for what are of use flowers?

Trees are not eternal. They have to propagate. Some mechanisms allow reproduction on place. For example when the tree was struck down or destroyed by frost (many mimosas froze in January, 1985), it propagate again from roots (it suckers from the roots ; ex : mimosas, maples, holm oak, seldom conifers, except the giant sequoia) or by the base of the trunk (it suckers ; ex : the sumac, some poplars). Others stretch by layering (a low branch is based on the soil and takes root ; example : some firs, willows) or in a descending line air roots which will take bedding in the soil (ex : Pippal, still called " multiplying " in some French-speaking lands). It is also possible to recall the techniques of assisted reproduction (transplant, layering of branches) to produce hybrids, mainly for the fruit trees, or clones (of the same sex in the case of species dioecious).

As all plants, trees reproduce by the germination of the female gametes (egg) with spermatozoons, which are produced by inflorescences, when mature (10 years old for the locust tree, 10 - 20 years for the birch, and 60 years for the beech or the sessile oak).

Contrary to the animals which are mobile and can meet for mating, trees are fixed! How to organise the meeting necessary to survival? It is a history of transport.

The water

Historically, the first vehicle of primitive transport was the water: the rainwater or the water of the dew picked up the ciliate spermatozoons to route them to gametes. Spermatozoons were concentrated in spores: the survivor of this period is the fern. The Ginkgo, which is qualified as living fossil, also draws inspiration from this principle.

The wind

250 million years ago (primary epoch, permien), conifers exploit another vehicle: the wind. The tiny male flower and reproduced in hundred of copies (grouped in catkins) ejects the pollen in abundant quantity. You undoubtedly pointed out the yellow pollen dispersed by some conifers, to the extent that the hoods of cars are colored. The meeting with the female flower is very unpredictable: especially since the ovule is bare, without the envelope of the ovary nor the stigma which makes easier access. These trees form the class of Gymnospermes, literally "in bare ovules". They are "anemophilous" ("they like the wind"; we find anemo in "anemometre", "instrument of measure of the wind").

Some conifers use the water however to increase the collecting area (the yew tree and the pine produce a drop "micropylar" at the entrance of the ovule). Fruits are cones which contain seeds between scales. Or the cone falls entire on the soil where it decomposes and liberates seeds, or the cone opens to disperse seeds to the liking of the wind.

As we know, nature evolves: in cretaceous lower, 130 million years ago, angiosperms, literally "in ovules protected by an ovary" appear: they produce seeds (themselves locked up in fruits). And these seeds have a survival capacity which augments chances of reproduction.


New evolution: willows change pollination by the wind (hence catkins), towards a pollination by insects (their catkins are slightly odorous).

Most angiosperms are "entomophilous" what means that they are pollinated by the honeybees, the butterflies and insects. Of course, pollination can take place with the return of insects in the spring, not winter when nature sleeps.

But then, how attract fowls (and sometimes the ants): flowers are broad (petals), coloured (immaculate, yellow, crimson white), fragrant, and sometimes nectariferous to reward the visitor come to gather them. Look at the magnolia (although its flower is primitive because it includes many stamens and carpels, as though the pollination was not probable) or the cherry tree. By rubbing the male organs (anthers, filled with pollen, at the end of the "filament", the whole constituting the cheesecloth), the insect carries, unknowingly, a little pollen on his legs or his back. On another flower, it rubs the female members and the meeting of the pollen with the egg can take place : the ovary which contains ovules is overcome of an extension, the style, ended by the stigma on which the pollen will germinate. A gamete unites with the egg and of this union the sprout containing the seed will be born. Another gamete unites with two stones of the ovule to form the egg white which will serve to accumulate reserves. After fertilization, the ovary is transformed into fruit. The fruit trees top up a refinement: they avoid the pollination on the same tree. The insect has to to go from a tree to other one to "cross " the pollination (case of the apple tree and of the pear tree).

The ripe fruit falls to the ground and the seedling takes root, not right away, because it is necessary to wait for the winter (as buds). Nature is well made!

But, in these conditions, the aerie does not stretch. It is here that another conveyance intervenes: birds. They have the good idea of feeding on fleshy fruits (for example of cherries) and to reject seeds here and there...

It happens that the male organs (stamens) and the females (ovaries) are carried by the same inflorescence: in that case, they mature in brought forward periods, to avoid the autopollination (incestuous pollination of the same flower). This flower is said hermaphrodite or bisexual. The most common example is the family Rosaceae to which belong the fruit trees (cherry tree, apple tree, etc.).

Flowers are sometimes separated on distinct trees (in that case the species is "dioecious", for example the tree of heaven or the Sweetgum).

Evolution always: at the beginning of the tertiary (650 million years), petals knit together and create subtle forms to trap insects (flowers papilionaceous of leguminous plants, flowers in the form of finger of paulownias).

The various reproduction mechanisms allowed to classify trees:

Science consisting in classifying species is called the taxonomy. The first classification tries were based on notions of reproduction: Andrea Cesalpino (1524-1603) became attached to seeds and to young plants (not obvious to classify a grown-up tree), Pierre Magnol, doctor and botanist of Montpellier (1638-1715), worked out another classification system by families, John Ray (1628-1705) took into account the fruit and among cotyledons of the seed, Joseph Pitton de Tournefort (1656-1708) took for criterion the form of the corolla of flowers. The current classification was worked out by the Swedish botanist, Carl von Linné, by 1730, and published in an exhaustive way in 1753 (Species Planturum): it is based on the number of stamens and of pistils and their arrangement. It is a "sexual system" to the great displeasure of very thinking minds of epoch! Errors of evaluation could be corrected as the knowledge of the reproduction mechanisms was refined. The species of trees (and more finely varieties) are classified in genera (example: maples, in Latin Acer), grouped in families (the family of maples is that of Aceraceae), grouped themselves in orders (angiosperms and Gymnospermes). The name of trees is a bit their calling card (see " the Latin names ")

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