PLANT ANATOMY: HISTORY, OBJECT OF STUDY, METHODS - BIOLOGY - 2024

The anatomy of plants in a strict sense, is the fundamental basis for the study of a great variety of plant tissues, being a tool of great importance in botany and in biological sciences in general. This discipline focuses mainly on the cellular study of tissues by microscopy from their origin to their development.

All reproductive tissues that are studied together in the field of plant embryology and palynology are often excluded. The way in which cells are put together and arranged with each other is of great interest in plant anatomy.

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Plant anatomy is closely related to other areas such as the physiology of plants and their morphology. The characteristics observed in most cases are differential between groups of plants and are used to establish phylogenetic relationships.

History

In its beginnings, plant anatomy also included the study of the morphology of plants and their external characteristics. However, since the mid-twentieth century, anatomy studies have been exclusively restricted to the study of internal organs and internal tissues, morphology being a separate discipline.

The first works on plant anatomy and botany, carried out with the help of the microscope, are due to Marcello Malpighi and Nehemiah Grew. By 1675 Malpighi had published his work Anatome plantarum, where he describes through illustrations some plant structures such as the stomata of the leaves.

For his part, by the year 1682 Grew published a work with very reliable illustrations on plant tissues, which demonstrate the accuracy of his observations. This work was entitled The anatomy of plants.

Starting in the 1960s, the development of microscopy represented a great advance in all areas of plant anatomy.

Microscopy and its use in plant anatomy

The study of plant structures has had a development closely related to the creation and evolution of microscopy. Since their invention in the 17th century, microscopes have evolved into the intellectual tool that shaped many areas of biological science.

One of the first areas to be favored with the development of microscopy was botany, especially in anatomical study. Experimental scientists Robert Hooke and Leeuwenhoek have been recognized as one of the first to observe microscopically and describe various structures during the 17th century.

In the works of Malpighi and Grew, microscopy played a fundamental role, allowing the development of these two valuable botanical works, making these important scientists of the seventeenth century the pioneers of plant anatomy and botanical micrography.

Since then, the study of plant anatomy has been developed together with microscopy. The latter was evolving according to the knowledge needs of man.

Microscopy is currently an essential tool in the study of plant structures, where it is used from simple magnifying glasses to advanced technology electron microscopes.

What does plant anatomy study?

Plant anatomy is responsible for the study of all tissues and forms of organization of the same, present in plants. This indicates that it evaluates both the tissues and internal cellular organization and the study of external structures.

Among the evaluated structures are: leaves, stems, barks, roots, stem and root tips, meristems and tissues after cell differentiation, cell arrangement in organs, among others.

Methods and techniques

The techniques applied to the study of the anatomy of plants are very varied. Each of them will depend on the tissue or organ that is being studied.

In general, permanent preparations for microscopic studies are indispensable as a source of elementary information both in research and in teaching. However, for the fixation of samples of various anatomical tissues, a series of basic techniques must be executed for their subsequent observation.

The latter are applied because the tissues and their components are difficult to differentiate clearly with direct observations.

All plants are made up of the same basic, dermal, fundamental and vascular tissues. Within these tissues, the way in which cells are organized differs markedly between plants and, therefore, the anatomical methods for processing them are different.

In general, the botanical material to be studied must meet certain characteristics, for example, that the structures are completely healthy and developed. In addition to this, they must not have external or internal structural damage and their coloration is typical of the species studied and that the specimen from which the samples are extracted is representative.

Fixation

The fixation process seeks to preserve the tissues and their morphological characteristics as similar as possible to when the tissue was alive. This can be achieved either with physical or chemical fixatives. The most widely used are simple fixatives such as ethanol, methanol or acetone, which fix by dehydration.

They work very well for small samples and can even preserve tissue pigmentation. Aldehydes such as formaldehyde, glutaraldehyde, and acrolein can also be used. Other coagulating fixatives include ethanol, picric acid, mercury chloride, and chromium trioxide.

