Botany - B.Ye. Yakubenko 2017

Part Three. Kingdom Plantae
Chapter VII. Systematics of Higher Plants
Laboratory Session Topic 12.32. Class Lycopodiopsida, Class Isoetopsida

General Remarks. Today, clubmosses (Lycopodiopsida) comprise over 100 species and 5 genera grouped into two classes: Lycopodiopsida and Isoetopsida (quillworts).

The Class Lycopodiopsida includes three orders: Asteroxylales, Protolepidodendrales, and Lycopodiales. Asteroxylales and Protolepidodendrales are extinct forms, whereas Lycopodiales are represented by extant species. All members of this class are homosporous, perennial herbaceous plants.

The class Isoetopsida includes heterosporous species. They played a significant role in forming the forest cover during the Upper Paleozoic. This class unites the orders Lepidodendrales, Selaginellales, and Isoetales.

Lepidodendrales, or scale trees, are extinct trees that formed coal deposits.

The order Selaginellales comprises modern perennial herbaceous plants with simple, ligulate leaves and adventitious roots arising from a slender stem. The sporophylls are gathered into strobili and bear Two Types of sporangia: microsporangia containing numerous small microspores, and megasporangia containing four megaspores. Within the sporangia, microspores develop into highly simplified male gametophytes, and megaspores into female gametophytes.

The order Isoetales includes extant perennial herbaceous plants with simple leaves. The plant axis, consisting of an apical leafy region and a basal corm (or rhizophore), is greatly shortened and undergoes secondary thickening. The leaves are linear-subulate with expanded bases, arranged spirally. The sporangia are large, located at the Base of the upper surface of the leaf. Quillworts grow in very wet soils or in Water.

Objects:

1. Class Lycopodiopsida (Lусороdioрsіdа), order Lycopodiales (Lусороdіаlеs), representative — stag's-horn clubmoss (Lусороdium сlаvаtum L.)

2. Class Isoetopsida (Іsоеtopsida), order Selaginellales (Selaginellales), representative — lesser clubmoss or spikemoss (Sе1аginella selaginoides L.)

Tasks:

1. Examine and study the External Structure of the stag's-horn clubmoss sporophyte.

2. Examine The structure of the stag's-horn clubmoss strobilus, study and draw

a longitudinal section.

3. Using the chart, examine and draw the STRUCTURE OF THE stag's-horn clubmoss gametophyte.

4. Examine and study the general appearance of the lesser clubmoss as a representative of heterosporous species.

5. Study and draw the structure of the selaginella strobilus.

Equipment and Materials: Microscope, magnifying glasses, permanent slides of longitudinal sections of stag's-horn clubmoss and selaginella strobili; herbarium specimens or living plants of stag's-horn clubmoss and selaginella, wall charts, dissecting needles, tweezers, razor blades.

Macroscopic studies of the clubmoss sporophyte. Using herbarium specimens or living plants, examine the external Morphology of the stag's-horn clubmoss sporophyte as the most typical representative of homosporous clubmosses.

Stag's-horn clubmoss is a perennial herbaceous plant, as seen from its creeping stem with robust dichotomously branched adventitious roots developing on rhizophores (Fig. 88). Upright, dichotomously branched shoots arise from the creeping stem, which can reach up to 3 m in length. Like the stem, these branches are covered with small, sessile, narrow phylloids (trophophylls) with subulate tips. A hand lens reveals the midrib. The tips of certain branches transition into elongated vertical stalks bearing strobili at their apex. The stalk is sparsely covered with modified leaves, and its tip typically forks into two branches—the stalks of the strobili. A single stalk bears two, or sometimes 3–4 strobili. A strobilus is a shortened SHOOT apex modified for sporulation. The sporophylls inserted on the strobilus axis are arranged spirally and fit closely together. They retain a leaf-like form, but differ from trophophylls by their expanded bases, subulate-tapered tips, and Kidney-shaped sporangia at the base. By gently separating a sporophyll with a needle under a magnifying Glass, its structure can be examined.

Upon completing the study, draw the sporophyte of the stag's-horn clubmoss and label the creeping stem, rhizophores, adventitious roots, dichotomously branched shoots, strobili, and phylloids on the drawing. Separately, draw and label the Components of the sporophyll with its sporangium.

