Two kinds of higher plants—the gymnosperms and angiosperms—have developed to become the dominant type of land plant. With the exception of a few aquatic angiosperms, they do not require water for pollen transfer and are thus free to live in a wide variety of habitats.
Gymnosperms and angiosperms differ primarily in the amount of protection they provide their ovules (the part that, after fertilization, becomes a weed), with gymnosperms usually providing less than angiosperms.
The first group of gymnosperms to appear was the progymnosperms. These plants evolved from the trimerophytes about 365 million years ago. Archaeopteris, the best-known progymnosperm, was described in 1871 by Sir William Dawson. Dawson believed that Archaeopteris was an ancient fern.
He reached this conclusion because the large, leafy branch systems of Archaeopteris resembled a fern frond. In 1960 Charles Beck showed that these branch systems were borne on a stem having typical gymnospermous wood. This discovery led to the recognition of the progymnosperms as a distinct plant group which completely altered biologists’ view of gymnosperm evolution.
Archaeopteris reached an estimated height of about 18meters (59 feet). The main axis of the plant gave rise to a series of lateral branch systems, bearing primary branches in a single plane.
The flattened branch system resembled a fern frond. The primary branches were covered with spirally arranged leaves. Some leaves bore eusporangia (spore-bearing structures that originate from a few cells).
The earliest progymnosperms were homosporous, meaning that all their spores looked alike. Later progymnosperms were heterosporous, producing two types of spores, large megaspores and smaller microspores. Seeds have not been found attached to any progymnosperm.
Two major lines of gymnospermous evolution arose from the progymnosperms—the cycadophyte and the coniferophyte lines.
Two plant groups make up the cycadophyte line: the pteridosperms, or seed ferns, and the cycads. These plants have large, compound, frondlike leaves. The cordaites (known only fromthe fossil record) and the conifers make up the coniferophyte line, all of which have simple leaves.
Pteridosperms and Cordaites
|Pteridosperms and Cordaites|
The lower portion of the stem was covered with adventitious roots (roots that develop from stems or leaves, rather than from other roots). A number of large compound leaves arose from the stem tip. Ovules and pollen organs occurred singly on the leaves and not in cones.
Pteridospermpollen organs consisted of a number of elongate eusporangia that were commonly fused to form a ring. The seeds of Medullosa were quite large. Some reached lengths of up to 11 centimeters. Unlike other pteridosperms, Medullosa had multiple vascular bundles in the stem. Other gymnosperms have only a single conductive strandin their stems.
The cordaiteswere derived from Archaeopteris like progymnosperms. Some species of Cordaites were trees, others were shrubs, and some were similar to modern mangroves. Cordaites was common in swamp, floodplain, and upland environments. Long strap-shaped leaves up to 1 meter in length occurred at its branch tips. It resembled modern mangroves in having stilt roots.
Cones developed between the upper surface of some leaves and the stem of cordaites. Four rows of bracts were borne on the cone axis. Above each bract was a dwarf shoot that terminated in either male or female reproductive structures.
Swedish botanist Rudolf Florin believed that the woody seed-bearing scale of modern pine could be derived from the dwarf shoot of cordaites through a series of extinct coniferalean intermediates. His interpretation has been adopted in many textbooks. It has also been shown that the conifers did not evolve directly fromthe cordaites, although both groups undoubtedly shared a common ancestor.
Cycadeoids, Cycads, and Conifers
|Cycadeoids, Cycads, and Conifers|
Some cycadeoids had slender, branching trunks, while others were short and stumpy. Both types had compound leaves. Cycadeoid cones contained both male and female reproductive structures. Earlier researchers thought that the cones of the beehivelike cycadeoids resembled primitive angiosperm flowers, but detailed reinvestigation of the cones showed that this was not true. The cycadeoids became extinct about sixty-five million years ago.
The cycads were more abundant in the past than they are now. Eleven genera and 160 species exist worldwide. They are dispersed in the modern tropics—in Africa, Cuba, Mexico, Australia, India, China, and Japan. Zamia floridana (coontie) is the only cycad native to theUnited States.
