Venus Flytrap

Pollination

Text 2

Retell one of the texts.

Read the texts using your dictionary.

Pair work. Ask and answer 6 questions to Text 1.

The transfer of pollen from the anthers to the stigma is called pollination. Cross-pollinating – when the pollen grains are carried away on the bodies of insects or simply blown by the wind and may land on the stigma of another flower. In self-pollinating plants the pollen comes from the same flower or another flower of the same plant.

Wind-pollinated flowers are adapted to their method of pollination by producing the large quantities of light pollen. They have anthers and stigmas which project outside the flowers. Some grasses have anthers which are not rigidly attached to the filaments and can be shaken by the wind. The stigmas of grasses are feathery and act as a net which traps passing pollen grains.

Grasses, cereals and many trees are pollinated not by insects, but by wind curents. The flowers are often quite small with inconspicuous leaf-like bracts, rather then petals. They produce no nectar.

The anthers and stigma are not enclosed by the bracts but are exposed to the air. The pollen grains, being light and smooth may be carried long distances by the wind and some of them will be trapped on the stigmas of other flowers.

Insect-pollonated flowers are considered to be adapted in various ways to their method of pollination. In the course of evolution the structure and physiology of a flower have been modified in ways which improve the chances of successful pollination by insects.

Most insect-pollinating flowers have brightly colored petals and scent, which attract a variety of insects. Some flowers produce nectar.

Text 3

If other plants can thrive on gases in the air plus water from the soil, why do Venus Flytraps eat insects? Flytraps actually get a good deal of their sustenance like other plants do, through the process of photosynthesis. During photosynthesis, plants use the energy of the sun to drive a reaction that converts carbon dioxide and water to sugar and oxygen. The sugar produced is then converted to energy in the form of ATP, through the same processes used by our bodies to process carbohydrates.

However in addition to synthesizing glucose, plants also need to make amino acids, vitamins and other cellular components to survive. To accomplish this, plants needs additional nutrients like:

nitrogen - to make amino acids, nucleic acids, proteins;

phosphorus - as part of the energy-carrying ATP molecule;

magnesium - as a co-factor that helps many enzymes function;

sulfur - to make some amino acids;

calcium - as an enzyme co-factor and to make plant cell walls;

potassium - to regulate water movement in and out of the plant.

In the bogs favored by Venus Flytraps, the soil is acidic, and minerals and other nutrients are scarce. Most plants can't survive in this environment because they cannot make enough of the building blocks necessary for growth. The Venus Flytrap has evolved the ability to thrive in this unique ecological niche by finding an alternate means of getting key nutrients like nitrogen. Living creatures like insects provide a good source of the nutrients missing from the soil, and they also contain additional energy-laden carbohydrates.

Carnivorous plants have to be able to:

attract insects

capture bugs

discriminate between food and non-food

digest their prey

All of these steps are accomplished through simple mechanical and chemical processes.In the case of the Venus Flytrap, the leaves forming the trap secrete a sweet nectar that draws in insects searching for food.

When an insect lands or crawls on the trap, it is likely to run into one of six, short, stiff hairs on the trap's surface. These are called trigger hairs, and they serve as a primitive motion detector for the plant. If two of these hairs are brushed in close succession, or one hair is touched twice, the leaves close down upon the offending insect within half a second.

Inanimate objects like stones, twigs and leaves that fall into the trap, or objects that are placed there, will not move around and fire the trigger hairs. If there is no further stimulation of the hair, the trap stays in its partially shut state until tension can be re-established in the leaves of the trap. This process takes about 12 hours, at which point the leaves spread apart again. The unwanted object either falls out as the leaves re-open or is blown out by the wind.

Once the trap fully closes, the leaves form an airtight seal so that:

digestive fluids and insect parts are kept inside the trap

bacteria and molds can't get in.

To make sure that the insects are contained within the trap, the edges of the leaves have finger-like cilia that lace together when the leaves press shut. These long, hair-like projections make the plant look like it has spiny teeth; but the cilia are really only used to latch the trap shut.

Once the insect is firmly ensconced in the trap, the process of digestion can begin. The trap secretes acidic digestive juices that:

Dissolve the soft tissues and cell membranes of the food.

Serve as an antiseptic to kill small amounts of bacteria inadvertently eaten or sealed in with the food.

Enzymatically digest DNA, amino acids and other cellular molecules into small pieces that can be taken up by the plant.

These digestive juices are secreted from glands on the inside surface of the trap, right onto the trapped prey. The insect is bathed in these juices over a period of five to 12 days, during which the insect is digested and nutrients are extracted..

The process continues until all that's left of the insect is its hard exoskeleton. Once the nutrients are depleted from the acidic bath, the plant reabsorbs the digestive fluid. This serves as a signal to reopen the trap, and the remains of the insect are usually either washed away in the rain or blown away by the wind.

Дата добавления: 2014-01-04