REPRODUCTION IN FLOWERING PLANTS

by
Prof. C. M. Sean Carrington,
Department of Biological & Chemical Sciences, University of the West Indies,
Cave Hill Campus, Barbados

This resource has been compiled to provide Caribbean examples relevant to the General Objective 3 in Unit 1 Module 2 of the CAPE Biology Syllabus and for UWI Preliminary Biology students.

Introduction

The origin of the flowering plants was described by Charles Darwin as "that abominable mystery". Flowering plants are the dominant plants on the earth today with an estimated quarter of a million species. Unlike lower plants like ferns and seaweeds which rely on spores to invade new habitats, flowering plants are dispersed by seeds and this undoubtedly accounts for their success. Lower plants alternate in their life cycle between distinct sporophyte and gametophyte stages. While these phases can be recognised in flowering plants as well, an understanding of this is not required for CAPE Biology and these concepts are best ignored at this level.

Flower structure

The flower is simply a collection of highly modified leaves. The outermost series is collectively termed the perianth and comprises sepals, which protect the developing flower, and petals, which often serve to attract pollinators. Within the perianth lie the actual reproductive organs. The stamens produce pollen from which the male gametes are ultimately derived. The carpels contain an ovary with ovules in which the female gametes develop.

Following the transfer of pollen to the stigma or receptive surface of the carpel, in a process called pollination, nuclei from the pollen will ultimately fuse with nuclei within the ovule's embryosac to form the seed.

In some flowering plants all the perianth parts are similar; there are no separate sepals and petals. This can often be seen in lily-like plants like the Spider Lily (Hymenocallis caribaea).

The flowering plants show great variation in flower structure and these characteristics are central to flowering plant classification. In most cases, flowers are not borne singly but grouped on a common stalk and termed an inflorescence. Various inflorescence types are recognised and while these do not concern us at this level be aware that these are very important in classification.

Sometimes the flower has closely associated with it a prominent (often coloured) leaf termed a bract. In bougainvillea (left) these bracts are more attractive than the actual flower which is pale and inconspicuous. Similarly, red bracts constitute the most conspicuous part of the inflorescence of the red ginger lily (right).

Overview of sexual reproduction

The details of pollen grain development and embryosac formation will not be discussed here as these are well covered in all textbooks. Some points, however, are worth emphasising.

In both pollen and ovule development, meiosis occurs which not only halves the chromosome complement of cells but involves crossing over and so a reshuffling of genes, ensuring genetic recombination.
Unlike other seed plants, the ovules are completely sealed within the carpel. This means that the pollen tube must actually bore its way down through the style into the ovary and this has led to the evolution in flowering plants of self incompatibility mechanisms to prevent the entry of "undesirable" pollen.

The flowering plants are unique in having a double fertilisation event. One nucleus from the pollen grain will fuse with the egg to form the diploid zygote which develops into a minature plant, the embryo. More unusual is the fact that a second nucleus from the pollen will fuse with 2 polar nuclei of the embryosac to form a triploid cell which develops into the endosperm, a tissue that will provide nutrition for the embryo. The seed that results then comprises the embryo, a minature plant ready to start a life of its own, with reserve materials either in a surrounding endosperm or transferred to the embryo's cotyledons, all within a tough seed coat.
One final unique feature of sexual reproduction in flowering plants is the development of fruit. Following fertilisation, as each ovule form a seed, the ovary swells to form the fruit. The biologist's definition of a fruit is simply an organ for dispersing seeds. This is quite different to the layman's concept of a fruit as something soft, juicy and edible. Fruit may split or remain intact. They can be hard and dry like a calabash or fleshy like a tomato or a cucumber. There is a complex system of recognised fruit types but you need not worry about these in this course.

Promotion of cross-fertilisation

To ensure the long term survival of any species it is advantageous to have a large and varied population on which natural selection can act. Such variability is promoted by cross-fertilisation and reduced by self-fertilisation. Not surprisingly flowering plants have evolved a number of mechanisms which maximise cross-pollination.

1. Separation of the sex organs
Some plants have unisexual flowers (i.e. contain either carpels or stamens but not both) rather than bisexual flowers. In dioecious species, plants are either male or female (e.g. nutmeg, marijuana). In monoecious species, there are separate male and female flowers but on the same plant (e.g. castor oil, breadfruit, corn, pumpkin).

Female & male flowers of Jatropha integerrima, a common garden shrub in the Caribbean Castor oil, Ricinus communis

2. Timing mechanisms
Sex organs may mature at different times. Protandry is where the anthers mature first while protogyny is where the stigma is receptive before the anthers mature. Both avocado and soursop are protogynous. The Anthurium bloom, which is not a flower but an inflorescence or collection of flowers, is another example. The fleshy spadix feels wet when the minute female flowers are ripe and dry and powdery when the pollen is being shed. Male and female parts are never ripe together.
Soursop flower with perianth (left), without perianth (centre), with anthers shed (right)

3. Specialised pollination mechanisms
Many plants have highly specialised pollination mechanisms, often adapted to a single insect, bird or bat pollinator. The highly specialised flower structure and manipulation of the behaviour of the vector ensure cross-pollination. Click here to see flowers with insect eyes!