Fixing mixtures are also used, of which there are more than 2000 published formulas, the most frequent being FAA, fixatives with chromic acid, Farmer and Carnoy mixtures, among others.

Always during this process, special care must be taken with the fixing time and the temperature at which it is done, since processes such as autolysis can be accelerated.

Therefore, it is recommended to perform it at low temperatures and at a pH close to the physiological of the tissue to avoid the formation of artifacts in the tissues that lend themselves to anatomical misinterpretations.

Dehydration

It consists of the elimination of the water content of the previously fixed plant tissues. This is often done with an increasing gradient of dehydrating agents that may or may not be paraffin solvent, with paraffin being one of the main agents to include.

Solvent dehydration of paraffin is carried out mainly with ethanol in a series of 30, 50, 70 and 95%.

After this process, the tissues are transferred to a paraffin solvent dehydrating agent. These agents generally make tissues translucent. The most common agents are xylene and chloroform. A concentration series is also used for these reagents.

Infiltration / embedding of tissues in paraffin

This operation is carried out in order to replace the dehydration medium with the infiltration / inclusion medium. This gives the tissue sufficient rigidity to make thin and firm cuts, due to the temporary hardening of the tissues and cavities that it presents. The most widely used material is histological paraffin.

Microtomy

The samples included in paraffin blocks are sectioned with the help of a microtome, which makes cuts thin enough to be observed under a microscope. All morphological structures are preserved after cutting in such a way that the study of the tissue is facilitated.

In general, the cuts are 1 to 30 microns thick. There are several types of microtome that are frequently used, including the benchtop microtome, freeze, cryostat, slide rotation, and ultramicrotome. Some of them with specialized diamond or glass blades.

Staining

The histological sections are stained to facilitate the observation and analysis of the different cellular components.

The colorants and staining techniques are applied depending on which structures are to be observed more easily. The most common dyes used in botany are safranin "O", fast green FCF, hematoxylin, Orange G, aniline blue and toluidine blue. The selection of one dye or another depends on the ionic affinity of the dye with the structure to be dyed.

Contrast stains such as the combination of safranin "O" and fast green FCF can also be used. Safranin stains cutin red, lignified walls, nucleoli, chromatin and condensed tannins, and suberin reddish-brown. While FCF stains the cellulosic walls look bluish and a purplish green tone to the cytoplasm.

On the other hand, toluidine blue dyed fabrics range from dark blue / reddish to light blue / pink.

Histochemical tests

Histochemical tests are used to reveal molecules or families of molecules present in the studied tissue and evaluate their tissue distribution "in situ".

These tests can be performed using chemical reactions to detect free or conjugated carbohydrates and enzymatic histochemical tests in which cellular enzymatic activity is detected even after chemical fixation of the tissue.

The final product of this set of techniques ends with the evaluation of the histological section prepared with microscopy tools. Optical or electronic microscopes can be used, either scanning or transmission. Many of these characters are very small (ultrastructural or micromorphological).

Other techniques include the maceration of plant tissues to separate their components and observe them individually. An example of this is the maceration of tissues such as wood, which facilitates the observation of tracheal elements and other structures and make a detailed analysis of them.

References

1. Beck, CB (2010). An introduction to plant structure and development: plant anatomy for the twenty-first century. Cambridge University Press.
2. Blanco, CA (Ed.). (2004). The blade: external morphology and anatomy. Universidad Nac. Del Litoral.
3. Megías, M., Molist, P., & Pombal, M. (2017). Atlas of animal and plant histology. Vegetable tissues. Department of Functional Biology and Health Sciences. Faculty of Biology University of Vigo. Spain. 12pp.
4. Osorio, JJ (2003). Microscopy applied to botany. Theoretical-practical course. Academic Division of Biological Sciences. Autonomous Juárez University of Tabasco.
5. Raven, PH, Evert, RF, & Eichhorn, SE (1992). Plant Biology (Vol. 2). I reversed.
6. Sandoval, E. (2005). Techniques applied to the study of plant anatomy (Vol. 38). UNAM.