The Class Polypodiopsida is composed of the horsetails, whisk ferns, and ferns, with most of the roughly 12,000 Polypodiopsid species being ferns.
The Polypodiopsids are believed to be the sister group to the seed plants. Ferns are different from mosses in that they have true leaves, roots and vascular tissue. They still retain some of their ancestral characteristics, however, like flagellated sperm.
For the ferns we are using a variety of ferns as well as the specific genus, Polypodium, to represent the anatomical features of this group. For horsetails you will study the genus Equisetum and for Whisk Ferns you will examine Psilotum.
Within the ferns the whisk ferns are the most primitive. They have many features that are similar to bryophytes but also have many that are shared with the other ferns like vascular tissue and a dominant sporophyte.
Whisk ferns have vascular tissue but lack true leaves and roots (which the other ferns have). Photosynthesis occurs in the aerial stems, and water and mineral absorption occurs in the horizontal underground rootlike stems (rhizomes), which receive water and nutrients from fungi through a mycorrhizal association. There are two phases in the life cycle of a whisk fern. The large asexual plants (sporophytes) produce spores that develop into very small colorless sexual plants (gametophytes), which are similar to rhizomes in overall appearance. Eggs and sperm are produced in special structures on their surfaces. Union of these gametes initiates the second sporophyte phase.
The genus Psilotum contains two species of pantropical plants with whisklike green stems and scalelike appendages called “enations,” which may represent reduced leaves, but they contain no vascular tissue.
Psilotum is a genus of fern-like vascular plants, commonly known as whisk ferns. Plants in this genus were once thought to be descended from the earliest surviving vascular plants, but more recent phylogenies place them as basal ferns. They lack true roots and leaves, the stems being the organs containing conducting tissue. The sporangium of a Psilotum is a little more complex than the sporangia of mosses and is often referred to as a synangium.
An illustration of Psilotum anatomy.
A Psilotum plant.
An example of Psilotum with yellow sporangia.
Psilotum sporangia releasing spores.
Microscopic view of a Psilotum sporangium. It is actually a collection of sporangia and so this structure is often referred to as a synangium.
Live Sample of a Whisk Fern, Psilotum
Note: Please do not tear up the plant to view under the microscope.
Examine the live sample of a Whisk Fern plant inspecting the leaves, stem and strobili (where present).
Capture images of the Whisk Fern for use in your laboratory worksheet.
Return the Whisk Fern to its original station location.
A fern is a member of a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having certain tissue that conducts water and nutrients, and having branched stems. Like other vascular plants, ferns have leaves. Most ferns are termed "true ferns"; they produce what are called "fiddleheads" that uncoil and expand into fronds. The group includes about 10,560 known extant species.
Fern fronds may develop spores, in structures called sori, on their underside and these fronds are called a sporophylls. A sorus (pl. sori) is a cluster of sporangia (structures producing and containing spores). These form a yellowish or brownish mass on the edge or underside of the sporophyll. In some species, they are protected during development by a scale or film of tissue called the indusium, which forms an umbrella-like cover.
Sori occur on the sporophyte generation, the sporangia within producing haploid meiospores. As the sporongia mature, the indusium shrivels so that spore release is unimpeded. The sporangia then burst and release the spores. The shape, arrangement, and location of the sori are often valuable clues in the identification of fern taxa. The presence or absence of indusium is also used to identify fern taxa.
Like all plants the gametophyte is the haploid stage in the alternation of generations. However, from the mosses, there is a shift in the dominance of the gametophyte. In the bryophytes the gametophyte is the dominant stage and is the largest stage. It is the stage that carries out primary photosynthesis. In ferns the gametophyte still has the capability of photosynthesis but it is small and is often short lived as a stage in the fern life cycle. Instead it is the sporophyte, that can be many of thousands times larger than the gametophyte, that is the primary producing stage of the life cycle. But the ferns still retain some of the traits you see in mosses. For instance, the sporophyte still grows out of the archegonia of the gametophyte and the sperm that are released from the antheridium are still flagellated.
In fern the gametophyte is often referred to as a thalus. It varies from species to species but in Polypodium it looks like a small heart with rhizoids coming off of it almost like a beard. It is also the case in Polypodium that the archegonia and antheridia are found on the same gametophyte and not on separate gametophytes.
The fern gametophyte highlighting the archegonium and antheridium.
Prepared slide of fern gametophyte
Compound Light Microscope
Examine the fern gametophyte slide with a microscope
Examine the gametophyte and locate the archegonium and antheridia
Capture images of the fern gametophyte for use in your laboratory worksheet.
Return the fern gametophyte slide to its original station location.
The sporophyte is the largest, most dominant and most recognizable part of the fern life cycle. The fertilized egg grows out of the archegonium of the gametophyte as a young sporophyte. As the sporophyte grows the gametophyte falls away and the sporophyte forms roots and starts to gather its own nutrients. It grows in the form of a fiddlehead first unfurling as a frond of a mature fern.
Ferns have true roots (unlike mosses) because they contain vascular tissue but it can still look unfamiliar because they also have a structure called a rhizome. A rhizome is a modified stem and it can be inconspicuous because it often grows underground in ferns. This rhizome grows horizontally and roots grow from this rhizome. As the rhizome grows just under the soil surface new fronds may develop and grow upwards out of the soil. In this way a single rhizome may have many fronds attached to it and are all from the same plant.
This image shows a young fern sporophyte (the top leaf structure) growing out of the thalus of the gametophyte.
This microscopic slide highlights the difference between the young sporophyte and the gametophyte. The light colored structure (labeled "B") is the gametophyte while the green leaf, stem and root (labeled "A") is the sporophyte.
As the fern grows the fronds unfurl from this unique structure called a fiddlehead (known as a crozier).
