This laboratory features seedless vascular plants. Unlike bryophytes, vascular plants have highly specialized water and food conducting systems, collectively forming a vascular system. Like bryophytes, though, the sexual reproduction of these plants results in the production only of single-celled spores, rather than continuing on the development of the complex structures known as seeds that are produced by the plants treated in Laboratories 14 and 15. Our treatment of seedless vascular plants includes ferns, which of course have large, complex leaves (megaphylls), as well as plants often called fern allies, whose leaves (microphylls) are primitive at best, or even lacking entirely. As seedless vascular plants, ferns and fern allies share the same general life cycle: The dominant diploid plant produces spores by meiosis, which grow into an independent but small and inconspicuous haploid plant, which, in turn produces sperm and egg cells which unite in the production of a zygote, which develops into an embryo that matures into a diploid plant again. Club mosses and a very few of the ferns are distinctive in having two kinds of spores (heterosporous) rather than one (homosporous). These attract our interest, because the same pattern is found in the reproductive development of seed plants
I. Fern Allies
Each of these plant groups may be considered a surviving remnant of plants that were much more important in the geological past. Fossils indicate that they were of especially great importance during the coal forming Mesozoic time period.
Figure 13-1. Psilotum. a. Growth Form. b. Dichotomous Branching and Sporangia. c. Detail of Sporangium.
Psilotum is generally thought to be similar to the first vascular land plants. The appearance may be fortuitous, for botanists are not convinced that Psilotum should really be classified with the fossil general Rhynia and Psilophyton. Some even think they may represent primitive ferns! Psilotum has no true leaves or roots, consisting of little more than stems. The underground stems are rhizomes equipped with water and mineral absorbing rhizoids. The green, photosynthetic, above-ground stems are distinguished from most other vascular plants by dichotomous branching; that is, each member of a branching pair tends to be equal rather than being distinguished as a primary stem and a secondary branch. The leaf-like scales contain no vascular tissue and are considered anatomically to be stem outgrowths. Sporangia are produced terminally on vanishingly short branches. Note Figure 13 -1.
Look at the demonstration slide of the sporangia.
Figure 13-2. Lycopodium. a. Growth Form. b. Cross Section of Strobilus.
The gametophyte of Psilotum contrasts with that of ferns in typically being heterotrophic, lacking chlorophyll and depending on a symbiotic fungus during its development. Despite being haploid, the gametophyte looks quite a bit like the rhizome of the sporophyte. Psilotum has only one contemporary relative, an epiphytic plant, Tmesipteris. (Epiphytes are plants that use other plants for support. Neither Psilotum nor Tmesipteris can survive in climates with an extensive dry or cold season.
B. Club Mosses
Unlike Psilotum, club mosses have real roots and much of their photosynthesis is accomplished with leaves, albeit primitive ones (microphylls). Contemporary club mosses are all small and relatively insignificant, but fossils suggest a history of tree sized club mosses of wide geographical distribution and great ecological importance. There are five contemporary genera of club mosses, three of which occur naturally in Wisconsin:
1. Lycopodium—club mosses in the strict sense
Lycopodium species are mostly evergreen understory plants that look more or less like conifer seedlings. The sporangia are produced at the base of modified leaves (sporophylls) usually clustered in terminal structures called strobili, popularly known as cones. Examine these structures both in the preserved material and prepared slides. Pay special attention to the appearance of spores for comparison with Selaginella below. Label Figure 13 -2.
The gametophyte of some species of Lycopodium is lobed and green, while that of others is subterranean, branching and dependent on a symbiotic fungus during its development.
2. Selaginella—Spike Mosses
Selaginella species are often very moss-like in appearance, but have a vegetative structure much like Lycopodium. An abundance of tree-sized spike mosses are also present in the fossil record. Most contemporary spike mosses grow in relatively open sites. On display are typical spike mosses as well as an atypical and remarkable species known as resurrection plant. Examine the living material of Selaginella. If present, the sporophylls (leaves which bear sporangia) will contain both megaspores and microspores, for Selaginella is heterosporous. In any event, you should be able to find both spore types in the prepared slide. Label Figure 13 -3, identifying the microspores, megaspores, and the microsporangia and megasporangia in which they are found.
Figure 13-3. Selaginella. a and b. Growth Form of Two Different Species. c. Cross Section Showing Spore-Bearing Leaves (Sporophylls).
