Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
ferns and allied divisions
As you look at the materials of the pterophytes, keep in mind the comparison of ferns with Marchantia in terms of alternation of generations and the various structures. Ferns have both xylem and phloem, the transport tissues for water and nutrients that are characteristic of all upstanding land plants. The gametophyte generation is small but still depends on water for gamete fertilization. Ferns are tough colonists of disturbed environments.
Other divisions of primitive vascular plants include:
Division Sphenophyta (horsetails): preserved Equisetum.
Division Lycophyta (club mosses): preserved Lycopodium and Selaginella
Division Psilophyta (whisk ferns): preserved Psilotum and slide.
Alternation of generations (sporophyte/gametophyte)
Sporophyte. The diploid sporophyte (2n) is the plant that you call a fern. There are several live examples in pots in the room. One of them has many runners, each of which would root in moist soil; this is a means of asexual reproduction.
The sporophyte consists of a root and a shoot with upright leaves called fronds. Some ferns have a rhizome, a horizontal stem from which the true roots extend. Most ferns are much taller than either liverworts or mosses because fern plants are sporophtyes, which have vascular tissue; mosses and liverworts are gemetophytes, which lack vascular tissue. Vascular tissue consists of two kinds of cells. Xylem vessels transport water up from the roots; phloem sieve-tube members transport nutrients made in the leaves to other parts of the plant.
Slide: Pteris rhizome x.s. Several oval vascular bundles are encircle a central pair. Large, thick walled cells in the center of each bundle are xylem tubes; they are probably stained pink. Surrounding them are small, thin walled phloem cells, also tubular; they are probably stained green. Outside the bundles are zones of brown cells called sclerenchyma, a strengthening tissue.
The mature sporophyte forms sporangia, and within them meiosis occurs to produce haploid spores (n). The sporangia are grouped together in clusters called sori (sorus, singular) on the undersides of the leaves. They look like rusty brown rosettes.
Look at the cross section of the leaf itself. The top and bottom are defined by cellular layers called upper and lower epidermis. The inside of the leaf is called mesophyll; it is divided into two parts, closely packed tall cells above that maximize light collection for photosynthesis--palisade mesophyll--and a meshwork of cells with open spaces below for gas exchange--spongy mesophyll. Openings into the air spaces are through stomata in the lower epidermis. Recall that the pores and air spaces in Marchantia were at the top, because that plant lies on the ground.
Slide: Cyrtomium falcatum: sorus on leaf, median l.s. You can see that each sorus has a central axis to which the sporangia are attached; in this example there is a common covering underneath the sporangia, and it is called an indusium. Each sporangium is encircled by a thick walled ring of cells called the annulus. The annulus is hygroscopic (readily absorbs and retains water); as the annulus dries out, tension develops until the structure bursts open and the spores are flung out as if by a catapult. This aids in dispersal of the tiny haploid spores so ferns can take over the world.
Gametophyte: The haploid spores germinate into a haploid prothallus (n), the gametophyte. It is a small, heart shaped plant which is photosynthetic. There are preserved examples; living specimens are on the soil in the terrarium.
Slide: Fern prothalium, male & female. Rhizoids extend from the underside into the soil. Antheridia are scattered toward the edge of the underside; these appear as multi-celled spheres and produce flagellated sperm. When it rains, sperm swim to the eggs in archegonia that are located near the cleft of the heart shape. Each archegonium contains one egg.
Slide: Fern prothalium, young sporophyte. When fertilization takes place, a diploid zygote (2n) forms. The zygote germinates and divides by mitosis to produce a new sporophyte plant. The young sporophyte consists of a primary root pointing down and a primary leaf pointing up. The bifurcating vascular bundle, a characteristic of sporophytes, is visible in the leaf. The sporophyte grows on the site of the archegonium as in Marchantia, but it is an independent plant, because it forms roots and photosynthetic green leaves.
Ferns are plants with independent haploid gametophyte and diploid sporophyte stages. Suppose that a fern sporophyte is heterozygous for a lethal recessive allele which prevents synthesis of chlorophyll.
A. What percent of the gametophytes of the next stage in the life cycle will die?
B. What percent of the next sporophyte generation will still carry the lethal recessive allele?
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Last updated 22 September 2007 (JHW)