Botany - B.E. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter VII. Systematics of Higher Plants
Laboratory Class Topic 11.31. Class Bryopsida, or True Mosses

General Remarks. Leafy mosses represent the next stage in the evolution of bryophytes. Their body is differentiated into a "leaf" and a "stem," hence their name. These mosses possess a more complex Structure than liverworts. Their stem is differentiated into distinct tissue blocks, their sex Organs are multicellular, and Fertilization occurs via an aqueous (liquid-drop) medium. The sporophyte is elevated into the air and features adaptations for spore dispersal. The protonema is multicellular, with oblique cross-walls, and contains METABOLISM/14.html">Chloroplasts with pyrenoids.

Objects of Study:

1. Hair-cap moss (Polytrichum commune Hedw.)

2. Bog moss / Peat moss (Sphagnum palustre L.)

Tasks:

1. Examine the general appearance of the female and male gametophytes of the hair-cap moss.

2. Study The structure of the archegonium and antheridia of the hair-cap moss.

3. Examine and study the STRUCTURE OF THE sporophyte.

4. Study the leaf structure of the bog moss.

Equipment and Materials: herbarium specimens of hair-cap moss gametophytes, slide preparations of cross-sections through the apex of female and male gametophytes, a slide preparation of a sporophyte capsule, magnifying glasses, a Microscope, and illustrative charts.

Macroscopic Studies. Using herbarium specimens and live material, examine the structure of the male and female gametophytes of the hair-cap moss. The hair-cap moss is a perennial herbaceous plant, up to 20 cm tall, with straight, unbranched stems covered with pointed green phylloids. The leaves in the lower part are brownish. Rhizoids are located on the "rhizome" (Fig. 86).

The common hair-cap moss is a dioecious plant. The male gametophyte is distinguished from the female by a brownish apex formed by modified leaves, known as the perianth, which resembles a peculiar "flower." Female gametophytes are covered with ordinary, uniform "leaves" and have a green apex.

After studying the herbarium and live material, draw the general appearance of the gametophytes and label the rhizoids, stem, phylloids, male and female gametophytes, archegonia, antheridia, and the modified apical leaves of the male gametophyte.

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Figure 86. Life Cycle of the common hair-cap moss

Method for Preparing a Cross-Section Slide of a Hair-Cap Moss Stem. To study the Internal Structure of the Water/115.html">Vegetative organs of the hair-cap moss, it is necessary to prepare thin cross-sections of the stem and leaf. To do this, take a small section from the middle part of the stem with green leaves, press all the leaves tightly against the stem, and insert it between pieces of elderberry pith. Using a well-sharpened razor blade, make a series of sections through the entire leafy SHOOT and transfer the sections into a drop of water.

Place the sections under low magnification of the microscope and examine them carefully. This Overview makes it possible to determine that the stem consists of two main parts: a thick brownish cortex on the periphery and a vascular bundle in the center.

Microscopic Examination of the Preparation. Switching the microscope to high magnification reveals that the outer cortex consists of small Cells with thick, reddish walls. These cells presumably provide strength and elasticity to the stem. On the inner side, the fundamental tissue of the cortex is visible, consisting of larger cells with thin walls and living contents. Starch grains and fat droplets can be found within them.

The central part of the stem is the vascular bundle, in which we can distinguish a peripheral region adjacent to the cortex. This region serves for the Transport of Assimilates and corresponds to the phloem. The phloem cells are living, elongated, thin-walled, and resemble sieve tubes, though lacking sieve plates.

The central part of the bundle is occupied by the xylem, which stands out due to its rounded, thick-walled cells that lack living contents and perform a water-conducting function. Between these elements lie 2–3 layers of starch-rich cells, which can be verified using an iodine-potassium iodide test. Overall, The Vascular System of this moss can be characterized as a protostele.

Now we proceed to examine the leaf. Thin sections of the leaf blade can be found on the same slide. The blade has thin, single-layered margins and a multi-layered middle region containing a vascular bundle—the midrib. On the upper side of the blade, light-green vertical columns composed of a single row of chlorophyll-containing cells stand out; these are assimilatory cells. Water is effectively retained between them and absorbed by these green cells.

Microscopic Examination of Sex Organs and Sporogenesis Preparations in Hair-Cap Moss. On a prepared slide of a longitudinal section through the male apex (see Fig. 86), elongated reddish leaves are noticeable, forming a unique little bud. These modified leaves protect the sac-like antheridia, among which numerous colorless, multicellular paraphyses are located. The antheredium has a short, colorless stalk by which it attaches to the stem apex. Its multicellular wall surrounds numerous colorless spermogenous cells.

On the longitudinal section slide of the female gametophyte apex, green leaves similar to the other stem leaves are also visible. Among them, locate the flask-shaped archegonia and filamentous paraphyses. Examine the structure of the archegonium closely and draw it separately at a larger scale. Its familiar constituent parts are clearly visible: a short stalk, a widened venter containing the egg Cell, and a neck filled with numerous transparent neck canal cells. The archegonium is covered by a multicellular, single-layered jacket. As you can see, the structures of the archegonium and antheridium in hair-cap moss and Marchantia are similar.