This is a fully unfurled, mature fern frond.
Live Samples of a Fern
Note: Please do not tear up the plants to view under the microscope.
Examine the live samples of fern plants inspecting the fronds, rhizomes, stem and sori (where present).
Capture images of the fern for use in your laboratory worksheet.
Return the Fern to its original station location.
Preserved sample of young fern sporophytes
Dissecting Microscope
Small dish (to hold the sporophyte and water)
Water
Obtain a small sporophyte
You will need to tease a single sporophyte away from the large mass of them in the jar
Place the young sporophyte in a small dish with water to view under the dissecting microscope
Examine the youn sporophyte identifying the sporophyte and the gametophyte it is growing out of
Capture images of the young fern sporophyte for use in your laboratory worksheet.
Return the young fern sporophyte to the jar and to its original station location.
When a frond matures it can become a sporophyll and develop structures called sori on the underside of the leaf. A sorus (singular for sori) is a collection of sporangia. Each sporangium is a structure that houses the spores that are made from meiosis. When they are mature each sorus will break open releasing its spores which can form into a new gametophyte. Different species of fern may have sori that are arranged differently on the sporophyll and have different arrangements of sporangia in the sorus. Some sori also have a shield on them that protects the sporangia. This shield is called an indusium and it can be a unique shape and diagnostic for many species.
Many of the mature fern fronds will develop sori underneath the leaf. We call these fronds sporophylls. Each of the yellow dots on this frond are sori.
This close up of an individual sorus reveals that it is a collection of round structures called sporangia. Each of these sporangia holds numerous spores that are maturing. These spores were formed by meiosis.
This diagram illustrates how an individual sporangium opens to release its spores. The dark ridge of cells (called collar cells) on the sporangium will change shape when the spores are mature causing the sporangium to open up.
This is an image of two sori that have a clear shield on top called an indusium. An indusium is a protective covering and can be unique to a fern species even though many species of fern lack an indusium entirely.
This is a microscopic cross section of a sorus that has an indusium. You can see how the indusium acts as a shield to the sporangia below it.
Fern plant sporophyll (a frond with sori)
Dissecting Microscope
With permission remove a small piece of a sporophyll of a live fern that contains sori on the underside
Examine the sori under the dissecting microscope
Capture images of the fern sori for use in your laboratory worksheet
Dispose of the fern sporophyll fragment in the trash
Prepared slide of a fern sorus (may be labeled to include "indusium")
Compound Light Microscope
Retrieve a slide of a sorus from the lab station
Examine the sori under the microscope. Note the arrangement of the sporangia and spores
Capture images of the fern sori for use in your laboratory worksheet
Replace the fern sori slide back at the appropriate station
Now that you have examined the gametophyte and sporophyte of a fern it is time to look at it as part of a whole in the life cycle. Like the mosses, club mosses and all of the other plants, ferns go through an alternation of generations life cycle. This means you will find the same stages but they are now modified from what the mosses have but are similar to the Lycopodiopsids in many respects. The sporophyte is the most conspicuous part of the life cycle and is considered the dominant stage.
Polypodium Life Cycle
Horsetail, (genus Equisetum), also called scouring rush, is the only living genus of plants in the class Equisetopsida. Horsetails grow in moist, rich soils in all parts of the world except Australasia. Some species produce two kinds of shoots: those with conelike clusters (strobili) of spore capsules and those lacking such structures. Some are evergreen; others send up new shoots annually from underground root stalks. Their hollow, jointed, ridged stems contain silicate and other minerals. The leaves are reduced to sheaths that clasp and encircle the shoots.
Equisetum is a "living fossil", the only living genus of the entire subclass Equisetidae, which for over 100 million years was much more diverse and dominated the understorey of late Paleozoic forests. Some equisetids were large trees reaching to 30 m (98 ft) tall.
You can find Equisetum plants usually in large numbers in ditches or along the edge of collected water (ponds, rivers, etc.).
These are the vegetative stems of Equisetum. They do not produce the strobilus but do most of the photosynthesis.
These are the reproductive branches of Equisetum. When conditions are right they will produce a strobilus at the top of the plant.
An illustration of the anatomy of Equisetum.
Live Sample of Equisetum (Horsetail)
Note: Please do not tear up the plants to view under the microscope.
Examine the live sample of Horsetail inspecting the nodes, internodes and strobilis (where present).
Capture images of the Horsetail for use in your laboratory worksheet.
Return the Horsetail to its original station location.
You have already examined strobili in the Lycopods and know that they are an important part of the story of plant evolution. However, the Equisetum strobilus poses an interesting question. Is this strobilus just a continuation of the evolution of the strobilus from the lycopods or is it convergent evolution? The question has to be asked because if lycopods are ancestral to the polypods (members of the polypodiopsids) then why do Psilotum and Polypodium not have a strobilis?
Evidence that this is convergent evolution is that the sporangia in the Equisetum strobilus form at the base of stems while those in lycopods develop at the base of leaves (microphylls).
Current research suggests that the strobilus first developed in the lycopods but may have independently arose in several groups as an environmental response for efficient spore dispersal.
An Equisetum strobilus.
An illustration of the parts an Equisetum strobilus. A) A whole view of a strobilus. B) A longitudinal section through the strobilus. C) A cross section of the strobilus. D) An individual Sporangiophore.
A microscopic view of a longitudinal section through an Equisetum strobilus.
A microscopic view of a cross section through an Equisetum strobilus.
Prepared Slide of a Equisetum Strobilus
Compound Light Microscope
Retrieve a slide of a Equisetum strobilus from the lab station
Examine the strobilus under the microscope. Note the arrangement of the sporangia and spores
Capture images of the Equisetum strobilus for use in your laboratory worksheet
Replace the Equisetum strobilus slide back at the appropriate station