Selaginella gametophytes develop within the spore wall! Microgametophytes develop in microspores, producing antheridia and sperm. Megaspores develop in megaspores, producing archegonia and eggs. Fertilization occurs by swimming sperm, as in many seed plants, the embryo uses the megagamophyte tissue for its first growth. Unlike seed plants, once it is formed, the megaspore receives no direct support from the parent plant, there is no dormant phase between the maturing of the embryo and the development of a seedling. Spike mosses seem to represent a bridge between spore and seed plants.
Quillworts are small plants with a very short fleshy underground stem topped by a crown of quill-like leaves (see Figure 13 -4). The leaves seem too long to be considered microphylls, but are simpler in structure and stem attachment than megaphylls. Quillworts are heterosporous. Quillworts are reasonably common in the shallows of Wisconsin lakes.
Figure 13-4. Isoetes.
C. Equisetum—Horsetails or Scouring Rushes
Once quite abundant in the fossil record with many different genera, only one genus (Equisetum) still exists. The sporophyte of Equisetum has jointed, ribbed, hollow photosynthetic stems with scale-like, non-photosynthetic leaves. Both stem branches and leaves occur at the joints in whorls. Rhizomes and roots are present. The stem may contain silica, which makes them rough to the touch and produces the scouring ability. Note the variation in strobili (cones) of the specimens on display. Some horsetails are dimorphic: some stems are vegetative only and others are non- green but have strobili. Other plants have strobili on green stems. Label Figure 13 -5a.
The sporangia are in paired umbrella-like structures called sporangiophores usually bearing six sporangia on their inner surface. These have been considered to be modified branches. Examine microscope slides of the sporangiophore. Label Figure 13 -5b and Figure 13 -5c.
Equisetum is homosporous. The gametophyte is green and bisexual with antheridium and archegonium. If available, look at the microscopic sexual structures present on the gametophytes. The spores also have special structures called elaters which aid in their dispersal. Examine the demonstration slide of the spores. Label Figure 13 -6.
The plants which formed the great coal deposits during Carboniferous times (345 to 280 million years ago) were mainly lycopods or horsetails. It is thought the prevailing environment favored the growth of swamp forests whose fossilized remains became coal beds. Later when the climate became cooler and drier, these coal forming plants became extinct with only a few species of small stature surviving today. The lycopod fossils include Lepidodendron (reaching to 40 m high). Fossil horsetails include Calamites and Sphenophyllum. Calamites reached to a height of 30 meters and 40 cm in diameter. Coal ball fossils are a conglomeration of many types of plants together with some of the original organic matter preserved.
Types of fossilization:
IMPRESSIONS—Only the impression may be left from the plant (or animal such as dinosaur tracks).
COMPRESSIONS—Organic matter may be preserved and sometimes can be removed intact from the rock in which it occurs,
Figure 13-5. Equisetum. a. Growth Form. b. Strobilus. c. Sporangia on Sporangiophore.
Figure 13-6. Equisetum Spore with Elaters.
PETRIFACTION—Internal cellular material is replaced with silica, iron pyrate, calcium carbonate but cell walls may remain intact such as found in petrified wood..
Observe the fossil specimens on demonstration. How can you distinguish lycopod from horsetail fossils?
What parts of the plants are shown?
Ferns are the largest group of seedless, vascular plants with over 10,000 species, the majority of which found in the tropics. They differ from fern allies by having large, complex leaves (megaphylls).
1. Vegetative Form.
Figure 13-7. Fern Leaves Illustrating Different Levels and Patterns of Compounding.
Note the great diversity of ferns on display. The vegetative part of a fern sporophyte typically consists of a horizontal underground rhizome from which arise clusters of fronds so large and divided that each leaf appears to be a leafy stem. Growing down from the rhizome are true roots. Look closely at the fronds of some of the fronds on the lab benches. Be sure you can identify the full extent of the frond (leaf) which is divided into stipe (or petiole or leaf stalk) and blade. The blade may be divided into pinnae (leaflets), which may be divided further into pinnules. Even additional levels of subdivision can occur. Leaves that have leaflets are said to be compound. See Figure 13 -7 for examples.
The central axis of a leaf or leaflet is called the rachis. Fern fronds may also be completely or partly dimorphic; that is, separated into reproductively fertile and vegetative leaves or leaflets. Find a fern with young leaves to see the distinctive way in which they unfold. An expanding fern leaf is called a crozier or fiddlehead. Look for a fern in which the rhizomes are above the ground surface, or dangle off the sides of the pot. Label Figure 13 -8.
Figure 13-8. Fern (Sporophyte Generation).
The fronds should have sori which are clumps of sporangia usually in distinct shapes. These sporangia may be covered with a lip of tissue called an indusium which is a specialized outgrowth from the leaf. All ferns except water ferns are homosporous.
