|BOTANICAL ELECTRONIC NEWS|
No. 122 December 16, 1995
We smile at these questions today, knowing that they are completely misguided, but they were valid questions to botanists in the 1700s and early 1800s. It wasn't until 1844 that Karl von Naegeli, a German botanist, steered questions about the fern seed in the right direction. By focusing his microscope on the undersurfaces of the prothalli (the membranes or scales that Lindsay originally reported), von Naegeli saw globose papillae containing dark, spiral filaments. He noticed that the papillae, when wet, burst at the tip and released the spiral filaments, which then began to wiggle and swim away. He knew that similar papillae and filaments had been found in mosses and liverworts, where they were called antheridia, in allusion to the "male" anther of the flower. Thus, von Naegeli adopted the name antheridia for the papillae he saw on fern prothalli. But where did the spiral filaments swim to?
This question was answered in 1848 by Michael Jerome Leszczyc-Suminski, a Polish count with a botanical bent. He found that the spiral filaments swam to another kind of papilla also located on the undersurface of the prothalli. This type of papilla, which we now call an archegonium, was flask-shaped with a long neck and a single, large cell at the base. When the sperm swam to the archegonium, they wiggled downwards between the neck cells and penetrated the large basal cell. After penetration, this cell (now known to be an egg cell) developed into an embryonic fern with roots, stem, and leaves. This baby plant eventually grew into a mature fern with spore-bearing leaves.
What developed from Leszczyc-Suminski's observations was the picture of fern reproduction still taught today. In a series of quick nutshells this is it: The spores (fern dust) are produced on the undersides of the leaves in sporangia. They are liberated from the sporangia, land on a suitable substrate, and germinate. They grow into prothalli that bear the sex organs-archegonia and antheridia-which produce egg and sperm, respectively (the prothalli of some ferns produce only one kind of sex organ). The sperm are released from the antheridia when water is present and swim to the archegonia and fertilize the egg. The resulting cell, the zygote, develops into an embryo with stem, roots, and leaves. This embryo grows by widening its stem and producing larger and larger leaves until a spore-bearing leaf eventually appears. At this point the process is complete.
This sequence of events is known as the fern life cycle-the bugbear of many Introductory Botany students. It has two distinct phases, or generations. The first is called the gametophyte generation because it produces the gametes or sex cells. The second is called the sporophyte generation because it produces the spores. The gametophyte consists of the prothallus, and the sporophyte consists of the "normal" fern plant we typically think of-the one with roots, stems, and leaves. Each generation develops from a single cell: the gametophyte from a spore, the sporophyte from a zygote.
One point must be made about these two generations, a point often dimly understood: The gametophyte is the sexual generation because it produces the sex cells, egg and sperm. In contrast, the sporophyte is the asexual generation because it produces asexual spores; it does not produce sex cells. Remember this the next time you spot a leafy fern luxuriating in the wild. What you are looking at is an asexual being, one that does not and cannot engage in sex. This point is difficult to grasp because we tend to equate, erroneously, our own bodies with that of the generation develops from a single cell: the gametophyte from a spore, the sporophyte from a zygote.
One point must be made about these two generations, a point often dimly understood: The gametophyte is the sexual generation because it produces the sex cells, egg and sperm. In contrast, the sporophyte is the asexual generation because it produces asexual spores; it does not produce sex cells. Remember this the next time you spot a leafy fern luxuriating in the wild. What you are looking at is an asexual being, one that does not and cannot engage in sex. This point is difficult to grasp because we tend to equate, erroneously, our own bodies with that of the fern sporophyte. But unlike plants, humans and other animals produce their gametes directly by meiosis; we have no intervening gametophytic (sexual) stage that produces gametes by mitosis.
But to return to the fern-seed. Botanists today realize that spores and seeds are completely different structurally. A spore consists of a single cell and contains no preformed embryonic parts. In contrast, a seed (typically) consists of hundreds or thousands of cells and contains stored food (the endosperm) and an embryo. Moreover, spores and seeds differ in what they give rise to. A fern spore gives rise to the prothallus of the gametophyte generation; a seed, to the baby plant of the new sporophyte generation.
These differences between spores and seeds seem so great that most of us are astonished when we learn that early botanists once seriously considered spores were seeds. But our astonishment is only proof that botany has progressed. Nowadays, it is the belief in the fern seed that walks invisible.
Selected References and Notes:
Author's address: Dr. Robbin C. Moran, Dept. of Systematical
Botany, University of Aarhus, Denmark
University of British Columbia Herbarium (UBC), Vancouver, B.C.
will be closed for fumigation from December 18, 1995 to January
Abstract. The Queen Charlotte Islands lie off the west coast of North America, and are characterized by a strongly oceanic climate, coniferous rain forest and rocky shores, with a low mountainous region raising to 1100 m. Its lichen flora is abun- dant and diverse, with numerous disjunctions and new taxa. Fifty-four crustose lichens and lichenicolous fungi are reported as new for British Columbia, among them six are new to Canada, and 18 are new to the North American flora. The new combination Porpidia ochrolemma (Vain.) Brodo & R. Sant. is made.
The most interesting reports: Lecidea crassilabra - previously
known only from Australia and New Zealand; Pyrenopsis tasmanica
- previously known from Tasmania and New Zealand.- AC:
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