COMPOSITES – multiple Sowers mean success

The family Compositac is the largest among dicotyledonous plants. It has a worldwide distribution and members adapted to a wide variety of climatic and ecological situations. The family name is derived from the characteristic way in which the flowers are grouped in ‘composite’ heads, or capitula, which form a shared base. Although other plant families have small flowers (or florets as they are often called) grouped closely together, the Compositae are unique in that each floret has no calyx as such, although there may be a ring of teeth or bristles. The head, however, is surrounded by several series of bracts, known collectively as the involucre. These initially protect the bud, and during flowering and fruiting may protect the capitulum from insect attack from below.

Flower structure

The five petals of each flower are fused into a corolla tube. The length of the component parts may be equal or obviously different, and some parts may be very small. Flowers with corolla tubes which have five ‘teeth’ of equal length are called disc flowers, while those flowers which have one or more corolla parts extended well beyond the tube are called ray flowers. The reason for these names can be found by looking at a daisy, which has a ‘disc’ of flowers in the centre and ‘rays’ around the outside.

There are three main arrangements of flowers on capitula. The capitulum may be made up of both disc flowers and ray flowers as in daisies, asters, chrysanthemums, marigolds and sunflowers, in which the disc flowers are to the centre with the ray flowers to the outside. The ray flowers generally extend beyond the involucral bracts. Alternatively, capitula may consist entirely of ray flowers as in dandelion, lettuce and chicory, or they may consist entirely of disc flowers and be said to be ‘rayless’ as are the capitula of thistles, burdocks and cornflowers. Some species, such as buttonweed, Cotula coronopifolia, appear to be rayless but this is because their small ray flowers do not extend beyond the involucral bracts and are therefore inconspicuous. On the other hand, close examination of capitula which appear to have ray flowers around the edge may reveal that they are in fact very large disc flowers. Some species of Centaurea, including knapweeds and cornflowers, are like this. The carline thistles, in the genus Carlina, have large coloured involucral bracts which look like rays.

Reproduction

Within the corolla tube are the reproductive structures. The male androecium consists of five anthers and the female gynoecium of one inferior ovary containing a single ovule. The anthers are fused into a tube and release their pollen on the inside. The style growing outwards from the ovary pushes the pollen out of the tube. At this stage the style has its two receptive stigmatic surfaces pressed close together, giving it a solid rodlike form. Only when the style has grown beyond the anther tube does it ‘open’ to reveal the two stigmatic surfaces. This very greatly reduces the chances of pollen from an anther being transferred to the stigma of the same flower.

Some flowers may not have any anthers and may be female or sterile if the gynoecium is non-functional. Such flowers are often outer ray flowers. Flowers which are structurally hermaphrodite may produce seed and pollen, pollen alone or nothing at all. They are rarely, if ever, entirely female.

As with the different types of corolla, giving disc and ray flowers, so flowers of different reproductive function may be borne on the same head. In this case the female flowers are to the outside of the head, the hermaphrodite flowers inside them and the male flowers to the centre. The flowers develop and mature from the outside inwards. This means that the outer female flowers are likely to be fertilized by pollen from other heads and from other plants since the pollen from the male flowers on the capitulum will not have been released. If this outbreeding mechanism should fail and they are not pollinated or fertilized, then the pollen released later from the flowers to the centre of the capitulum may be transferred to them. This may be considered as a sort of ‘fail-safe’ mechanism.

A flower in which the ovule is fertilized by a pollen grain produced by its own anthers tends to produce seed and then offspring very similar to. itself because it has only its own genetic material to use. Such plants will survive if they can find an ecological niche similar to the parents. In turn they will produce similar offspring. Although this gives a short-term advantage for the rapid colonization of suitable sites, in the long run it is an evolutionary dead end. Only plants in which pollen from other flowers fertilize the ovules will tend to acquire different combinations of genetic material. Only they will yield offspring which have the potential to survive in different situations. So in the long run an outbreeding mechanism can lead to evolution and survival.

As the flowers on the composite head are ready for fertilization at different times, several different pollination events, spread over several days, must take place. This increases the chances that the pollen will come from different plants, which increases the chances that new combinations of genetic material will occur. Thus, a single flowering head may yield seed produced by cross-pollination with several other plants from the surrounding or nearby populations. Such a system is good from the evolutionary point of view as it tends to conserve the genetic information within the population, while effectively ‘reshuffling’ it.

Dispersal of seeds

The fruit of composites is a hard dry achene. The greatly reduced calyx of each floret often forms a collection of bristles or hairs at the tip of the achene, a structure called a pappus which acts as a tool for dispersal of the achene. Wind catches the pappus and it floats away bearing the achene. In some genera the involucral bracts fold downwards and the achenes, each with its pappus, form the well known ‘clock’ typical of the dandelion.

Wind dispersal can lead to a widespread ran- dom distribution of the seeds. This may deposit the seed in new and possibly unfavourable situations. If they have an adequate new combination of genetic material they may be successful and so extend the range of ecological habitats colonized by the plant.

In other genera the pappus is very reduced or absent. Many of these plants have barbs or hooks on the seed, or on the capitulum. If the hooks catch in the fur or feathers of animals the seed may be dispersed to the sites frequented by the animal. If these are fairly constant then the seed may well be distributed to an area similar to its origin. This will aid the rapid colonization of similar ecological habitats.

Adaptations to environments

With these breeding systems for maintaining and producing variation and efficient dispersal mechanisms it is hardly surprising that the family Compositae is so successful. It contains a very large number of species which live in a wide variety of places and have a great breadth of form and structure.

The genus Senecio has an African species, S. adnivalis, which grows in the Ruwenzori mountains and reaches a height of 6m (20ft). S. kleinia from the Canary Islands, has a fleshy stem and grows to a height of up to 1.8m (6ft). In Britain the groundsel species of Senecio are generally only several centimetres high and are annual weeds.

The familiar lettuce, is a cultivated variety of a slightly less palatable wild species of Lactuca. Many of the composites are attractive to grazing animals. Some genera have reduced their attraction by the possession of unpleasant milky sap or spiny leaves and capitula as in the thistles. Perhaps the most extreme anti-grazing and anti-dehydration mechanism is that shown by Launea arboresceus which can survive attacks from anything bar camels. It is known as camel grass and only produces flowers or foliage leaves within a tangled ball of reduced spiny leaves borne on woody stems. It is the botanical equivalent of barbed wire.

Uses by man

The root systems of composites vary. The dandelion has its well-known tap root, the dahlia has tubers and daisies have fibrous roots. Root types may vary within genera. The genus Leontodon, has species with tap roots, fibrous roots and tuberous roots. The possession of root tubers is a great asset to the horticulturist and the gardener, for they enable easy propagation of the plant.

Many composites are familiar as ornamental garden plants. Dahlias, chrysanthemums and asters are widely grown and bred. Greater head size has been but one of the characters favoured by their breeders. In the wild this is often a disadvantage. Not only could the head snap off but greater amounts of seed and genetic material could be lost if insect infestation of the head occurred. However, if a lot of small flowering heads are produced then the damage may be contained to one or a few of them.

Apart from lettuce, chicory, salsify and the ornamentals, the two most important composite genera are perhaps Helianthus, the sunflower, the seeds of which yield oil of considerable economic impotance, and Pyrethrum which yields the important insecticide of the same name.

The family Compositae is successful and diverse. Much of its success must be due to the capitulum of the flowers which characterize the family. This aggregation of individual flowers into a single head gives the plant an elevated evolutionary status as a ‘superplant’.

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