus the cone scales grow together to form a fleshy berry-like fruit, while in Podocarpus and Taxus each singly borne seed becomes partly enclosed in a brightly coloured cup, the aril. The woody cone is, however, the most characteristic type of fruit in gymnosperms.
As in angiosperms, there is wide variation in the interval between flowering and seed maturity and dispersal. Because of the lengthy gap between pollination and fertilization in pines, mentioned earlier in this chapter, the total period between pollination and cone maturity is usually about two years in this genus; among tropical pines, average periods are 23 months in Pinus kesiya (Armitage and Burley 1980), 18–21 months in P. oocarpa (Robbins 1983 b). In Agathis robusta it takes 16 months from pollination to cone maturity (Whitmore 1977), in Araucaria cunninghamii up to 24 months (Walters 1974), in Araucaria hunsteinii 21–24 months (Evans 1982). In several temperate genera development is completed within a single season e.g. in 5 months in Pseudotsuga menziesii (Allan and Owens 1972).
Development of unpollinated female cones with fully formed but usually empty seeds occurs in a number of gymnosperm genera. Parthenocarpy is common in Abies, Juniperus, Larix, Picea, Taxus and Thuja. It is rare in pines (Kozlowski 1971).
Seed Dispersal in Gymnosperms
In the typical gymnosperm cone, ripening and drying of cone and seed causes the cone scales to open and release the seeds. Dispersal is by wind, assisted by the presence of seed wings in some genera e.g. Pinus. In some species of pine, the “closed-cone pines” e.g. P. radiata, there is usually an interval of months or years between ripening of cone and seed and the opening of the cone to release the seeds. In a few cases, such as the interior provenances of Pinus contorta, the cones open only when subjected to the heat of occasional fierce forest fires. On the other hand the cones of Abies and Araucaria disintegrate readily on the tree within a few weeks of ripening.
Seed dispersal by animals is less common, but the “berries” of Juniperus and the fleshy fruits of Podocarpus are examples. In addition seeds of temperate conifers are collected and stored by rodents and some may germinate before being eaten.
At one extreme, certain species of mangrove are viviparous, the seeds germinating before they separate from the parent. At the other extreme, seed of some species may remain dormant but alive for many years, capable of germinating if an event occurs to break the dormancy state. The subject of dormancy is discussed later in this chapter. Between the viviparous and the deeply dormant seed types occur many types of seed which are capable of germination soon after seed shed provided that environmental conditions are suitable.
Just as fertilization initiates the transformation of the ovule into the ripe seed, so does germination transform the embryo within the seed into the independent seedling. For the purpose of laboratory testing, germination is defined as the emergence and development from the seed embryo of those essential structures which are indicative of the seed’s capacity to produce a normal plant under favourable conditions (Justice 1972).
At maturity and seed shed many seeds have lost the greater part of the moisture which they contained in earlier stages. For example the embryo and female gametophyte of Pinus lambertiana contain as much as 50 % moisture content (fresh weight basis) shortly after fertilization, but by the time of natural seed dispersal the moisture content of the embryo is reduced to 23 % and of the female gametophyte to 38 % (Krugman et al. 1974). Reduced metabolic activity is associated with the drying of the seed, so that the embryo is in a temporarily resting or quiescent state, which in non-dormant seeds can be easily reactivated by suitable conditions. These conditions are (1) Adequate moisture (2) Favourable temperatures (3) Adequate gas exchange and, for some species, (4) Light (Krugman et al. 1974). There is considerable variation between species in the optimum levels of the different factors and there is frequently an interaction between them. Some examples of optimum temperatures for different species are given in Tables 9.1 and 9.2 in Chapter 9.
Germination consists of three overlapping processes:
(1) Absorption of water mainly by imbibition, causing a swelling of the seed and eventual splitting of the seedcoat, (2) Enzymatic activity and increased respiration and assimilation rates which signal the use of stored food and translocation to growing regions, (3) Cell enlargement and divisions resulting in emergence of radicle and plumule (Evenari 1957, cited by Krugman et al. 1974).
In most seeds the radicle of the embryo is close to the micropyle, where absorption of water is easier and quicker than through the seedcoat. As the radicle swells, it exerts pressure on the seedcoat which commonly splits first at this point to free the radicle. This gives rise to the primary root which grows down into the soil and soon produces lateral roots. Subsequent stages depend on whether the species exhibits epigeal germination e.g. Pinus – the hypocotyl elongates and the cotyledons are lifted above ground – or hypogeal germination e.g. Quercus – the hypocotyl is undeveloped and the cotyledons remain on or in the ground. In hypogeal germination the cotyledons can have only a food storage function, or a haustorial function (in species in which food is stored in the endosperm e.g. palms, Scorodocarpus), while in epigeal germination they may also perform a valuable photosynthetic function during early growth of the seedling.
