Botany online 1996-2004. No further update, only historical document of botanical science!
Further results concerning intracellular structures could not be gained before the use of new preparation techniques (studies dating from the mid-fifties to the mid-sixties). Freeze etching proved to be an optimal method for the depiction of membrane surfaces. The resolution is high enough to recognize single macromolecules or macromolecular complexes, especially if they are arranged in the membrane in regular patterns as two-dimensional crystals .
The use of glutaraldehyde as a conserving agent allowed the depiction of tube-shaped structures, the microtubuli. They are part of the cytoskeleton and are essential for the movements of organelles and other intracellular particles, for the transport of chromosomes and as structural elements of flagelli (9+2 structure).
Among the peculiarities of a plant cell is the cell wall, whose layered structure could be confirmed by electron microscopical analysis. Layers occur also in collenchyma, stone or corked cells.
The use of highly specific markers or probes to localize certain molecules or classes of molecules is another important technique. We will discuss this problem later. It represents a bridge to the molecular level. To stress this further, it is also spoken of the molecular architecture of a cell.
The attempts to analyze electron microscopic pictures with the aim to reveal the three-dimensional shape of intracellular structures are independent of the mentioned methods. The huge amounts of data needed for this make computers indispensable. The biggest and up till now not completely solved problem is the structure recognition by the computer. A machine cannot decide, wether a particle of a certain grey colour is actually part of a structure recognizable by the observer or not. The single steps of a method for the construction of a three-dimensional model of dictyosomes developed by W. MEINHARDT, J. LOCKENHAUSEN, W. J. DALLAS and U. KRISTEN ( Phillips Forschungslabor Hamburg and Institut für Allgemeine Botanik, Hamburg, 1986) are explained in the following. An important step, the conversion of the electron microscopic picture into a sketch of the outline remains to be done by hand.
The cell wall is in many places perforated by pits as has been mentioned before. Plasma bridges, so-called plasmodesmata run through them and connect the protoplasts of neighbouring cells. The pits and plasmodesmata can occur in various shapes. They may have a very narrow lumen, may be branched and/or very numerous; they may also have very wide lumina and occur only sparsely. Especially striking pits can be found in the different cell types of the vascular tissues, they have already been mentioned. Examples are the pores in sieve plates or the pit areas in xylem elements. As we have been seen before, a number of cell and tissue types can be identified by means of light microscopy. But the use of the electron microscope allows a closer examination of the differences between certain cell types.