Reproduction |
Reproduction in Flowering Plants may be either sexual or asexual. Sexual reproduction involves the fusion of male and female reproductive cells known as gametes. In asexual reproduction no fusion of gametes takes place and the reproductive entities are vegetative bodies such as corms, tubers, bulbs, cuttings etc.
Sexual reproduction may be dealt with in a number of stages:
• pollination—the transfer of pollen from anthers to stigma
• fertilisation—union of male and female gametes
• development of the seed
• development of the fruit
• germination of the seed.
Pollination
The transfer of pollen, which carries the male gamete, to the stigma takes place in several ways. Many plants are pollinated by wind including the conifers (pines and their allies), she-oaks and grasses, and many northern hemisphere trees such as birches, alders and oaks. These plants produce large quantities of pollen and release it into the air, and it may float or be blown onto a compatible stigma. The stigmas of wind-pollinated plants are commonly large and feathery, which gives them a better chance of trapping pollen.
Insects are the most important pollinators of flowers. They visit to collect nectar or pollen or both, and at the same time incidentally transfer some pollen from one flower to another. Insects locate flowers by odour and then are influenced by colour and shape. It is known that bees preferentially visit yellow or blue flowers whereas moths, which emerge in the evening, are attracted to white or cream flowers, which are more readily seen at that time.
Birds, particularly honeyeaters, are important pollinators of flowers with tubular corollas, to which they are attracted by the presence of copious nectar. Pollen catches on the head feathers of the birds as they probe for nectar and then is carried to other flowers. Birds seem to be attracted to red flowers, but they visit other colours if nectar is available. Small animals such as pygmy possums, glider possums and dibblers are also believed to pollinate certain species.
Insect- and bird-pollinated plants usually have large colourful flowers and produce nectar. In contrast, the flowers of wind-pollinated plants are commonly small, green or brown, often with no petals and lacking nectar.
Fertilisation
At maturity most pollen grains contain three nuclei, two of which are sperm, or male gametes, while the other, known as the tube nucleus, appears to be involved with the growth of the pollen tube.
The mature ovule contains an embryo sac, and the most usual type has eight nuclei enclosed in seven cells. The egg cell, or female gamete, flanked by two other cells is at one end of the embryo sac; in the centre are two polar nuclei and at the other end the three remaining cells.
When a pollen grain germinates on the stigma, the pollen tube emerges and grows down through the style, through the carpel wall and into the ovule and embryo sac. The tube nucleus moves down with the advancing end of the pollen tube followed by the two sperm nuclei (Fig. 12). The sperm nuclei enter the embryo sac, and one fuses with the egg cell to form the zygote, which is the general name given to a fertilised egg in all organisms. The second sperm nucleus fuses with the two polar nuclei to form the primary endosperm nucleus, which divides very rapidly to produce a tissue called the endosperm.
Fig. 12 Diagrammatic longitudinal section of two united carpels after pollination; the pollen tubes can be seen growing down the style.
Germination of the pollen depends on a chemical interaction between exudates from the wall of the pollen grain and the surface of the stigma. A favourable reaction between the exudates enables the pollen to germinate and the pollen tube will grow down into the style. In such a case the pollen is said to be compatible. If the reaction results in the suppression of pollen germination, the pollen is incompatible. Many plants are self-incompatible, which means that their own pollen will not germinate on the stigmas of the same individual. The plants must then be cross-pollinated with pollen from another individual of the same species in order to be fertilised. Usually stigmas reject pollen from other species, but when different species do fertilise one another they are said to have hybridised.
Development of the seed
Following its formation, the zygote divides and develops into an embryo, this process, at least in part, absorbing nutrients from the endosperm. A mature embryo consists of an axis (the shoot–root system of the future) and either one or two cotyledons, sometimes called seed-leaves. The presence of these seed-leaves is the basis for the names of the two major traditional groups of Flowering Plants, the Monocotyledons (monocots) and Dicotyledons (dicots). However, the Dicotyledons have now been reassessed, with a number of groups separated out (see Chapter 6).
Generally a dicot embryo will have two cotyledons and a monocot embryo only one. In dicots the remaining food stored in the endosperm may be absorbed by the cotyledons, which become thick and fleshy as in beans, peas and wattles, or the endosperm may remain in the mature seed as in Ricinus communis (Castor Oil Plant) and some members of the Chenopodiaceae (Saltbush Family). Monocot seeds all contain endosperm that is absorbed through the cotyledons during germination. Mature embryos are surrounded by a seed coat or testa (Fig. 13) but in some monocots, such as the grasses, the testa is fused with the fruit wall. The hilum (Fig. 13) is the scar on the seed marking the point where the funicle was attached.
Fruits either form from the gynoecium of a single flower or, less often, from an inflorescence. Simple fruits develop from a single carpel or a syncarpous gynoecium, and are dealt with in more detail below. A flower with free carpels gives rise to an aggregate fruit (Pl. 2b) and examples include the strawberry and raspberry. (See the section on the family Rosaceae.) Multiple fruits are formed from an inflorescence; two examples are the pineapple and the fig. The pineapple consists of many fleshy units attached to a central axis, and the pattern on the tough outer skin shows the boundaries of the individual fruitlets. In the fig, the flowers (and later the small fruitlets) are enclosed in a hollow inflorescence axis which becomes fleshy as it matures (Pl. 17c).
