RETURN TO PLANT STRUCTURE III: FLOWERS AND FRUITS
RETURN TO PLANT STRUCTURE III: FLOWERS AND FRUITS
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Fundamentally, the angiosperm flower differs little from a gymnosperm strobilus. The main difference is that in angiosperms, the megasporophylls (which formed the major structure in the cone) are reduced and have enclosed the ovule. The megasporophylls have therefore formed the carpel. The structural difference may not be much, but the biological difference is great. Inside the carpels, new environmental conditions have been created for the growth of the ovules and for the process of fertilization. As a result many fundamental changes have taken place in the structure of the female gametophyte and the process of fertilization itself.
The strobili of gymnosperms (cones) evolved to protect ovules and pollen from predators. Insects were particularly attracted because they too were protected and sheltered and there was an abundance of pollen grains on which they could feed. Moving from one strobilus to another facilitated cross-pollination. Unfortunately they also ate the ovules, this must have led to evolution of greater protection of the ovules through natural selection. This led in angiosperms to the enclosure and reduction of megasporophylls to produce carpels which protected the young, succulent ovules. This protection also allowed other changes; there was no longer any need for special defence modifications, such as thickening of the integuments, and formation of thick sclerotic seed coats. As a result the ovules became smaller and simpler - with an even more reduced gametophyte and were capable of much faster development (pine reproduction from pollination to seed release can take up to 15 months). This speed of development would have been of great benefit in the seasonal climate that is thought to have existed. Relatively little energy is used in the construction of the ovule and gametophyte.
Development of carpels also led to the development of the stigmatic surface. The stigma took over the function of catching pollen grains and simulating the growth of pollen tubes, (previously a function of the micropyle in gymnosperms). This also allowed the ovule to become simplified. The stigma creates a new barrier to "undesirable" pollen thereby assisting cross fertilization. Finally, the formation of the carpels on the stem meant that they could be joined together so that many ovules could be enclosed within one structure and in turn the carpels could be enclosed by other parts of the flower.
Much of the evolution which occurred after this initial important step, did so in line with evolution of the pollinating insects - co-evolution. The earliest seed bearing plants (gymnosperms) were pollinated passively by wind. Ovules exuded drops of sticky, sugary sap from the micropyle to aid pollen capture - like the modern conifers. It is thought that the earliest pollinating insects were sap and resin feeding beetles which discovered the supply of protein-rich pollen and the sticky ovule exudate. Once they returned regularly to the sites of this newly found food supply they began to inadvertently carry pollen to the ovules. For some plants this pollination was more effective than wind pollination alone.
The evolution of more specialized insects - bees and wasps, butterflies, moths and flies - led to a great advance in the process of cross pollination including genetic diversity by increasing the constancy of flower visitation. The insects involved have highly developed instincts of flower constancy - in other words, they can differentiate between flowers, and will often visit one species. exclusively for as long as they can obtain food from it. It is only at this point that they will transfer to a second species, and then visit that exclusively. This feature of "pollination insects' evolved at the same time as the flowers themselves were becoming more specialized. In particularly, flowers were concealing the nectar and pollen to a greater extent and the dimensions of flowers were standardized in line with the dimensions of the body and proboscis of the specific pollinators. (Those flowers which are more specialized produce less pollen.) This explains much of the diversity of both insects and flowering plants - each dependent upon each other and adapted in terms of shape and timing of their abundances. This use of insects and other animals has resulted in species becoming highly specialized and isolated. This isolation has led to the creation of the great diversity of this, the largest group of land plants.
Unlike most animals, land plants cannot move about to seek partners for mating. The flowering plants evolved a set of features that allows "movement" for reproduction, these features are embodied in the flower. By attracting insects and other animals and directing their activities the angiosperms ensure that a high frequency of cross fertilization occurs. The more attractive the plants were to the beetles the more frequently they would be visited and the greater the number of seeds produced. Any mutations that would result in an increase in the frequency of the visitation or the efficiency of pollination offered immediate selective advantage. One such modification was the evolution of specialized glands called nectaries within the flowers secreting a nutritious sugary fluid attractive to insects called nectar. Attraction of insects to the flowers raised a new problem, protection of the ovule from predatory insects. The selective pressure was for greater ovule protection and the closed carpel was probably a direct result of this. Greater ovule protection may also be the selective pressure for the evolution of the inferior ovary.
