Tree by David Suzuki & Wayne Grady

Author:David Suzuki & Wayne Grady
Language: eng
Format: epub
Publisher: Allen & Unwin
Published: 2018-07-09T16:00:00+00:00

Douglas-fir forest

The mature fern disperses thousands of spores. Those that fall on moist, shaded ground immediately begin to grow, but not into discernible ferns; they grow into low, flat plants called gametophytes, a few centimeters in diameter, bearing on the undersurface of their leaves organs that produce not spores but the normal plant sex organs—the male antheridia and the female archegonia—more typical of those now found on conifers. These “hidden” sex organs “wed” to produce a seed that, when fertilized, grows into a fern. This complicated and indirect method of reproducing may have arisen to ensure a fall-back position for the family if climatic conditions suddenly became unfavorable to either strategy, spore production or seed dispersal.

Although climatic conditions changed drastically at the end of the Carboniferous period—the global average temperature plunged from 20°C (68°F) to 12°C (54°F)—and the huge plants died out, the fern family line has continued virtually unbroken to the present, which is why we have ferns in such numbers today. There are, worldwide, more than twenty thousand species of them, including at least one living fossil, the field horsetail, smaller than its ancestral monsters but one of the most widely dispersed of its kind. Some modern ferns are not so small: the beautiful tree ferns of the tropics often reach heights of 30 meters (100 feet) or more, and the giant horsetail (Equisetum giganteum) grows to 10 meters (33 feet). Most, however, are under 1 meter (3 feet), having returned to the size of their forebears before the Carboniferous period. Fungi still reproduce solely by means of spores; gymnosperms, like our tree, all of which are descended from ferns, went the seed-producing route. Hofmeister showed conclusively that conifers were the evolutionary link between ferns and the flowering plants.

Gymnosperm means “naked seed,” from the Greek gymno, “naked” (Greek athletes performed naked in gymnasiums) and sperma, “seed.” (Sperm whales are so called because it was once thought that the white, fatty substance in the whale’s head was semen.) In the gymnosperms, the ovules within which the seeds develop lie openly on the cone scales, not covered by protective carpels as they are in the later flowering plants, or angiosperms (“enclosed seeds”). The seed-producing organs in conifers are still called sporophytes, a term held over from the spore-producing organs of ferns. And in the horsetails and club mosses, the spores are contained in strobili, Latin for “cones.”

Gymnosperms evolved from ferns by getting a cambium. They also improved the strength of their stems, increased the amount of cellulose and lignin as stiffening agents, and filled the hollow centers with dead wood. Why they did so is a matter of speculation. Their adaptation may have been in response to the cooler, drier climate that followed the Carboniferous period; tough outer bark and a more efficient way of getting water from the roots into a lofty canopy would have been distinct evolutionary advantages. And developing elaborate root systems would have been a better way than relying on rhizomes for gathering increasingly scarce ground water.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.