Figure 13-9. Fern Sorus Section (Leaf Section at Top).
Look at the demonstration slide of a cross section of a sorus with an indusium. Label Figure 13 -9.
Figure 13-10. Fern Sporangium.
Examine a prepared slide showing sporangia and see the special cells which aid in spore dispersal (annulus and lip cells). Label Figure 13 -10.
Do spores have a haploid or diploid chromosome number?
What shape are the spores?
These will be dispersed and eventually produce the prothallus or gametophyte.
3. Aquatic Ferns
Look at the displayed water ferns which are heterosporous (Azolla, Salvinia and Marsilea).
Most ferns are homosporous with spores which produce a gametophyte (prothallus) with both antheridium and archegonium. Note the shape of the gametophyte. It lacks stems, roots and leaves. Rhizoids should be present, however. Look for the sexual structures in the fresh material and in the prepared slide. Label Figure 13 -11.
Figure 13-11. Fern Prothallus (Gametophyte Generation).
How thick is the prothallus?
Is it photosynthetic?
2. From the demonstration slide of a prothallus cross section, distinguish between the antheridium and archegonium (Figure 13 -12a and Figure 13 -12b).
When the egg in the archegonium is fertilized by a sperm, the first growth of the new sporophyte begins. Label Figure 13 -12c.
Figure 13-12. a. Fern Prothallus and First Growth of Sporophyte. b. Fern Archegonium Detail. c. Fern Antheridium Detail.
Do the antheridia and archegonia mature at the same time?
Do they resemble bryophyte organs?
3. After fertilization and growth of the zygote, how long is the sporophyte dependent upon the gametophyte?
C. Fern Identification
Use the fern key on the next page to differentiate the species which are on your identification list.
Key to Ferns on Common Plant Identification List
1a. Fertile (spore-bearing) fronds or pinnae, brownish, lacking green or photosynthetic tissue, sharply differentiated in size and color from green sterile (vegetative) fronds or pinnae.
2a. Fertile frond entirely brownish and separate from sterile frond above ground, although both arising from the same rhizome.
3a. Fertile fronds hard and smooth; veins of sterile fronds much branched and forming a network of closed loops Onoclea sensibilis SENSITIVE FERN
3b. Fertile fronds soft and covered with reddish-brown wool; smallest veins of pinnae lobes of sterile fronds forking and forming a "U" which ends at the margin
Osmunda cinnamomea CINNAMON FERN
2b. Fertile pinnae small, brownish, appearing in middle or at end of green frond.
4a. Entire frond twice compound; fertile pinnae at top third of frond
Osmunda regalis ROYAL FERN
4b. Entire frond once compound; fertile pinnae at about the middle of the frond
Osmunda claytoniana INTERRUPTED FERN
1b. Fertile fronds or pinnae green and photosynthetic, similar in size, shape, and color (except for the brownish sori) to the sterile fronds, or specimen lacking fertile fronds.
5a. Stipe (petiole) of frond forking into two recurving branches; sporangia along upper edge of pinnules, covered by its reflexed margin Adiantum pedatum MAIDENHAIR FERN
5b. Stipe straight, continuing as mid-rib or rachis of the blade.
6a. Fronds compound two or more times.
7a. Blades broadly triangular and horizontal; sori forming a continuous band around margins of pinnules Pteridium aquilinum BRACKEN FERN
7b. Blades oblong and vertical; pinnules sterile (small and turning brown if fertile) Osmunda regalis ROYAL FERN
6b. Fronds once compound or only deeply lobed.
8a. Blades usually more than 50 cm long and once compound with more than 12 pairs of pinnae; smallest veins of pinnae lobes forking and forming a "U" which ends at margin.
9a. Pinnae with small tufts of woolly hair at base
Osmunda cinnamomea CINNAMON FERN
9b. Pinnae smooth at base Osmunda claytoniana INTERRUPTED FERN
8b. Blades less than 50 cm long, deeply lobed or with 1-3 pairs of pinnae at base; veins forming closed loops or U branches which do not reach margins of lobes.
10a. Fronds thick and green through winter; sori round, without indusia; veins forming a "U" Polypodium virginianum POLYPODY FERN
10b. Fronds thin, sensitive to light frost; sori absent from green fronds; veins forming a network of closed loops Onoclea sensibilis SENSITIVE FERN
prothallus (young gametophyte)
sorus (pl. sori)
pinna (pl. pinnae)
sorus (pl. sori)
indusium (pl. indusia)