2.4 Longitudinal section through an ovule of Pinus during the period of pollen tube development preceding fertilization. (USDA Forest Service)
2.5 Examples of germination in two West African Sterculiaceae. (A) Epigeal in Mansonia altissima. (B) Hypogeal in Cola nitida (after De La Mensburge 1966). (De La Mensburge, CTFT Nogent sur Marne)
In epigeal germination anchoring of the young plant by the radicle is followed by rapid elongation of the hypocotyl which arches upwards above the soil surface and then straightens; simultaneously the cotyledons and plumule are exposed, to which the seedcoat may or may not still be attached. The plumule then develops into the primary shoot and photosynthetic leaves. In the subtype “durian germination” (Ng 1978) the hypocotyl elongates but the cotyledons are shed while still enclosed within the seedcoat (e.g. Durio zibethinus, Strombosia javanica). In hypogeal germination, the cotyledons remain in situ underground or on the ground while elongation takes place in the plumule. In the subtype “semihypogeal germination” (Ng 1978) the cotyledons are exposed but remain on the ground. The two main types, epigeal and hypogeal and the two subtypes, durian and semihypogeal are the result of the four possible combinations of two independent variables: hypocotyl elongated or not and cotyledons exposed or not. All four combinations occur in the humid tropics.
Even in non-dormant seeds there is considerable variation, between species and individuals, in the speed of germination, from a few days to several weeks; much of this is due to different rates of imbibition in the first stage. Many tropical rain forest species, with high moisture content and permeable seedcoats at seed fall, must germinate within a few weeks. If they fail to find suitable conditions soon, they lose viability and die.
The term “dormancy” refers to a condition in a viable seed which prevents it from germinating when supplied with the factors normally considered adequate for germination – suitable temperature, moisture and gaseous environment. A viable seed is defined as one which can germinate under favourable conditions, providing any dormancy that may be present is removed (Roberts 1972).
Dormancy in nature serves to protect the seed from conditions which are temporarily suitable for germination but which quickly revert to conditions too harsh for survival of the tender young seedling. Thus a seedcoat relatively impermeable to moisture prevents germination during isolated showers in the middle of a long dry season, while permitting it during a sustained rainy season. In the cool temperate zone the type of embryo dormancy which can be removed only through exposure to low temperatures facilitates subsequent germination in spring, while preventing it in autumn, when the resulting seedling would be unlikely to survive the winter.
The strength of dormancy has been observed to vary according to latitude and provenance, and from year to year even in seed from the same parent. There is also differential dormancy within the same species and seedlot, so that germination is staggered over a more or less extended period of time. In the Malaysian woody flora, about 50 % of species complete germination within six weeks or less, so that the differential is not very great, but in some species such as the hard-seeded leguminous Parkia javanica the germination period may extend from 1 week after sowing for the first seed to two years for the last (Ng 1980). Differential dormancy and staggered germination insures against the entire seed crop being destroyed by a single climatic catastrophe or pest.
In nature a number of external factors may work, more or less slowly, to end seedcoat dormancy. These include alternate heating and cooling, alternate wetting and drying, fire and the activities of animals, soil organisms, fungi, termites and other insects. Dormancy due to embryo immaturity will come to an end if the embryo is given time and suitable conditions in which to mature after seed shed.
The exact mechanisms of physiological dormancy of the embryo, and of the processes which can terminate it, have been widely investigated but underlying causes are still little understood (Krugman et al. 1974). There is good evidence that growth-promoting hormones, of which gibberellin is a well known example, and growth-inhibiting hormones interact in the maintenance or breaking of dormancy. In temperate climates the balance between inhibitors and growth-promoters is altered by a combination of low temperature and high moisture maintained over a period of time which varies from species to species. This combination is provided naturally during winter, the season least suitable for growth. It can trigger biochemical changes in the embryo which lead to the breaking of dormancy, the initiation of embryo metabolism and growth, and the subsequent germination of the seedling.
