Celulas Y Tejidos

Cells and Tissues or

What Is Seen Through a Microscope ?

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http://www.biologie.uni-hamburg.de/b-online/e04/04.htm

• The Structure of a Plant Cell

• Tissues

• Meristems

• Literature

In 1838, M. SCHLEIDEN postulated the cell theory, which states that every plant is organized from cells.In 1839, T. SCHWANN showed that it applies to animals as well and in 1855 VIRCHOW coined the phrase:

"Omnis cellula e cellula"

, i.e. latin for "each cell stems from another cell". The sentence is of universal validity and is among the few dogmata of biology. The German botanist H. v. MOHL was the first to observe the propagation of plant cells by division in 1835. Around the middle of the 19th century, the cellular organization of plant tissues was mostly outlined. The growing perfection of microscope construction and use, of cutting and conservation techniques as well as the use of selective dyes allowed the reproducible depiction of vegetable cells and tissues. Illustrated textbooks were published. J. v. SACHS' textbook "Lehrbuch der Botanik" (A Textbook on Botany; first edition published in 1868) and its sequel "Vorlesungen über Pflanzenphysiologie" (Lectures on Plant Physiology) were outstanding and pointing the way. Illustrations derived from it were - often simplified - adopted by many following textbooks, thus proving the fact that well-done scientific experiments form the basis on which the following research is founded.

Since these times, microscopic courses belong to the repertoire of basic botanical teaching. A reference textbook, that is still valid today is E. STRASBURGER's "Kleines Botanisches Praktikum" (A Short Botanical Course; first edition published in 1884).

The examinations of the 19th century were mostly restricted to the observation and interpretation of longitudinal and cross-sections through different plant organs. In the 20th century it was concentrated more on problems of the development of special tissues in the course of ontogenesis and evolution (phylogenesis). It was and is tried to understand the plant body as a three-dimensional structure, and to get an idea of the meaning of the spatial arrangements of single tissues towards each other.

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General Features of Plant Cells: Shape and Content

Cells come in a large variety of forms. There is no such thing as a typical plant cell. The typical plant cell given in some textbooks displays a compendium of features of different cell types. All cells do, nevertheless, share a number of features, while some are typical only for plant cells. A third group of features distinguishes undifferentiated from differentiated cells.

Every cell is surrounded by a membrane. Both animal and plant cells contain a nucleus that is missing in bacteria and blue green algae. It is thus distinguished between eucaryotes (organisms that have a nucleus) and procaryotes (organisms without a nucleus). Contrary to animal cells plant cells are almost always surround by a cell wall and many of them contain a special group of organelles: the plastids, of which the chloroplasts are the most striking organelles. Cell walls can also be found with bacteria and blue green algae, though neither their chemical structure nor their synthesis is the same as that of plants. It does seem to be an analogous feature, i.e. a structure with the same function, which did not derive from a common earlier stage.

The content of a plant cell (the protoplasm) changes in composition in the course of growth and development. Differentiated cells are marked by a voluminous vacuole.

Besides the clearly visible structures like cell nucleus and chloroplasts, a lot of other granular particles can be seen through a light microscope, some of which can be dyed selectively. Sometimes, different shapes (long, round, etc.) can be distinguished. The majority of these particles has a size that is at or below the power of resolution of a conventional light microscope and their identification was impossible before the use of the electron microscope. Mitochondria are a typical example.

The organelles may contain inclusions like crystals, fat droplets, starch grains or others, a feature that is especially common in specialized cells.

Cells are no static objects. Very often, plasma currents occur that can be detected by the movements of organelles like chloroplasts or different granules. Most of these movements have a direction and sometimes it even looks as if single particles would move along rails. I will show later that molecular evidence for the truth of this impression exists.