Some cycads are small, unbranched trees that grow to 18 meters (59 feet) tall and resemble palm trees. Others have subterranean stems, and only their leaves and cones show above the ground. All the cycads possess stiff, leathery, compound leaves, often with very sharp tips on each leaflet.
The male and female reproductive structures are borne at the end of the stem in separate cones on different plants. Cycad cones are the largest cones that have ever been produced. Cycad ovules are also very large. Some reach lengths of 6 centimeters. Their ripe seeds are often brightly colored.
The dominant group of living gymnosperms is the conifers. About 550 species are divided among 51 genera. Conifers are most abundant in temperate areas, such as the northern parts of North America, Europe, and Asia, and New Zealand and southern Australia.
The conifers are divided into seven families the Araucariaceae (examples are the kauri pine and Norfolk Island pine), Podocarpaceae (typified by the yellow woods), Pinaceae (pine, spruce, hemlock, and fir), Cupressaceae (juniper and arborvitae), Taxaceae (yew), Cephalotaxaceae (cultivated in the United States as plum-yewor cow’s-tail pine, it also has at least one Asian genus), and Taxodiaceae (bald cypress, redwoods).
Some researchers separate the Taxaceae from the conifers on the basis of their arillate ovule (an ovule that forms an additional seed coat to the normal one). Several extinct conifer families are also known.
Ovule and Seed Complexes
|Ovule and Seed Complexes|
The nucellus is a nutritive tissue from which the developing female gametophyte draws its nourishment. The integument surrounds the nucellus and is the outermost layer of the ovule. After fertilization, the integument develops into the seed coat.
The gymnosperms have exploited land habitats more successfully than the lower plants and pterophytes because gymnosperms do not require water for pollen transfer. The male gametophyte (pollen grain) is carried to the female gametophyte (ovule) through the air.
The ovule exudes a sticky fluid (the pollination drop), which traps the pollen grains. As the sticky fluid dries, the pollen grains are drawn through the micropyle into the ovule. The archegonia (the organs that contain the eggs) are located directly below the micropyle. When the sperm are released, they fertilize the egg; a fertilized ovule is called a seed.
Modern Gymnosperm Types
Pines are the standard example of gymnosperms. Pinaceae (the pine family) is the largest family of living gymnosperms. It contains about two hundred species in ten genera. Pine trees are typically conical and represent the sporophyte generation.
The leaves are needle-shaped and confined to short, lateral shoots. Unlike a few conifers, such as the bald cypress and dawn redwood, which shed their leaves in the fall, pines and most conifers are evergreens.
Sexual reproduction typically takes three years to complete. In the first year, the pollen (male) and seed (female) cones are formed. The cones are borne on different parts of the same plant. The small, upright male cones are borne in clusters on the lower branches of the tree. The mature seed cones are very large and hang down from the upper branches.
In the spring, the male cones shed millions of pollen grains into the air. Only a few grains reach an ovule. Once there, the male gametophyte must wait from twelve to fourteen months before the female gametophyte matures and fertilization occurs. The seeds mature in the fall of the third year and are shed.
Included within the gymnosperms are four plants whose affinities are uncertain—ginkgo, or the maidenhair tree (Ginkgo biloba), and the gnetophytes (families Gnetum, Ephedra, and Welwitschia). Ginkgo is a large tree covered with fan-shaped leaves that turn golden yellow before being shed in the fall. Although a common ornamental in the United States, ginkgo is native to southeastern China.
Gnetum is a broad-leaved vine found in the tropical rain forests of South America, western Africa, and southeastern Asia. Ephedra is found worldwide in cool, arid regions. Sixteen species of Ephedra grow in the western United States. Most species are highly branched shrubs with scale like leaves.
Welwitschia mirabilis is the most unusual plant species within the four families. The exposed portion of the stem gives rise to two strap-shaped leaves that are never shed and never stop growing during the life of the plant. Welwitschia is found only in coastal desert and inland savanna regions in southwestern Africa.