Plants and their extracts have been used for centuries to relieve pain, aid healing and kill bacteria and insects. They have also been employed in perfumery, cosmctics and religious rites. Plant chemistry became an established discipline in universities at the end of the nineteenth century and since then many new structures have been discovered. The number of natural products obtained from plants has now reached over 100 000 and every year new chemical compounds are being discovered. Although their functions in plants have not been fully established, it is known that some substances have grow th regulatory properties whilst others are involved in pollination and seed dispersal. Many are important as antifungal and antiherbivore agents with further compounds being involved in the symbiotic associations in plants. Detailed information on individual plant chemicals is available in Ha&Dictionary of Natural Products and the Phytochemical Dictionary.
The complex nature of these chemicals, which are usually produced in various types of secretory structures, is also influenced and control led by genetic and ecological factors and significantly, by the mode of extraction from the plants. The type of secretory structure is an important characteristic of a plant family. Detailed anatomical description of these structures is relevant to the market value ofthe plants, the verification of authenticity of a given species and for the detection of substitution or adulteration. It also provides a guide to the method of processing.
Microscopical investigation of plant structures is an important pan of the complex biological research process which includes plant growth and development, genetics and breeding. There is endless variability in form and structure: observing these with the microscope allows us to discover Nature in one of its most powerful forms.
Plant material received for these investigations was examined fresh or as chemically-fixed and stained sections for light microscopy (LM), chemically-fixed and critical point dried (CPD) for scanning electron microscopy (SEM) or cryogenically fixed for cryo-SEM. No single technique was ideal for all purposes since each had its own limitation and risk of introducing artifacts but by employing several methods these difficulties were reduced lo give a clearer understanding ofthe structures. For LM. thin transverse sections (T/S) were cut from wax-embedded material and stained to provide colour contrast, however, the involved chemical processes invariably caused some damage to the sample. This was alleviated by examining fresh, unstained sections either by differential interference contrast (DIC) or. if sufficient contrast was attainable, bright field illumination (BF) where no chemical or staining processes were employed. SEM allowed high magnifications together with improved depth of field giving an overall view of the structures. Since SEM is incapable of recording colour, all images produced using this method were manually enhanced using computer techniques.
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