Plants in this division are somewhat more complex, and better adapted to a land environment, than mosses, though still restricted to growth in moist areas. These plants contain some specialized vascular tissue and, like all vascular plants, have a dominant sporophyte generation.
A fern sporophyte most often consists of a horizontal stem, called a rhizome; roots; and prominent erect leaves, called fronds. At maturity sporangia, consisting of a stalk and capsule, develop on the fronds, often in clusters called sori. Each sorus may be covered by a protective flap of tissue growing from the leaf surface, called an indusium.
Meiosis of cells within the sporangia results in the production of spores. When the sporangia within a sorus are mature the indusium shrivels to expose them, the wall of each sporangium capsule opens, and the spores are released to the surroundings.
The spores germinate to produce a fern gametophyte, which is a small, photosynthetic prothallus, attached to the land by rhizoids. ( A prothallus is a relatively undifferentiated plant body, which is small and flattened.) As it matures, each prothallus will produce antheridia and/or archegonia. Sperm produced within the antheridia swim across a surface film of water to reach, and fertilize, an egg within an archegonium. The zygote produced from fertilization grows into an embryo within the archegonium, and then develops into an independently growing sporophyte.
The life cycle of a fern can be viewed here, 2.
1. Sketch a frond from the fern specimen provided by your instructor, labeling the sori on its lower surface.
2. Obtain one sori-bearing pinna (leaf segment) from a fern frond. Observe a sorus on this pinna with a dissecting microscope and draw it, labeling the sporangia and the indusium.
3. Using a compound microscope, examine a prepared slide of a fern leaflet bearing a sorus. Draw this, labeling the leaflet, the sporangia, and the indusium.
4. Examine a prepared slide of fern sporangia. Draw a sporangium and label the stalk and the capsule. Also label the annulus (the row of thick-walled cells running across the top of the capsule) and the spores contained within the capsule.
5. Obtain a slide of fern gametophytes carrying archegonia and antheridia. Each gametophyte is capable of producing both archegonia and antheridia, but they are produced at different times. Draw one heart-shaped prothallus, labeling the rhizoids, and the archegonia clustered near the notch of the "heart". Also draw a second prothallus, labeling the rhizoids again, and the antheridia scattered across its surface. A composite prothallus with both antheridia and archegonia can be viewed here.
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