Flower structure

Pollination

Angiosperm life cycle

Fruits

Seeds

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The Basics of Flowers

What are the characteristics of angiosperms that make the group so unique and appealing? the evolution of flowers and and fruits of course.

Most of you probably have had one or more lessons on the parts of the flower early in your career. Still, a quick review should be helpful:

Although the model above displays all the potential 'parts' of a flower, many specialized species may not contain all structures in a single flower or may in cases fuse parts to increase the probability of pollination as you can see from the examples below.

Specifics on parts:

Petals = surround the carpels and stamens. Generally they are pigmented to attract potential pollinators but are too thin to do much in the way of protection. Often they have markings to guide pollinators or may change colors to signify when feeding time is over ( already pollinated; no more donors needed)

Sepal = combined called calyx , are actually modified leaves, thick and waxy, which may form a protective barrier during development of the flower i.e. deter insects

Note in this example of a daffodil, the sepals are modified and are fused to the petals, which also have fused to form a tube-like structure 

The daffodil flower is perfect meaning it has all the standard parts and the plant is monoecious, carrying both sex organs.

If flowers are imperfect, the plant can be:

either pistillate or staminate ( male only)

In dioecious plants there are separate female and male plants, in monoecious both male and female flowers can be found on the same plant.

II. Not all flowers however have this typical structure

Flowers which are WIND pollinated generally don't expend much energy on petals or pigmentation. Instead, since wind is an inconsistent pollinator, they use their reproduction budget to produce more flowers, with exposed stigmas and anthers loaded with pollen.

The grass flower shown below is actually a very advanced design which minimizes any extraneous tissue and maximizes the probability of pollen interception and dispersal. It is placed high above the blades on thin stems which shake in the wind.

When mature, the stigmas a large and feathery. The one on the left is just emerging.

Wind pollination is most efficient where there are dense populations of the same species. Grasses form large perennial populations which allow this strategy to work.


Orchids, unlike the grasses above, are rare. In the tropical rain forests,(often) existing as epiphytes high in the trees, they are fairly isolated from one another.

They need to be noticed easily by potential pollinators ( birds & insects) and would certainly opt for a carrier which is true to the species ( high fidelity = will only carry that species pollen and not everyone's else's unwanted pollen at that same time period) .

Thus they need to be easily distinguished while in flight from other orchids, and must be attractive enough to entice their pollinator far distances from the last orchid ( of the same species) visited.

The orchid is a very advanced design, with fusion of petal parts and the production of specialized male anthers. Instead of many anthers, all are fused to produce an pollinia, which is "picked" up by one large pollinator ( bird, bee) and brought to another orchid of the same species hopefully. This prevents self pollination of the orchid, increases the probability that a visitor will come to this widely dispersed plant, and if only one arrives, that it contains enough pollen for reproduction to continue on.

 

TO better appreciate how specialized flower adaptations can become, read the below abstract:

The flower of the vine Mucuna holtonii, above, sports a small, concave "mirror" that reflects most of a bat's echolocation and guides the animal to its nectar. Picture: Courtesy of Marc Holderied/University of Erlangen/Germany |

4/30/99 : VINE USES PETALS TO LURE BATS

A Central American vine that relies on bats for pollination attracts them by using its flower petals like tiny satellite dishes to bounce the animals' sonar signals back at them. The Mucuna vine is believed to be the first plant species found to use such a mechanism.

Bats navigate by bouncing signals off of objects. The signal is so high-pitched it is not audible to humans. Researchers found that each blossom of the Mucuna vine contains a special petal with a concave acoustical "mirror." The petal directs signals back toward any nectar-feeding bat that chirps in its direction. The discovery was made by Dagmar and Otto von Helversen, a husband-and-wife team from the University of Erlangen in Germany. They reported their findings in today's issue of the journal Nature. "This is a very elegant, very interesting, very beautiful story, and if other researchers verify this, it will appear n all biology textbooks as a classic example of adaptive evolution," said Karl Niklas, a professor of plant biology at Cornell University and editor in chief of the American Journal of Botany. The von Helversens tested their theory by removing the acoustic guide petals from some blossoms and stuffing others with cotton pads. They found that 20 percent or less of those blossoms were visited by bats, compared with about 75 percent of the untouched flowers.

