Leaf - biology.
Publié le 11/05/2013
Extrait du document
«
The mesophyll, sandwiched between the upper and lower epidermis, consists of many thin-walled cells that are usually arranged in two layers.
The palisade layer is next to the upper epidermis.
It consists of cylindrical cells that are packed closely together.
Next to the palisade layer and making up most of the thickness of the leaf bladeis the spongy layer. The spongy layer consists of roundish cells that are packed loosely together and have numerous air spaces between them.
In most plants the spongy layer extends down to the lower epidermis.
However, in certain grasses, irises, and other plants whose leaves grow straight up and down, the spongy layer iswedged between two palisade layers of mesophyll.
Like the guard cells, all the cells of the mesophyll contain chloroplasts.
E Veins
Running through the middle of the mesophyll and branching out to all of its cells are veins.
The veins extend into the petiole and connect with other veins in the stem ofthe plant.
A major function of the veins is to help support the leaf blade.
Each type of plant has a characteristic pattern of veins forming lines and ridges in the blade.
The veins of a leaf are made up of two specialized tissues, xylem and phloem.
Xylem usually forms the upper half of the vein.
It consists of tubular open-ended cellsthat are arranged end to end.
The walls of the cells are thick and rigid.
Xylem conducts water and dissolved minerals to the leaf blade from the rest of the plant.
Phloem lies on the underside of the vein.
It is made up of thin-walled tubular cells with tiny openings at their ends, somewhat like a sieve.
These cells are also arrangedend to end.
Phloem carries food manufactured in the blade to the rest of the plant.
III LEAF GROWTH AND LEAF FALL
A leaf has a limited life span, usually living for only a single growing season in most deciduous plants and seldom more than a few years in evergreen plants.
Intemperate regions, leaves develop and grow during spring and early summer.
In autumn they grow old, change color, and die.
In nonwoody plants (low in xylem) theleaves wither and fall away because of decay and various external conditions.
Woody plants (rich in xylem) lose their leaves as a result of characteristic changes in thebase of the leaf.
In tropical regions that have distinct wet and dry seasons, the formation and fall of leaves depend on moisture conditions rather than temperature.Contrary to popular belief, evergreen plants also shed their leaves.
However, evergreens are never bare because the old leaves are pushed out only as new leavesdevelop.
A Budding
All leaves develop from buds, which are located at the nodes, or joints, of a plant stem and at the end of a plant stem.
Contained in the buds are areas of rapidlygrowing tissue, called the meristem. The meristem gives rise to the first recognizable signs of the leaf.
In the spring the buds shed their outer covering and open, exposing the leaves.
As leaves develop, they are arranged on the stem in one of three ways: alternate, opposite, or whorled.
The arrangement provides an equal distribution of leaf weighton the stem.
It also prevents overlapping so that each leaf can receive adequate sunlight.
B Color Changes
In addition to chlorophyll, leaf cells also may contain other pigments.
These pigments account for the color of autumn leaves.
Among the pigments found in leaves areyellow xanthophylls, yellowish-orange carotenes, and red and purple anthocyanins. Leaves also may contain tannins, which give them a golden-yellow color in autumn.
Like chlorophyll, xanthophylls and carotenes are contained in tiny granules in some leaf cells.
Although these pigments are present throughout the leaf's lifetime, theircolors are usually masked by the green of chlorophyll.
In the autumn, however, chlorophyll production decreases, and the yellow and orange pigments become visible inthe leaves.
Eventually all pigment production stops, and the leaves turn brown.
Unlike xanthophylls and carotenes, anthocyanins are not contained in granules but are dissolved in the liquid part of leaf cells.
In some plants, such as coleus and redcabbage, anthocyanins are always present, giving the leaves a reddish or purplish color.
In other plants, anthocyanins are not present throughout the life of the leaf,but are produced only under certain conditions.
In oak and maple leaves, for example, sugar accumulates in autumn.
This accumulation is believed to result in theformation of anthocyanins and the production of vivid colors in the leaves.
C Leaf Fall
The leaves of evergreens continue to function and manufacture food throughout the year.
In deciduous plants, however, the leaves stop functioning in the autumn anddrop off.
Leaves may be killed by frost, but changes due to age and growing conditions occur well before then.
Decreased day length, reduced light intensity, lowertemperatures, lack of water, and decrease of growth-promoting substances in the plant all contribute to the decline of the leaves.
The changes start in the weakest partof the petiole, at the base.
During autumn the cells in the base of the petiole begin to disintegrate and die.
As a result, the leaf blade is supported only by the veins inthe petiole.
Soon the vascular bundles become plugged, decreasing the flow of water, food, and minerals to and from the leaf blade.
When the blade is disturbed, as bywind, it breaks off the plant at the base of the petiole.
IV IMPORTANCE
Unlike leaf-bearing plants, animals cannot manufacture their own food.
For this reason, animals must receive their nourishment, either directly or indirectly, from leaf-bearing plants.
For example, cattle, sheep, and horses eat the leaves of grasses and other plants.
These animals, in turn, are consumed by various carnivorous animals,including humans.
Humans also eat many kinds of leaves directly, including artichoke, cabbage, lettuce, and spinach.
In addition to being a source of food, leaves provide many useful products.
For example, the leaves of tea plants are made into a beverage, and the leaves of thyme,sage, and parsley are used for seasoning foods.
Tobacco leaves may be smoked or chewed.
Drugs are obtained from the leaves of foxglove, witch hazel, senna, andmany other plants.
Oils extracted from the leaves of geranium and citronella plants are used in manufacturing perfumes and soaps, and oils from mint and wintergreenleaves are made into flavoring extracts.
Tannins, chemical substances used in preparing leather, are derived from sumac leaves, and dyes are made from indigo andhenna leaves.
The leaves of many plants may be used as fertilizer.
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.
All rights reserved..
»
↓↓↓ APERÇU DU DOCUMENT ↓↓↓
Liens utiles
- A transparent leaf instead of the mouth 2016/2017
- Maple Leaf Rag - analyse de l'oeuvre.
- Alligator - biology.
- Amphibian (animal) - biology.
- Basilisk - biology.