The Structure of a Plant Cell

To depict the structure of a plant cell, an epidermal cell of an onion will be used as an example. The epidermis is the final tissue that covers all organs above ground. The cells of the onion epidermis are common specimens on the first day of a German basic botanical course. Since they contain no chlorophyll, they are actually no "typical" plant cells.

The picture above shows an onion"s epidermal cells. They are elongated and the ratio of length to width can vary strongly. Each cell is enclosed by a wall. In the region of the cell poles and where three cells adjoin, large intercellular spaces can be observed. Elsewhere a pectin-containing middle lamina cements neighboring cells together like bricks. The cell wall is perforated at regular intervalls, so that adjoining cells are in contact. The holes of the perforation are called simple pits and the plasma cords that run through them plasmodesmata . The surface of the epidermal cells seems to be folded, an effect that is caused by the water-repellent, waxy cuticle.

Protoplasm, Cytoplasm and Cytosol: The Cell's Content

The "living" content of a cell, the protoplasm , is surrounded by a membrane called plasmamembrane or plasmalemma. The protoplasm is usually next to the cell wall, so that the plasmalemma can hardly be seen. To display it, the cells are transferred into a high salt or sugar solution. As a result the protoplasm shrinks and detaches itself from the wall. The process is reversible and is called plasmolysis. This behavior is due to the properties of the membrane and the plasma. It is reviewed in more detail elsewhere. A substance that causes plasmolysis is called plasmolyticum and - depending on its chemical composition (potassium ions or calcium ions, for example) - the protoplasm takes on different shapes. The plasmolyticum has accordingly an influence on the properties of the membrane. The properties of the plasmamembrane differ from that of the tonoplast. The tonoplast is the membrane that surrounds the vacuole. The difference is especially striking if cells with a colored vacuole content are used. Often the vacuole is criss-crossed by numerous plasma cords. The plasma cannot therefore not simply be viewed as a solution that is influenced by the rules of hydrodynamics alone. Rather, it contains viscous, structure-determining components, whose chemical, physicochemical and structural properties have only been recognized recently and in fragments.

Cytoplasm and Caryoplasm

The nucleus is a rather conspicuous part of nearly every living plant cell. Its structure separated from the rest of the cell by the nuclear envelope, a membrane system that consists of two discrete membranes as can be seen on electromicroscopic images. The nuclear content is called the caryoplasm while that of the rest of the cell is called cytoplasm. But these terms are only valid at certain stages of a cell's life cycle. In the course of cell-division and mitosis, the nuclear envelope disintegrates and the nucleus is replaced by the chromosomes. It makes consequently no sense to speak of caryo- and cytoplasm during these stages.

The nucleus of plant cells is usually of a round or elliptic appearance, sometimes it is also shaped like a spindle. One nucleus per cell is the rule, but cells with two or more nuclei are no rare exception. The cells of certain algae of the genus Chladophora have many nuclei, they are polyenergid. The nucleoli that can often be perceived after staining are substructures of the nucleus. They, too, disintegrate during cell division and mitosis and do not reshape before a new nucleus has been formed.

Plastids

Plastids are organelles that occur only in plants. Their most prominent members are the chloroplasts. Others plastids are the colored chromoplasts and the colorless leucoplasts as well as their proplastids. Proplastids are vestigial bodies that are generated during germ cell development due to degeneration of plastids, for example. They may differentiate into complete plastids during the development of the plant embryo. Their ripening into chloroplasts occurs usually only after light exposure.

© M. Knee

Chloroplasts contain the green plant color chlorophyll. They are the places where photosynthesis takes place. Chloroplasts enable the plants to convert solar energy into chemical energy. Because of this process, plants are called primary producers. The existence of consumers, like most animals, depends on them. Chloroplasts occur in most cell types, but only in organs above ground. They can be especially well observed in tissues consisting of a single layer as in the leaflike structures of some mosses (Funaria hygrometrica or Mnium hornum) or in the water plant Vallisneria). Here they are rather large and of a lens-shaped appearance. During daytime, when the light is

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