The vine grows along creeks and in rain forests. Its short-lived two-inch-long, whitish-green flowers are timed to open every half hour to an hour. The acoustic device rises into its reflective position only when the night-blooming flowers are ripe for pollinating, thereby discouraging bats from visiting a flower that isn't ready to give up its pollen. Source: Associated Press

 

Comparing Primitive and Advanced

Overall, how can we distinguish the simple from advanced flowers? the chart below will give you a good start

Primitive

Advanced

spiral placement of petals, sepals etc.

radial symmetry: petals and parts in a circle-like or pie like arrangement

 

lots of flower parts

free parts ( petals, anthers)

perianth or petal display large

beetle pollination, after all they were thought to be the first pollinating insects out there.

bisexual containing both male and female parts

hypogynous: or superior ovary: this means that the ovary sits above the base of the petals, exposed to potential herbivores !

single flower on a stalk

all flowers alike( if many)

whorled placement of parts

symmetry bilateral (zygomorphic = can only be cut in one direction to get to equal halves i.e.. mirror image): this helps direct the pollinator to a specific location

reduced numbers

fused parts

perianth missing in specialized plants

wind or bee pollination

unisexual

epigynous or inferior ovary: means the ovary sits below, protected by the tissue above

inflorescences ( see pollination page for significance); many flowers on a single stalk - the big attraction

division of labor..

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For the following flowers determine whether they are primitive or advanced? what characteristics help you define their status


Messing with Nature......a better understanding of the genetics of flower development has permitted humans to get into the act of flower sculpture

Lifelines: De-pipping the Pippin

Christopher Surridge is a biological sciences editor of Nature.

. Yao, J-L., Dong, Y-H. And. Morris, B.A.M. Parthenocarpic apple fruit production conferred by transposon insertion mutations in a MADS-box transcription factor. Proceedings of the National Academy of Sciences USA. 98, 1306&.;1311 (2001)

Pip-less is pointless to a plant, but deliciously desirable for farmers

Conventional breeders in search of a commercial seedless apple may have been pipped at the post by geneticists. Jia-Long Yao's team at the Horticulture and Food Research Institute of New Zealand in Auckland have identified mutations in a single gene that give rise to apples sans seeds.

Several centuries' worth of plant breeding has produced a small number of seedless apple varieties, including the Wellington Bloomless, Spencer Seedless and Rae Ime. But these apples yield small, poor-quality fruit ; for a plant, making seedless fruit is a pointless waste of energy.

For the farmer seedless fruit has many agronomic and economic advantages. Consumers will pay more for it ; as evinced by the mark-up on seedless grapes. And seedless varieties crop without the need of pollinators, so they do not depend on the presence of particular insect species during flowering ; insects that can be affected by disease and poor weather conditions.

If hand-pollinated, Wellington Bloomless, Spencer Seedless and Rae Ime produce twice the normal number of seeds. But their flowers are so stunted that they fail to attract insect pollinators. These distorted flowers gave Yao's group a clue to the cause of the apples' seedlessness.

Instead of petals and anthers, Rae Ime blooms have additional sepals and styles. This arrangement is highly reminiscent of a classic flower mutation in the model plant Arabidopsis thaliana which robs the flowers of petals and anthers.

The four organs that make up a flower ; carpels, petals, anthers and styles ; usually form concentric circles, or whorls, with styles in the centre and carpels on the outside. This positioning is achieved by the overlapping production of different members of a 'MADS-box' family of proteins in the developing flower.

One of these proteins, 'PISTILLATA' (encoded by the gene PISTILLATA), is manufactured in the second and third whorls of simple flowers. It converts organs that would otherwise become carpels and styles into petals and anthers respectively.

Molecular genetics in any form of tree is difficult and time-consuming, not least because trees grow slowly. But, taking the hint from Arabidopsis, Yao's group identified a PISTILLATA-like gene in Granny Smith apples.

They found that a length of extra DNA in the middle of this gene renders Rae Ime apples incapable of producing the PISTILLATA protein. They discovered similar defects in the PISTILLATA gene of Wellington Bloomless and Spencer Seedless.

PISTILLATA has been extensively studied in Arabidopsis, and so you'd think researchers would have noticed its involvement in 'parthenocarpy' ; fruit production without fertilization &.; before now. But apples and Arabidopsis produce different types of fruit.

Arabidopsis, a weedy variety of cress, produces a very simple type of fruit, a 'silique'. This develops from the ovaries alone, in which PISTILLATA is never involved. Apples, on the other hand, form 'pome fruits', with seeds embedded in fleshy tissue derived from sepals, petal and anthers.

PISTILLATA must somehow exert a block on the development of pome tissue ; a block that fertilization relieves in normal apples. Perhaps we can now look forward to the pip-less pear (another pome fruit) and even the stone-less plum.

Macmillan Magazines Ltd 2001 - NATURE NEWS SERVICE