Research on the physiology of tropical tree species has, unfortunately, been only a fraction of that done on temperate species. There is no reason to suppose that the combination of low temperature and high moisture – the technique of “stratification” when applied artificially – would have any effect on tropical seeds possessing embryo dormancy (if they exist). In the dry tropics, on the contrary, a combination of conditions typical of the season least favourable for growth – high temperature and low moisture – would seem more likely to trigger the breaking of embryo dormancy and lead to germination in the following rainy season. In fact, seedcoat dormancy by itself appears to provide adequate protection for species of the dry tropics.
From the forester’s point of view dormancy has some disadvantages. Delayed and irregular germination in the nursery is a serious constraint on efficient nursery management (Bonner et al. 1974). Much research has therefore gone into devising effective artificial treatments to remove dormancy, in order to ensure that the seeds germinate quickly and evenly in the nursery beds. These treatments are described in Chapter 8.
On the other hand, dormancy confers certain advantages. Not only does it improve the chances of survival in nature, as mentioned previously, but it preserves the seed against temporarily unsuitable conditions such as may occur during the period between seed collection and storage. High quality orthodox but non-dormant seeds, dried to the appropriate moisture content and stored at the correct temperature, should have as long a life in storage as dormant seeds, but dormancy provides an insurance against the loss of viability during transport and processing which can easily occur in non-dormant seeds in less than ideal conditions.
Hazards of Seed Production
Both the quantity and the quality of seed crops may be greatly affected by external factors. Climatic factors can affect the abundance of flowering and thus indirectly of seed production. There is some evidence that above-average temperatures and a modest degree of moisture stress in spring and early summer can induce abundant formation of flower buds in temperate regions (Krugman et al. 1974). In Nigeria good seed years occur in Triplochiton scleroxylon following a particularly dry (30 % or less of average rainfall) August, the month when there is a diminution of rainfall between the heavy early and heavy late rains (Howland and Bowen 1977).
More extreme cases of unseasonal climate usually reduce the crop of flowers or fruits. Late spring frosts in temperate regions kill flowers or young fruits and abnormally high temperatures or drought may have a similar effect. Even if death and premature shed of whole fruits does not occur, a proportion of the seeds may abort later. Mechanical destruction of flowers or fruits may be caused by exceptionally high winds or hailstorms. Continuous rain at the time of pollen dispersal has a particularly adverse effect on the amount of seed set, whether pollination is by wind or insects. Tectona flowers during the rainy season and this may account for the low average rate of fertilization of 1 – 3 % reported from Thailand over the period 1967 – 72 (Hedegart 1975). Rain discourages flight of the pollinating insects as well as washing off pollen grains from the stigma before they germinate. Continuous wet weather during the pollen dispersal season is considered the main factor responsible for the characteristically poor seed crops of Pinus merkusii in Indonesia and Malaysia. Tamari (1976) reported that over 90 % of Dipterocarp flowers in Malaysia failed to develop into fruits.
Birds, mammals, insects, fungi and bacteria all do damage in flowering as well as fruiting stages. Insects are probably responsible for the most serious losses in the greatest number of species. For example, the weevil Apion ghanaense destroys a large part of the flowers and seeds of Triplochiton each year (Jones 1975). The larvae of Pagyda salvaris may destroy as much as 90 % of the flower buds of Tectona in some years (Hedegart 1975). Two species of the Bruchid genus Amblycerus can destroy many seeds of Cordia alliodora, but damage can be reduced by collecting the seeds three weeks before natural seedfall (Tschinkel 1967). The weevil Nanophyes sp. may attack up to 60 % of the seeds of Terminalia ivorensis (Lamb and Ntima 1971). Cone worms of the genus Dioryctria have been known to damage 60 % of maturing cones and seeds of Pinus elliottii and P. palustris in the southern USA (Krugman et al. 1974) and the same genus can also cause severe damage on Pinus merkusii seeds in the Philippines (Gordon et al. 1972). Larvae of Agathiphaga, a genus of moths, may destroy over 50 % of the seeds in cones of several species of Agathis in Queensland and the western Pacific islands (Whitmore 1977). Seeds of many dry area species of Acacia and Prosopis suffer serious damage from Bruchid larvae (Armitage et al. 1980). Birds and mammals, especially squirrels, may consume considerable numbers of seeds in some years, although they also perform a useful service in seed dispersal. Losses from pests and diseases do not normally have a serious effect in years of abundant seed production, but in years when flowering is poor for climatic reasons they can convert a light crop into complete failure.
Chapter 2 SEED AND FRUIT DEVELOPMENT, GERMINATION, DORMANCY Introduction Some knowledge of the biology of seeds is essential to their proper handling. The use of seed for artificial