Microscopic Examination of the Sporophyte. On the slide of a Cytology/practical/54.html">Longitudinal section of the sporophyte, locate the stalk, which transitions into an expanded apophysis at the apex. A small constriction separates it from the capsule. The capsule itself consists of the urn and the operculum (lid) that covers it. The urn has thin walls, with a columella running through its center, which expands at the apex into a membranous epiphragm. The latter caps the urn, protecting it from moisture and diaspores. Surrounding the columella like a sleeve is the sporangium, filled with small brownish spores. The sporangium is suspended around the columella by green filaments called trabeculae. At the apex of the urn, along the margins, is a toothed peristome, whose hygroscopic properties ensure spore dispersal.

Macroscopic Study of Bog Moss (Sphagnum). Using herbarium specimens and provided material, examine and draw the external appearance of the bog moss. It is easy to distinguish a fairly robust stem up to 20 cm long in this plant. On it, locate and dissect Three types of branches: apical branches, which form the HEAD (capitulum); middle branches—normally developed and greenish, which determine the color of the moss; and lower branches—elongated and hanging downward, which facilitate upward water movement since Sphagnum lacks rhizoids.

Method of preparing a cross-section slide of the stem of peat moss (Sphagnum palustre). To study the Anatomical Structure of the stem, you need to prepare its cross-section. To do this, take a small sphagnum plant and carefully strip the stem of its leaves and branches using tweezers. Cut off a 1.5–2 cm segment and place it into a slit previously made in an elderberry pith to a depth of about 2 cm. Clamp the stem tightly between the two halves of the pith. Level The surface of the pith using a scalpel or razor blade. Using a sharp razor, make a series of thin cross-sections. With the help of a magnifying Glass or under low microscope magnification, select the best sections: those that are very thin and capture the peripheral part of the sphagnum stem. Place the section in a drop of water and cover it with a coverslip. Secure the slide on the microscope stage.

Microscopic examination of the slide. Under low and high microscope magnification, examine and sketch the Anatomical Features of the peat moss stem. The slide clearly shows that the stem is externally covered by large, thin-walled hyalodermis cells. Beneath the hyalodermis lie several layers of parenchymatous cells with thickened cell walls, known as the sclerodermis. This tissue performs a mechanical function, providing hardness and strength to the sphagnum stem. The core consists of large-celled parenchyma with intercellular spaces. Certain cells are elongated and perform a conducting function, effectively acting as a conducting strand, though they are barely noticeable on a cross-section, except perhaps for their smaller size (Fig. 87).

Figure 87. Life cycle of peat moss (Sphagnum palustre)

a — sporogonium; b — gametophyte; m — Meiosis: 1 — protonema; 2 — portion of the shoot;

6 — spermatozoid; 7 — sporogonium capsule; 8 — sporogenous tissue;

3 — archegonial branch; 4 — antheridial branch; 5 — antheridium; 9 — sporangium; 10 — columella; 11 — sporogonium seta

Figure 87. Life cycle of peat moss (continued):

12 — pseudopodium; 13 — gametophyte with sporophyte; 14 — cross-section of the stem; 15 — medulla (core); 16 — sclerodermis; 17 — hyalodermis; 18 — leaf; 19 — chlorophyllose cell; 20 — hyaline cell;

21 — wall thickening; 22 — pores

Method of preparing cross-section and whole-mount slides of the “leaf” of peat moss (Sphagnum palustre). To do this, take a single leaf or a branch from the middle of the plant and place it into a slit in the elderberry pith, level the surface of the pith, and make a series of sections. Select the best section, place it in a drop of water, and cover it with a coverslip. Right next to it on the glass slide, place the plucked leaf from the middle branch in a drop of water and cover it with a coverslip. Secure the prepared slides on the microscope stage using clips.

Microscopic examination of the slides. While examining the whole-mount slide of the sphagnum leaf, note that the leaf is single-layered and lacks a midrib. The leaf cells are of two types. Some of them are colorless, large, and dead — these are hyaline or water-storage cells. You can easily distinguish them by the presence of Cell wall thickenings and pores, through which they absorb and retain a significant amount of water, which is why they are also called water-storing cells. Along the edges of the hyaline cells, locate smaller cells filled with chloroplasts; these are the chlorophyllose (photosynthetic) cells (see Fig. 87).

Conclusion. Bryophytes are archegoniate plants in whose life cycle the gametophyte (haploid) is dominant over the sporophyte (diploid). The gametophyte is adapted to life in conditions of excessive moisture, while the sporophyte is adapted to a drier aerial environment. Perhaps this was nature's tentative step toward evolving a terrestrial plant type.

Self-Check Questions

1. What classes is the Division Bryophyta divided into?

2. What does the protonema develop from in mosses?

3. What is the chromosome set of bryophyte gametophytes?

4. What marks the beginning of gametophyte development?

5. What marks the beginning of sporophyte development?

6. Does the bryophyte sporogonium lead an independent lifestyle?

7. To which class do the Jungermanniales (leafy liverworts) belong?

8. What subclasses are mosses (Bryopsida) divided into?

9. In which mosses is the protonema filamentous, and in which is it thalloid (plate-like)?

10. Which of the mosses lack rhizoids?

11. What conditions are essential for the fertilization process in mosses?

12. What are the constituent PARTS OF THE sporophyte in haircap moss (Polytrichum)?

13. What type of Cell Division accompanies spore formation?

14. Which mosses are responsible for peat formation?

15. Name the Structural components of archegonia and antheridia.