Plant Anatomy: A Practical Guide - Panyuta O.O. 2019

Topic 2. Tissues
Laboratory Work No. 5. Meristems, or Forming Tissues

Higher plants, which possess the highest degree of structural Organization, consist of a combination of specific tissue types and Organs. Tissues are groups of Cells similar in Structure, united by a common origin and shared Functions. Plant tissues vary in structure and origin, and perform distinct physiological functions. Based on these features, they are divided into various types.

Tissues are classified according to Cell shape, the density and compactness of their arrangement, their functions and metabolic activity, The Nature of their differentiation, and other characteristics.

The most widely accepted and commonly used Classification is the anatomical-physiological classification of tissues, which is based on the origin, structure, and function of the cells. According to this classification, the following tissues are distinguished: meristematic tissues (or Meristems), Dermal Tissues, Vascular Tissues, mechanical tissues, assimilatory tissues, storage tissues, aeriferous tissues (or aerenchyma), and Secretory Tissues.

Higher plants possess tissues that develop at a specific stage of their ontogeny. The body of a newly developing embryo consists of identical cells that only later give rise to various tissues.

Theoretical Background. Plants grow throughout their entire lives. Growth processes cease only under unfavorable conditions—for example, during autumn and winter in temperate latitudes, or during periods of drought. Plant growth is localized in specific Regions of the body known as growth points or growth zones. Meristems, or meristematic tissues, are concentrated within these growth points.

Depending on their origin, meristematic tissues are divided into Primary and secondary.

Primary meristematic tissues are formed at the early Selection/3.html">Stages of development of the plant Organism or its individual organs, whereas secondary meristems arise later and can develop from permanent tissues. In this case, a peculiar rejuvenation process of the plant organism takes place.

Meristematic tissues consist of small, living cells that tightly adhere to one another without any intercellular spaces. The thin, delicate walls of meristematic cells are composed primarily of hemicellulose, pectic substances, and a small amount of Cellulose. The entire cell is filled with Cytoplasm containing a Nucleus and nucleoli. In shape, the cells of meristematic tissue are parenchymatous. A characteristic feature of meristematic cells is their ability to divide rapidly over a short period of time.

Meristematic tissues are best studied in the growth points of the plant body where new organs or parts are formed—for example, at the apex of a stem or ROOT, where the SHOOT apex is located.

Depending on their position, meristems are classified into apical (terminal), intercalary (insertional), and lateral.

Apical meristems are located at the growth points of the stems and roots of vascular plants, whereas intercalary meristems are situated in the internodes, where they drive the lengthwise growth of the organ. Lateral meristems ensure the regeneration of certain tissues and the radial thickening of the organ. Lateral meristems include the cambium, which forms a continuous ring surrounding the wood on all sides. As a result of tangential division of cambial cells, the woody stem thickens.

Secondary meristems arise from permanent tissues, which distinguishes them from primary meristems. For instance, the cork cambium, or phellogen, is a secondary meristem because it originates from the Cells of the epidermis, cortical parenchyma, or phloem.

Apical meristems are found at the growth points of stems and roots in vascular plants. The structure of the apical meristem varies among different plant species. For example, in mosses and pteridophytes, the apical meristem terminates in a single apical cell, whereas in some Algae, it has a cylindrical shape with a rounded apex. The stems of liverworts and green mosses contain a biconvex apical cell that appears triangular in longitudinal section and is outlined by two arcs in planar view.

In ferns and horsetails, the apical meristem consists of just a single tetrahedral cell.

In seed plants, the growth points of the stem and root have a nearly identical structure. They lack a single apical cell and instead consist of several cell layers that form tissues. Externally, the growth point is covered by a layer of dividing parenchymatous cells. This cell layer, called the dermatogen, gives rise to the epidermis. Beneath it lie one or several cell layers that form the periblem, the cells of which develop into the primary cortex. The central part of the growth point is formed by the pleroma.

It gives rise to the central cylinder of the plant.

However, this STRUCTURE OF THE growth point is not characteristic of all seed plants. For instance, in conifers, the dermatogen cannot be clearly distinguished within the growth point.

Objective: to examine the Structural Features of meristematic tissues.

Materials and Equipment: light microscopes, Glass slides and coverslips, dissecting needles, forceps, glass rods, filter paper, distilled Water, Javelle water, methylene blue, plant material.

Slide. Structure of the Shoot Apex of Canadian Waterweed (Elodea canadensis Michx.)

The aquatic plant elodea is a highly convenient subject for studying the growth points of stems and roots.

Using dissecting needles under a magnifying glass, carefully detach the small leaves from the tip of the stem. The studied specimen must be kept in a drop of water to prevent it from drying out and shriveling. Under a magnifying glass, even at tenfold magnification, the shoot apex is clearly visible along with its margins, which feature slightly noticeable protrusions of leaf primordia in the form of tiny Teeth or serrations.

To make the specimen clearer, place it in a drop of Javelle water for a few minutes. Afterward, rinse the specimen with water to wash away the Javelle water, place it in a drop of water on a Microscope slide, stain it lightly with methylene blue, and examine it under a microscope. With light staining, The Cell walls remain colorless, while the cell contents turn blue. As a result, each cell becomes highly distinct and sharply separated from the others.

Under the microscope, it is evident that the tip of the shoot apex consists of nearly identical cells whose cavities are filled with cytoplasm containing large nuclei (Fig. 23).

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Fig. 23. Structure of the Elodea stem apex:

A - stem apex (surface view) with leaf primordia, B - Cytology/practical/54.html">Longitudinal section of the stem apex

The cells of the apical meristem are arranged in regular rows parallel to the surface. Below the apex, prominent bulges known as leaf primordia are visible; they are smaller near the apex and larger further away. As they grow and expand, they develop into leaves.

Thus, one can trace The Development of a leaf and observe that its primordium forms as a result of the proliferation of meristematic tissue. Only later, when the cell number in the leaf primordium increases significantly, does Cell Differentiation become noticeable alongside The Emergence of the vascular tissue located in the center of The Leaf as the midrib.

Note. The stem apex can also be studied using horsetail as a model organism, which is commonly found along the banks of marshes, small rivers, and water bodies.