Plant Anatomy: Practical Course - Paniuta O.O. 2019

Topic 2. Tissues
Laboratory Work No. 9. Secretory Tissues. Laticifers

Theoretical Background. Secretory Tissues are plant tissues in which secretory products are formed and accumulated.

Laticifers are long, thin, branched tubes consisting of one or many Cells. Laticifers composed of a single Cell are called non-articulated, while those consisting of several or many cells are called articulated.

Non-articulated laticifers develop from special cells that are laid down in small numbers already within the seed, specifically in the inner part of the primary cortex of the cotyledonary embryo. As the embryo develops and the plant grows, the laticifer also elongates and branches. A fully formed laticifer represents a single giant (in length) multinucleated cell with a vacuole. Non-articulated laticifers are characteristic of the spurge family (Euphorbiaceae), certain mulberry plants, spindle trees, and other species.

Articulated laticifers are formed from a series of sac-like cells that may be closed on all sides and communicate only through pores, or, as is the case in most plants, the transverse walls between the cells dissolve and partially or completely disappear, persisting only at the margins. In some plants, such as celandine, one or several openings form in the transverse walls, and similar openings also appear in adjacent, abutting laticifers.

Lateral branches of some laticifers may connect with the branches of other laticifers via bridges, or anastomoses, thereby forming a complex network of articulated laticifers.

Articulated laticifers are characteristic of many plants from the poppy family (Papaveraceae), aster family (Asteraceae), and certain genera of the spurge (Euphorbiaceae) and banana (Musaceae) families, among others.

The walls of laticifers are mostly thin, cellulosic, elastic, and easily stretchable. The lumen of the laticifer is filled with a parietal layer of Cytoplasm containing a Nucleus, leucoplasts, and small vacuoles. In articulated laticifers, each branch contains an individual nucleus. Plant latex consists of Water (50-82%) containing A wide variety of substances in solution or emulsion, such as Alkaloids, Enzymes, calcium malate, Proteins, Essential Oils, resins, rubber, etc.

Objective: to examine the Structural Features of secretory tissues.

Materials and equipment: light microscopes, Glass slides and coverslips, dissecting needles, forceps, glass rods, hot plate, test tubes, filter paper, chlor-zinc-iodine, iodine, distilled water, plant material.

Slide. Non-articulated laticifers in the stem of spurge (Euphorbia sp.)

To study The Structure of non-articulated laticifers, one can use any of the spurge species. The material for study should be prepared in advance and preserved in alcohol, because in alcohol-fixed material the latex coagulates and does not spill out from the laticifers when the stem is sectioned.

Several transverse and longitudinal sections should be made from the spurge stem and placed on separate glass slides. First, examine the STRUCTURE OF THE laticifers in cross-section. The laticifers appear as thickened, shiny rings scattered among the parenchymal Cells of the primary cortex and the phloem (Fig. 54). Under high magnification, it can be seen that the interior of the ring contains latex and rod-shaped starch grains.

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Fig. 54. Transverse section of spurge laticifers:

1 - parenchyma cells, 2 - laticifers with thickened walls, 3 - starch grains

To examine the laticifers more clearly, make a new section from the alcohol-preserved material, place it in a drop of chlor-zinc-iodine, cover with a coverslip, and observe under the Microscope. The laticifers stain purple, indicating that the walls of the vessels are cellulosic.

If the slide is prepared from fresh material, you should also examine

the latex that oozes out of the laticifers during sectioning, which represents an emulsion consisting of water mixed with A large number of resin droplets. The latex also contains starch in the form of regular rods or rods with bulbously swollen ends. To reveal the Chemical Nature of these inclusions in the latex, add a few drops of iodine to the preparation. The small resin droplets will stain golden-yellow, and the starch grains will stain blue. Young starch grains appear as rods, onto the opposite ends of which starch is later deposited, which is why the starch grains acquire a spherical shape at the ends. Plastids, which stain yellowish with iodine, can often be seen near the starch grains. After examining the slide, make a drawing.

add a drop of iodine to the slide.

Fig. 55. Cytology/practical/54.html">Longitudinal section of spurge laticifers:

1 - parenchyma cells, 2 - laticifers, 3 - starch grains, 4 - cytoplasm

Slide. Non-articulated laticifers in the stem of spurge (Euphorbia sp.) in longitudinal section

The longitudinal section should be made from a piece of a thick stem split lengthwise in half, so as to capture the cortex. It is recommended to make several sections, as long as possible, and arrange them on the glass slide in the order they were cut. For better Differentiation of the individual PARTS OF THE laticifers,

Laticifers are scattered among the cortical parenchyma cells. They are very easily recognized by their thickened walls, which stand out in relief among the thin-walled cortical parenchyma cells (Fig. 55).

Examining the preparation under high magnification reveals cytoplasm within the laticifers that appears coagulated in alcohol-fixed material and stains yellowish with iodine. Starch grains are dispersed throughout the cytoplasm, as clearly visible in the cross-section. Some of the tubes on the slide are branched, yet lack transverse walls between the individual branches. Consequently, this represents a single giant, vigorously growing multinucleate cell containing cytoplasm and cell sap, which constitutes the latex itself. Laticifers typically originate during the Embryonic Stage, and their number remains constant throughout the plant's life.

Slide. Articulated laticifers in the rhizome of greater celandine (Chelidonium majus L.)

Greater celandine is a common plant that is readily available in either fresh or preserved form.

Laticifers are present in all parts of this plant, but they are most distinct in the rhizome. Therefore, collecting the rhizome is essential when harvesting material.

The typical articulated structure can only be observed in longitudinal section; consequently, work with this specimen should begin by preparing a longitudinal slice.

The celandine rhizome should be split lengthwise in half. Because its laticifers—much like those of spurges—are located predominantly in the cortex, the section must be cut to capture the cortex along the entire length of the rhizome segment. Laticifers appear as a series of vertically arranged, elongated cylindrical cells. The transverse walls between these cells lie flat or obliquely and are perforated by extremely minute pores. Thus, in celandine, a laticifer consists of numerous segments that are easily distinguishable, particularly where the diameters of adjacent segments differ.

Slide. Articulated laticifers in the ROOT of Spanish salsify (Scorzonera hispanica L.)

Material for slide preparation must be prepared in advance by fixing it in alcohol.

The laticifers in salsify are located in the cortex; therefore, sectioning must necessarily include the cortical tissue.

First, several cross-sections should be made and examined under high magnification.

Laticifers are distributed in groups alongside sieve tubes within the cortex. Their walls are thickened, and their lumens are filled with a dark content that coagulates upon alcohol Treatment. Some laticifers are cut transversely and appear as rings, whereas others appear as longitudinal segments. This Variability is due to the fact that the laticifers here are highly branched and form a dense network.

After examining and sketching the laticifers in cross-section, a new preparation—a longitudinal section through the root—is made. Even to the unaided eye, dense longitudinal strands formed by groups of laticifers can be seen running along the root among the cortical parenchyma cells.

When viewing the preparation under the microscope at low and high magnifications, one can observe that the laticifers form a dense network that stands out clearly against the background of the cortical parenchyma cells.

Articulated laticifers develop from vertically aligned meristematic cells whose transverse walls gradually dissolve. The resulting cells then undergo extensive growth, producing lateral outgrowths. When these lateral outgrowths come into contact, the partition between them dissolves, thereby forming a dense network of laticifers. Treating the preparation with zinc-chlor-iodide reveals that the laticifer walls stain blue-violet, confirming their cellulosic composition.