Botany - B.E. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter VII. Systematics of Higher Plants
Laboratory Class Topic 12.33. Division Equisetophyta

General Remarks. Horsetails constitute a microphyllous branch in the evolution of Water/159.html">HIGHER SPORE PLANTS. They are the first group in evolutionary history to develop true leaves of telomatic origin.

The division is divided into four classes, three of which are extinct. The Class Sphenopsida is a modern plant group comprising a single family, a single genus—horsetail (Eguisetum) and 32 species distributed worldwide. In Ukraine, 9 species of horsetails grow.

All species are perennial herbaceous plants growing in waterlogged meadows, fields, bogs, along river and lake banks, and less frequently in forests. Some horsetail species (field and woodland horsetails) produce Two Types of shoots—spore-bearing and chlorophyll-bearing. Other horsetail species (winter horsetail, marsh horsetail, water horsetail) lack a distinct spore-bearing SHOOT; instead, strobili develop at the apex of the chlorophyll-bearing shoot in early summer.

Object: field horsetail (Еquisetum аrvеnsе L.)

Tasks:

1. Examine and study The Structure of the horsetail sporophyte.

2. Study the STRUCTURE OF THE field horsetail strobilus and spores.

3. Examine the Anatomical Structure of the horsetail stem.

4. Examine the structure of the male and female horsetail gametophytes using wall charts.

5. Draw the observed PARTS OF THE field horsetail in your workbook, label their constituent parts, and sketch its developmental cycle.

Equipment and Materials: MBR-1 or Biolam microscopes, hand lenses, scalpels, slide preparations, live and herbarium material, fixed strobili and stem fragments, and other supplies.

Macroscopic Study of the field horsetail sporophyte. Using herbarium specimens or living plants, examine the structure of the spore-bearing and chlorophyll-bearing shoots of the field horsetail. The specimens show that the subterranean part of the plant is a brown, sturdy rhizome articulated into nodes and internodes, bearing tubers that store nutrient reserves accumulated over the summer. Bundles of slender adventitious roots extend from the rhizome nodes. The horsetail propagates vegetatively via the rhizome, often becoming a troublesome weed in agricultural fields.

Two shoots rise vertically from the rhizome: a brown spore-bearing shoot and a green trophophyllous shoot. At the apex of the unbranched spore-bearing shoot is a whorl of fused, awl-shaped, pointed leaves lacking chlorophyll. Using a hand lens, let us examine the structure of the strobilus. In a young strobilus, its surface is formed by tightly appressed hexagonal sporangiophores (shields) arranged in horizontal whorls. As maturation proceeds, the strobilus axis elongates, and the shields expand and pull apart, facilitating spore dispersal.

Using forceps, dissect and remove a single sporophyll—a shield with its stalk—from the strobilus. Around the stalk, along the edge of the shield, hang 8–9 cylindrical sporangia that open by means of a longitudinal slit.

Draw the rhizome with tubers and adventitious roots, the spore-bearing shoot with the strobilus, and the sporophyll with its stalk, shield, and sporangia.

Now examine the structure of the green trophophyllous shoot. Detach a single segment from the stem, consisting of a long internode and a node. The segment easily separates just above the node, where the intercalary meristem is located. The internodal cavity is visible from the lower side of the segment; a ring of small, pointed leaves, fused at the base into a sheath, is situated above the node. The leaves are almost devoid of chlorophyll, meaning the stem and lateral branches perform the assimilation function. Draw the trophophyllous shoot and label the stem, node, internode, branches, and leaves on your drawing.

Figure 90.

Microscopic Studies. Preparation Techniques. Using fixed or living material, prepare a cross-section of the stem. To do this, clamp a small piece of the stem in elderberry pith and, using a sharp razor blade, make a series of sections and place them in a drop of water on a Microscope slide. Select the best section, cover it with a coverslip, and place it on the microscope stage under low magnification. Within the field of view, you will observe that the stem has a hexagonal outline (Fig. 91). Externally, it is covered by a single-layered epidermis consisting of colorless Cells; beneath the epidermis, mechanical collenchyma tissue is concentrated in the stem ridges (Ribs), while green chlorenchyma is located in the furrows. An air cavity lies beneath the chlorenchyma, and cortical parenchyma cells are situated between and below the collenchyma and chlorenchyma. The center of the stem is occupied by a vascular cylinder with a pericycle along its periphery. Vascular bundles of the collateral type are located in the fundamental parenchyma of the stem. These bundles are situated opposite the stem ridges and consist of two narrow strands of xylem laterally adjacent to the phloem, which faces the pericycle. The central stem cavity lies further inward.

Draw the stem cross-section, labeling the epidermis, collenchyma, chlorenchyma, pericycle, vascular bundles, and the central stem cavity.

Figure 91.

To prepare a slide of spores with elaters yourself, shake spores from a dry strobilus directly onto a microscope slide and, without adding a coverslip, place it on the microscope stage. Breathe gently on them to provide moisture, and observe the hygroscopic movement of the elaters. Sketch a spore with its elaters.

Using wall charts and textbook illustrations, examine the structure of the male and female gametophytes and sketch them, indicating the thallus, rhizoids, antheridia, and archegonia.

Conclusion. Modern horsetails are perennial herbaceous plants with stems articulated into nodes and internodes, bearing reduced, whorled leaves. Spores are produced in specialized spore-bearing zones known as strobili. Morphologically, the spores are identical, which allows horsetails to be classified as homosporous plants; physiologically, however, the spores are differentiated and germinate into subterranean or surface-dwelling dioecious gametophytes.

Self-Assessment Questions

1. What distinguishes the sporophyte of horsetails from that of clubmosses?

2. Which generation is dominant in The life cycle of horsetails?

3. Can horsetails be classified as heterosporous plants?

4. How many wall layers does a horsetail spore have?

5. Into what classes is the division Sphenophyta divided?

6. Of what soil types are horsetails indicators?

7. What contributes to the Vegetative Reproduction of horsetails?

8. What are the Anatomical Features of the horsetail stem?

9. Name the most prominent horsetail species in the flora of Ukraine.