Several Aristolochia flower types Aristolochia, Dutchman's pipe, is a woody vine with several species native to the Caribbean, all with strange, bent, tubular flowers. Flies are attracted by a putrid scent emitted by the flower and enter its tunnel-like interior. Downward pointing hairs allow them to proceed further but not escape. The flowers are protogynous so on this first day the stigmas are receptive but no pollen is shed. On Day 2, the stigmas wither and the pollen is now shed, coating the entrapped flies. The next day the hairs blocking the flies' escape whither, setting the "prisoners" free. They now visit a new freshly-open, "fragrant" flower. The story is repeated again but on arrival they bring pollen to the receptive stigmas of these Day 1 flowers and so achieve cross-pollination!
Orchids also provide good examples of highly evolved flower-pollinator relationships - the stuff of natural history documentaries.

4. Heterostyly
In some species a plant bears one of two morphological forms of a flower. Flowers may have long styles with anthers near the flower base (pin form) or short styles with anthers borne on high (thrum form). These arrangements prevent selfing when a pollinator visits and ensures cross-pollination between plants with the different flower types. The Red Cordia, Cordia sebestena, (right) shows this.

5. Self incompatibility
When a pollen grain germinates on a stigma of the same species a recognition process takes place. This involves molecules from the stigma entering the pollen grain and vice versa. This process is under genetic control and results in pollen from the same plant or a different plant of similar genetic make-up being rejected. The pollen grain may be prevented from germinating or the growth of its tube may be inhibited within the style. Many plants including cabbage and tobacco show this.

Asexual reproduction

Sexual reproduction in flowering plants involves meiosis and the fusion of gametes, usually from different plants, and results in seedlings which differ from their parents. In asexual reproduction, exact copies of the parent plant result as the new plants are formed simply by mitosis. Sometimes flowers are involved in this process but usually only the vegetative parts of the plant are involved and so this is termed vegetative reproduction.

Plants have a range of perennating organs, which store reserves and are produced to enable the plant to tide over periods of unfavourable environmental conditions, e.g. the dry season in the tropics, winter in temperate regions. Examples of these are the rhizomes (underground stems) of ginger, the stem tubers of English potato, the corms (vertical stem tubers) of eddo, the root tubers of yam and sweet potato and the bulbs of onions and lily-like plants. These can be used as a means of propagating the plant in question.
	corm	bulb

Plants may also send out creeping branches or runners (Water lettuce), or may produce adventitious buds along the edges of leaves (Bryophyllum sp.). Most Agave spp. (Maypole, Dagger, Lapitte) have sterile flowers but then produce minature plants or bulbils on the pole-like inflorescence.

Water lettuce, Pistia stratioites Bryophyllum sp. Agave sp.

Propagation

Artificial vegetative reproduction at its simplest involves inducing stem cuttings to form roots by maintaining these in a humid environment. Treating the cuttings with a synthetic auxin such as indole-3-butyric acid (typically diluted with inert talcum powder) often speeds up the induction of adventitious roots. If cuttings do not readily root it may be necessary to use the technique of air-layering where rooting is induced on a branch still attached to the parent plant. This involves removing a ring of bark, packing the exposed area with moist soil and keeping this sealed with plastic film until roots form, at which time the rooted cutting is removed and planted. Some woody cuttings are very difficult to root. In such cases, one must graft a bud (budding) or shoot cutting (grafting) onto a root stock. Such vegetative propagation is necessary when an exact copy of the plant is desired. This is true for particular varieties (more correctly cultivars) of woody plants like mango and hibiscus. Since sexual reproduction leads to recombination, seed from such plants will not produce offspring with identical traits to the parent and vegetative propagation is required.

In plant tissue culture, sterile portions (explants) of a plant are transferred to sterile medium containing agar, mineral nutrients, vitamins, sucrose and plant hormones like auxins and cytokinins. This induces cells to return to a meristematic dividing phase and produce a disorganised proliferation of cells termed a callus. By later changing the culture medium, calli can then be induced to form shoots and roots and so regenerate lots of tiny plants. This technique can propagate plants on an industrial scale and has been used in the Caribbean for banana propagation. It does require skilled personnel, specialised facilities and can take a considerable time to regenerate plantlets and then to grow them to a field-ready stage. It is also well-documented that genetic variation, termed somaclonal variation, can develop during the tissue culture process so that the final plants may not be identical. These techniques of in vitro propagation are especially important for the generation of disease-free planting material.

Callus of sugarcane


Female & male flowers of Jatropha integerrima, a common garden shrub in the Caribbean	Castor oil, Ricinus communis


Water lettuce, Pistia stratioites	Bryophyllum sp.	Agave sp.