Bees, wasps, butterflies and moths are long tongued insects which often have flowers as their only source of nutrition. Their rise and diversification was as a direct result of angiosperm evolution. Consequently these insects profoundly influenced the evolution and diversification of the angiosperms. Some of the insects visit only a very restricted range of flowers although given plant species is almost never totally dependent on only one pollinator and a pollinator is almost never totally dependent on only one type of flower. A plant with a narrow range of pollinators tends to become specialized for the characteristics of that group of pollinators. Many modifications which evolved were to encourage constancy of visitation; a) to make the flower more clearly distinguishable from other flowers - i.e. distinctive shapes, colours and odours to guide the specific pollinators and b) structural changes to discourage unwanted visitors - a major modification being fusing of the petals to form a corolla tube whereby nectaries at the base of the tube are only accessible to long-tongued insects. Mutations resulting in more efficient pollination led to other structural changes such as fusion of the carpels allowing a single deposit of pollen to result in the simultaneous fertilization of a large number of ovules.
Development of the corolla tube and the fused carpel occurred independently in members of many diverse families thus as flowers diverged in some characteristics they converged in others. By the early Tertiary period a course of mutual evolution involving flowers with fused petals and long tongued, flower constant insects (mainly bees) was well established.
Two types: (1) large flower borne singly e.g. magnolia, poppy, wild rose; or (2) small flowers gathered in an inflorescence e.g. dogwood, elder. Beetles sense of smell is more highly developed than the visual sense thus beetle pollinated flowers often have strong fruity or spicy odours (as opposed to the sweeter odours of bee, butterfly and moth pollinated flowers) and white or dull colours. Most have inferior ovaries well away from the chewing jaws of beetles.
Bees are the most important group of pollinating insects. Adults live on nectar and collect pollen to feed the larvae. Mouth parts, body hairs and other appendages are specially adapted to collect and carry these food materials. There are some 20,000 species of bees virtually all of which visit flowers for food. Bees have a high degree of constancy to particular flowers. They recognize colours, odours and outlines. Their visual spectrum is different from humans - they can see ultraviolet but cannot distinguish red (they see it as black). Bee pollinated flowers have bright showy petals often of blue or yellow (never pure red) often with a distinctive pattern on them for quick recognition. The patterning may be a "honey guide" to indicate the position of the nectaries. Flowers often have distinctive markings in ultraviolet. Nectaries are characteristically located at the base of the corolla tube accessible only to a specialized sucking organ. Flowers often have a conspicuous landing platform. Some evolutionarily more advanced flowers e.g. orchids have passageways that ensure the anthers and stigma touch the bee's body at a particular point and in the right sequence as it enters and leaves the flower.
An incredibly specialized pollination technique called pseudocopulation is found in orchids of the genus Ophrys. The flower resembles a female bee, wasp or fly. The male insects emerge before the females and the orchids flowering occurs at this time. The male insects attempt to copulate with the flower and as a result carry pollen away and transfer it to the next flower visited.
Similar in many respects to bee pollinated flowers as the insects are guided by a combination of sight and smell. At least some species. of butterflies can see red as well as blue and yellow thus flowers may be orange or red. Most moths are active at night and a typical moth pollinated flower is white and has a heavy fragrance emitted after dark. e.g. Nicotiana. Other colours which stand out against a dark background in the evening are used e.g. yellow - Oenothera hookeri (evening primrose), pink -Amarylis belladonna
Nectary found at base of a long slender corolla tube only accessible to the long tongues of moths and butterflies. Moths do not enter the flower but hover above inserting the tongue so moth pollinated flowers do not have landing platforms and complex internal machinery seen in some bee pollinated flowers.
Some birds regularly visit flowers to feed on nectar, pollen or insects and many act as pollinators. Humming birds are the main bird pollinators of the Americas. No bird pollinated plants in Europe. Pure red flowers are inconspicuous to most insects, often pollinated by birds. Birds have a poorly developed sense of smell but good vision. Bird pollinated flowers usually have little odour but are colourful with red and yellow the most common colours. Flowers are large or grouped in inflorescences to offer large amounts of nectar. Examples include fuchsia, passion flower, hibiscus, eucalyptus and members of the cactus, orchid and banana families.
An important plant part where the reflection of light has a clear function is the petals of flowers. Petals are conspicuously unwettable. Flowers are absolutely dependent upon being seen and recognized by insects. They have to absorb and re-radiate specific bands of the spectrum. As a result they must be unwettable - as water can clearly influence the reflection of light. The mechanism used differs from that in leaves. They are not covered in a thick layer of wax or are covered in trichomes, they rely on the shape of the epidermal cells. The cells are usually papillate ( = having blunt projections or protuberances) and may have surface striations - daffodil). The pigments (eg anthocyanins) are contained in the vacuoles of these cells. Underneath the epidermal cells is a layer of unspecialized, unpigmented but reflective cells, these reflect the light back out with the unique surface scattering it.
Flowers are extraordinarily diverse but some evolutionary trends can be recognized. Many trends related to greater and greater specialization of insect pollination mechanisms, - towards bilateral symmetry, - fusion and elaboration of floral parts, - the compound inflorescence. Other trends related to changes in numbers of seeds produced, protection and dispersal, the inferior ovary and different fruit types. Some taxa have become highly specialized for wind pollination eg. grasses.