The fruit wall, or pericarp, generally develops from the carpel wall or, as in apples and rose hips, it may include the floral tube. The pericarp is sometimes clearly differentiated into three layers and then the outermost layer is the exocarp, the middle one the mesocarp and the inner the endocarp. The three layers are present in a cherry: the skin is the exocarp, the flesh the mesocarp and the stone that encloses the seed the endocarp. In fruits that are dry at maturity the layers are incompletely differentiated. Sometimes the perianth remains attached to the fruit and may enlarge as the fruit grows. In such cases, the terms ‘fruiting perianth’ or ‘perianth persisting in fruit’ are used.
At maturity, fruits may be fleshy, or hard and dry. Those with dry pericarps are either dehiscent (split open to release their seeds) or indehiscent. Sometimes a dry fruit produced by a multilocular ovary splits up to form several fruitlets, as in Correa and some other members of the family Rutaceae.
In botany the term fruit includes many so-called vegetables such as peas, beans, cucumbers, capsicums and pumpkins. The section dealing with the Rosaceae describes some familiar fruits belonging to that family. The accompanying list of fruit types commonly encountered in literature dealing with plant identification includes some well-known examples. The fruit types are defined in the glossary.
In the natural environment fruits show a variety of adaptations that aid their dispersal, and hence the spread of their seeds. Birds commonly eat fleshy fruits and then often deposit the seed well away from the fruit source. The fruits of plants such as Acaena (Sheep’s Burr, Bidgee-widgee, Fig. 70) and some species of Medicago (Medic) have awns that become caught in the fur of animals, not to mention people’s socks, and the fruits may be carried some distance from the parent plant. The feathery tufts on thistle and daisy fruits, and the wings on fruits of elms, birches and ashes assist in their dispersal by wind. Some plants form pods that open explosively causing the seeds to be catapulted out. The seed of the coconut palm is well protected by a thick fibrous husk, and coconuts have drifted on ocean currents to many remote islands. Other plants that live close to beaches are known to spread in the same way.
Fruit dry
| Fruit dehiscent | |
| Follicle | Banksia (Fig. 44b), Grevillea (Fig. 48b), Hakea (Fig. 52), Macadamia, individual fruitlets of Crassula (Pl. 1h) and Brachychiton (Pl. 22e) |
| Legume | bean, lentil, pea, peanut, Lathyrus (Fig. 65b), Senna (Fig. 56) |
| Capsule | Callistemon (Fig. 81), Eucalyptus (Figs. 85, 87), Lilium (Pl. 9e), Melaleuca (Pl. 19j), Papaver (Pl. 4e) |
| Samara | individual fruitlets of Liriodendron (Pl. 6d) |
| Silicula | Capsella (Pl. 24b, e) |
| Siliqua | Brassica (Pl. 23g) |
| Fruit indehiscent | |
| Nut | acorn |
| Cypsela | individual fruitlets of the daisies, including Senecio (Fig. 121), Tagetes (Fig. 117), Taraxacum (Fig. 124), Galinsoga (Pl. 33e), Hypochoeris (Pl. 32k) |
| Achene | individual fruitlets of Ranunculus (Pl. 2b, c) |
Fruit fleshy
| Drupe | cherry, date, plum, Rhagodia (Pl. 26d) |
| Berry | Acca (Feijoa, Pl. 19g), guava, passion fruit, many Solanaceae (Pl. 30, including tomato and eggplant) |
Germination
If the seed of a pea or bean is soaked in water for a time the testa can be removed easily (Fig. 13). Inside are the two fleshy cotyledons and if these are opened out the embryo plant can be seen between them. The cotyledons are joined to the axis of the embryo at a point known as the cotyledonary node. The axis above the node is called the epicotyl, which bears the apical meristem and often the first pair of leaves as well (Fig. 13d). Below the node the axis is called the hypocotyl and the embryonic root at its lower end is the radicle.
Fig. 13 Seed, germination and early growth of a bean. (a–c ×0.3, d ×1.7)
At germination the young root usually emerges first and develops sufficiently to anchor the young plant in the ground. In plants such as beans, wattles, and eucalypts the hypocotyl elongates and pushes the seed above the soil surface, whereupon the cotyledons emerge from the testa, spread out and become green; then the epicotyl elongates and the two foliage leaves spread out and begin to grow. In plants such as the edible peas the epicotyl elongates first and the seed and cotyledons remain in the soil. During germination the plant absorbs nutrients from the cotyledons or endosperm until it can sustain itself by photosynthesis (see Chapter 5). Monocot seeds are very variable in structure but the general principles apply, and in all cases the single cotyledon absorbs nutrients from the endosperm to support the growth of the young plant.
Many seeds will not germinate immediately after their release from the fruit. They enter a period known as dormancy, which under natural conditions ensures that germination will not occur until the chance of survival for the seedling is greatest. A number of plants, particularly those in regions where the winters are very cold, produce seeds that must be held at a low temperature for some time before they will germinate. This process, known as stratification, can be simulated by storing the seeds in a refrigerator. In nature the low soil temperature ensures that germination will not occur until conditions are more favourable.
Some seeds, such as those of wattles, have very tough testas that must either gradually rot away or be cracked open by the heat of a fire. After fire, large numbers of seedlings will emerge in the open habitat, free of competition and, with extra nutrients available from the burnt vegetation, these seedlings have an excellent chance of survival. Many Australian plants produce seeds that can apparently remain viable in the soil for a long time and germinate after fire. Research has found that smoke promotes germination independent of heat. The effect can be transferred to the glasshouse by watering seed trays with ‘smoked water’ prepared by passing the smoke from burning vegetation through a container of water.
The testas of the seeds of some desert plants contain inhibitory chemicals, which must be leached away by the soil water before germination will take place. In another group of plants, the seeds need to pass through the intestine of a bird or animal before they will grow. Sometimes this process can be simulated by acid treatment but in other cases the effect of passage through the animal is not understood.