Fig. 88

Microscopic studies of the clubmoss strobilus. Using a prepared permanent slide of a Cytology/practical/54.html">Longitudinal section of a stag's-horn clubmoss strobilus, examine its structure. Under low magnification, the axis of the cone, composed of parenchymatous Cells, is visible. Attached to the central axis are sporophylls bearing kidney-shaped sporangia on short stalks. The slide shows that the sporangia are filled with spores of identical size and shape (isospores).

Draw a longitudinal section of the stag's-horn clubmoss strobilus, and label the strobilus axis, sporophylls, sporangia, and spores on your drawing.

Macroscopic studies of the clubmoss gametophyte. Examine the general appearance of the stag's-horn clubmoss gametophyte on the chart. As seen from the drawing, it is pear-shaped and brownish in color; its narrow end bears rhizoids, while its upper surface develops antheridia and archegonia. The antheridia are embedded in the gametophyte tissue, whereas the necks of the archegonia protrude from it. Make a drawing and label: the gametophyte body, rhizoids, antheridia, and archegonia.

Macroscopic examination of the lesser clubmoss (Selaginella). Using living plants or herbarium specimens, examine the external structure of Selaginella selaginoides as a representative of heterosporous clubmosses (Lycopodiophyta). This is a small herbaceous plant with a creeping stem covered with lanceolate, spirally arranged leaves, which are particularly dense and arranged in four rows at the tips of the branches, where the strobili are also located. The latter are cylindrical and dense. Adventitious roots can also be observed on the stem, originating from rhizophores. Draw the general appearance of Selaginella, and label the following on your drawing: dichotomously branched stem, leaves, strobili, and adventitious roots.

Microscopic examination. Using prepared microscope slides of longitudinal sections of a Selaginella strobilus under low magnification, observe and study its structure.

The slide shows that the strobilus contains two types of sporophylls: microsporophylls and megasporophylls. Microsporophylls are located in the upper part and bear microsporangia with numerous microspores on short stalks. Megasporangia are oval, located in the lower part of the strobilus, and contain four (sometimes fewer) megaspores (Fig. 89).

A small appendage, the ligule, characteristic of the representatives of this class, can be seen at the base of the sporophylls.

Draw the strobilus, showing its axis, sporophylls with ligules, microsporangia with microspores, and megasporangia with megaspores.

It is known from theoretical studies that the spores of the lesser clubmoss (Selaginella) begin to germinate while still inside the sporangia and then fall onto damp soil. Microspores develop into microscopic male gametophytes consisting of a reduced antheridium and a single vegetative Cell. The entire male gametophyte remains enclosed within the spore wall, which bursts only after the spermatozoa have matured.

Figure 89. Life Cycle of Selaginella

The Development of the female gametophyte also begins inside the megasporangium (megaspore), which later opens via a tri-radiate fissure, simultaneously revealing the multicellular tissue of the female gametophyte. Tufts of rhizoids and several archegonia embedded in the gametophyte tissue appear on the exposed part. A distinctive feature of Selaginella gametophytes is that they do not shed their spore walls. This played an important role in The Emergence of seed habit (angiospermy) in flowering plants.

Fertilization requires at least a drop of water. An embryo develops from the zygote, and an adult plant develops from the embryo.

Conclusion. Main distinctive Features of the Division Lycopodiophyta: dichotomous branching, spiral arrangement of phylloids, appearance of true roots; presence of both homosporous and heterosporous forms. In heterosporous forms, isospores develop in strobili, germinating into perennial, subterranean, achlorophyllous gametophytes bearing archegonia and antheridia.

In heterosporous forms, two types of sporangia are formed in the strobili: microsporangia with microspores and megasporangia with megaspores. Microspores develop into highly reduced microscopic male gametophytes, while megaspores develop into less reduced female gametophytes.

The transition from homospory to heterospory is a progressive step in the evolution of vascular plants.

Self-Assessment Questions

1. Which generation is dominant in The life cycle of clubmosses?

2. What vegetative Organs does the sporophyte of clubmosses possess?

3. Into which classes is the division Lycopodiophyta divided?

4. Are there any heterosporous species among clubmosses, and to which class do they belong?

5. Where are sporangia with spores formed in clubmosses?

6. What is the difference in the structure of the gametophytes between staghorn clubmoss (Lycopodium clavatum) and lesser clubmoss (Selaginella selaginoides)?

7. Is there a difference in the strobilus structure between Lycopodium and Selaginella?

8. Name fossil forms of clubmosses.

9. What develops from a microspore of Selaginella?

10. What develops from a megaspore of Selaginella?