This laboratory features seedless vascular plants. Unlike bryophytes, vascular plants have highly specialized water and food conducting tissues that collectively form a vascular system. Like bryophytes, though, the sexual reproduction of these plants results only in single-celled spores, rather than the complex structures known as seeds. This treatment of seedless vascular plants includes ferns, which of course have large, complex leaves (megaphylls), and fern allies, whose leaves are primitive (microphylls), or even lacking entirely.
Ferns and fern allies share the same general life cycle: A dominant diploid plant produces spores by meiosis, which grow into an independent haploid plant that is small and inconspicuous. The haploid plant, in turn produces sperm and egg cells that unite to produce 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 attention because the same pattern is found in the reproductive development of seed plants
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 especially important during the coal forming Mesozoic time period.
Figure 23-1. Psilotum. a. Growth Form. b. Dichotomous Branching and Sporangia. c. Detail of Sporangium.
Psilotum is probably similar to the first vascular land plants. The similarity may be coincidental, for botanists are not convinced that Psilotum should really be classified with the fossil general Rhynia and Psilophyton. Some even think Psilotum species might represent primitive ferns! Psilotum has no true leaves or roots, and consist of little more than stems. The stems that occur underground are called rhizomes and are equipped with water and mineral absorbing rhizoids.
The green, photosynthetic, above-ground stems are distinguished from those of most other vascular plants by dichotomous branching; that is, each member of a branching pair tends to be equal (as opposed to one being a main stem and the other a secondary branch). The leaf-like scales contain no vascular tissue and are considered anatomically to be merely stem outgrowths. Sporangia are produced terminally on vanishingly short branches. Compare Figure 23-1 with the potted Psilotum in the lab.
Look at the demonstration slide of Psilotum sporangia.
Psilotum gametophytes typically contrast with fern gametophytes by being heterotrophic, lacking chlorophyll and depending on a symbiotic fungus during 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.
Unlike Psilotum, club mosses have real roots and much of their photosynthesis is accomplished with primitive microphylls. Contemporary club mosses are all small and relatively insignificant, but fossils show 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:
Figure 23-2. Lycopodium. a. Growth Form. b. Cross Section of Strobilus.
ycopodium—club mosses in the strict sense
Most Lycopodium species are 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 preserved material and prepared slides. Pay special attention to the appearance of spores for comparison with Selaginella below. Label Figure 23-2.
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.
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 23-3, identifying the microspores, megaspores, and the microsporangia and megasporangia in which they are found.
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. Also, 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.
Figure 23-3. Selaginella. a and b. Growth Form of Two Different Species. c. Cross Section Showing Spore-Bearing Leaves (Sporophylls).
Figure 23-4. Isoetes.
uillworts are small plants with a very short fleshy underground stem topped by a crown of quill-like leaves (see Figure 23-4). 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.
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 23-5a.
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 23-5b and Figure 23-5c.
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 that aid in their dispersal. Examine the demonstration slide of the spores. Label Figure 23-6.
The plants that 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 balls are a conglomeration of many plants 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 23-5. Equisetum. a. Growth Form. b. Strobilus. c. Sporangia on Sporangiophore.
ETRIFACTION—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. Answer Q1 and Q2 on the answer sheet.
Figure 23-6. Equisetum Spore with Elaters.
Figure 23-7. Fern Leaves Illustrating Different Levels and Patterns of Compounding.
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 megaphylls and by having photosynthetic gametophytes.
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 23-7 for examples.
Figure 23-8. Fern (Sporophyte Generation).
he 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 23-8.
Figure 23-9. Fern Sorus Section (Leaf Section at Top).
ertile fern fronds bear 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.
Look at the demonstration slide of a cross section of a sorus with an indusium. Label Figure 23-9.
Figure 23-10. Fern Sporangium.
Examine a prepared slide showing sporangia and see the special cells that aid in spore dispersal (annulus and lip cells). Label Figure 23-10.
Answer Q3 and Q4 on the answer sheet.
The spores will be dispersed and eventually produce the prothallus or gametophyte.
Look at the displayed water ferns, which are heterosporous (Azolla, Salvinia and Marsilea).
Figure 23-11. Fern Prothallus (Gametophyte Generation).
Most ferns are homosporous with spores that 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 23-11. Answer Q5 and Q6 on the answer sheet.
From the demonstration slide of a prothallus cross-section, distinguish between the antheridium and archegonium (Figure 23-12a and Figure 23-12b).
When a sperm fertilizes the egg in the archegonium, the first growth of the new sporophyte begins. Label Figure 23-12c.
Figure 23-12. a. Fern Prothallus and First Growth of Sporophyte. b. Fern Archegonium Detail. c. Fern Antheridium Detail.
Answer Q7 through Q8 on the answer sheet.
After fertilization the zygote grows into a sporophyte plant. During the time it is dependent on the gametophyte it is considered an embryo. Answer Q9 on the answer sheet.
Use this fern key to learn distinctions between the fern species that are on your identification list.
Key to Ferns on the 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
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
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
strobilus (pl. strobili) or cone
crozier or fiddlehead
sorus (pl. sori)
indusium (pl. indusia)
pinna (pl. pinnae)
sorus (pl. sori)
indusium (pl. indusia)
Answer Sheet, Laboratory 23
Q1 How can you distinguish lycopod from horsetail fossils?
Q2 What plant parts can you find in the fossils? (Leaves, stems, seeds, etc.)
Q3 Do fern spores have a haploid or diploid chromosome number?
Q4 What shapes are the fern spores?
Q5 How thick is the fern prothallus?
Q6 Is the fern prothallus photosynthetic?
Q7 Do the antheridia and archegonia mature at the same time?
Q8 Do these reproductive organs resemble those of the same name found in bryophytes?
Q9 Based on the size of the gametophyte, how long do you think the fern embryo stage lasts?