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Methods of Plant Anatomical Studies (With Diagram)!
It goes without saying that the aid of a compound microscope is indispensable for the studies of the minute structures of plants. Very thin and uniform sections are taken for observation under the microscope.
In case of cylindrical bodies like the axis sections are taken both at right angles to the axis (transverse or cross-section) and along or parallel to the axis (longitudinal section). Longitudinal planes are of two types, viz. that dividing the axis radially is known as radial plane; and that at right angles to the radial plane is the tangential plane (Fig. 481). For detailed studies sections along all the planes mentioned above are necessary.
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Normally preparations are made free-hand with the help of a sharp razor. For specialised studies an apparatus known as microtome is employed. For making sections by microtome the materials have got to be killed, fixed and ultimately embedded in paraffin, which involves long-drawn elaborate processes.
Nevertheless microtome has positive advantage for two reasons, viz. first, the sections are always uniformly thin, and, secondly, serial sections may be obtained. In the studies of developmental anatomy microtome is of inestimable value.
Another method, called maceration, is also employed for working out the detailed structure of the individual cells and elements. This process involves complete or partial dissolution of the ‘cementing material’—the intercellular substance lying between the cells, by chemical treatment.
Thus the cells become isolated and can be conveniently observed as three-dimensional bodies. The cells tend to create two-dimensional concept in sections. Moreover, many characters of the cells and particularly the nature of the wall with various types of localised thickenings become much more clear. Different techniques of maceration have been in use.
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Other methods now employed are whole mounts, peel mounts and scrape mounts. Often materials are mounted entire without maceration or sectioning, though trimming
of excess of tissues or reduction of the sizes of the materials may be necessary.
This method is commonly employed for observation of lower plants like algae, fungi, thalloid bryophytes and prothalli of ferns. In higher plants it is used conveniently for the study of leaf-epidermis and stomata, hairs and trichomes, small flowers and pollen grains.
In case of leaf-epidermis and stomata practically small portions are peeled off and mounted as usual. So it may be called peel method as well. Apart from cell-contents, scraping of portions reveals clear nature of tissues like cambium, tracheids and tracheae.
The value of the magnifying apparatus light microscope has already been mentioned, but it has its limitations. Transparency to visible light is a positive handicap in observing the cells in living state. Polarised light microscope has proved quite helpful particularly in case of structures like the cell wall with highly oriented molecules. Phase- contrast microscopes with monochromatic light source developed later were very useful in watching the process of cell division—undoubtedly a dramatic biological phenomenon.
In modern times a new tool—the electron microscope with high resolving power—200 times higher than that of light microscope, has enabled us to understand the finer details of many structures, as well as in discovering many new structures hitherto unknown. Here a beam of high speed electrons is focused by electromagnetic lenses in place of a beam of light.
The unit of measurement in a light microscope is a micron, written as pm, one thousandth part of a mm, whereas the unit in case of an electron microscope is an Angstrom, written as Å =one ten thousandth part of a micron or ten millionth of a mm or 01 nm (nanometre).
So one µm = 10,000Å or 1,000 nm. Structures up to 15Å (1.5 nm) can be studied under electron microscopes compared to about 3,000A (300 nm) visible under highest power light microscope.
