Although the bryophytes and lower tracheophytes have evolved various adaptations to terrestrial life, water is still needed for the MICROGAMETE (sperm) to swim to the MACROGAMETE (egg) for fertilization. Thus, these plants are essentially aquatic as far as sexual reproduction is concerned. The higher tracheophytes have developed two major evolutionary advances in their reproductive cycles that have freed them from the ancestral aquatic environment.
- the development of seeds
- the development of a microspore (male gamete) and
microgametophyte (POLLEN GRAIN) that do not require water as a medium for fertilization
Reproduction by seeds is one of the principal distinctions between higher and lower tracheophytes and is generally viewed as a significant factor in the success of the higher groups.
General Characteristics
All seed plants are heterosporous and the greatly reduced gametophytes that develop from the spores are always parasitic (dependent) upon the sporophyte, which is quite conspicuous and dominant. In some cases, both microspores and megaspores are produced by the same plant (monoecious). In others (dioecious), microspores and megaspores are produced by different sporophytes. Ultimately microgametophytes, still enclosed by the microspore wall, are released and carried by wind or animals, especially insects, to the megasporangium (PISTIL). At the stage in which they are shed from the microsporangium (ANTHER), the male microgametophytes are called POLLEN GRAINS. Instead of falling to the ground and germinating there, the pollen grains are carried and deposited in contact with the megasporangium (PISTIL). This process of transport is known as POLLINATION.
The megasporangium or NUCELLUS, which contains the female gametophyte (egg), is protected by a covering (integument) and the whole structure - the potential seed - is called an OVULE.
Because the macrogametophyte (egg) is buried inside the ovule, the sperms of most seed plants must get to the egg through an outgrowth of the pollen grain called the POLLEN TUBE. The pollen tube enzymatically digests its way through the tissue enclosing the female gametophyte (goes through the neck of the pistil), enters the ovule through an opening in the integument known as the MICROPYLE, and breaks open, delivering the sperm.
After fertilization, the seed is formed and ready to leave the parent sporophyte. Under favorable conditions, the seed germinates and the embryo within it develops into a new sporophyte plant.
THE GYMNOSPERMS
The GYMNOSPERMS include ancient lines of plants, some of which extend back to the lower Carboniferous Period (290 + million years ago). For many years the gymnosperms were considered to form a natural taxonomic group of closely related plants derived from a common ancestor. The evidence now available seems to suggest that the gymnosperms include at least two evolutionary lines that arose at different times and are genetically independent of each other. One of these lines led to the CYCADOPHYTA and consists of smaller plants, usually with PINNATE leaves and UNBRANCHED stems. They possess a large PITH area, little WOOD, and a thickened CORTEX. Another evolutionary line led to the CONIFEROPHYTA. This group contains larger plants which BRANCH considerably and have SIMPLE leaves. The stems have a SMALL PITH area, ABUNDANT WOOD, and a relatively SMALL CORTEX.
In addition to the characteristics already given for the Pterophyta, the gymnosperms typically produce naked seeds, usually borne on the upper side of open scales that are produced in cones. All are heterosporous and the sporophytes consist of true roots, stems and leaves. The sporophyte body is usually large (mostly trees) and woody with an active VASCULAR CAMBIUM producing considerable secondary xylem and phloem. Woody tissues are composed mostly of single-celled elements, the TRACHEIDS.
All are perennial and almost all are evergreen. With regard to reproduction, gymnosperms are characterized by various methods of pollination, the production of pollen tubes, and independence from water as a medium for fertilization.
Relationships of the Gymnosperms
Both evolutionary lines apparently originated from some ancestral heterosporous tracheophytes such as psilophytes. The lines of development separated early and both have evolved via the seed ferns. Although both are of similar age, the Coniferophyta have advanced much further in their evolutionary series. Such advances include nonmotile sperms, simple leaves, small pith and cortex, a freely branched stem, and emphasis on vascular tissue.
Economic Importance
The gymnosperms, especially the conifers, are an important source of lumber and lumber products. Among the principal timber-produ-cing trees are pine, Douglas fir, spruce, redwood, cedar, fir,
cypress, and hemlock. Wood pulp for use in paper is also obtained from several conifers such as pines and spruces. Numerous other materials of economic importance, such as turpentine, tannins, charcoal, tar, various oils, resin and methanol are derived from different conifers. Turpentine is the principal solvent for many paints and varnishes; tannins obtained from hemlock, are used in the tanning industry, for ink making, and in the manufacture of certain drugs; and resins, extracted from pines, are used in the manufacture of linoleum, perfume, varnishes and other products. Conifers such as spruce and fir are used as Christmas trees.
Cordaitales
This is an extinct group from the Paleozoic. Evidence indicates
some were tall trees over 100 feet or more. They contained narrow, undivided leaves with parallel veins. Internally the stems of more primitive genera resembled cycads and consisted of large
pith, scanty wood, and a large cortex. Advanced genera contained small pith, much wood produced by extensive secondary growth, and a thin cortex. Such forms have stems that resemble conifers and they are presently classified as such. Micro- and Megasporangia were borne in cones rather than on vegetative leaves - again suggestive of conifers.
Coniferophyta
Most numerous and widespread of the gymnosperms, they comprise some 50 genera and 550 species. They include such genera as pine, spruce, cedar, redwood, cypress, larch or tamarack, yew, fir, and hemlock. Most are trees, some are shrubs. There is a large variance in size. All are characterized by the production of cone-like strobili - thus their designation as cone-bearing plants. Most are monoecious, few dioecious; in none are both male and female organs borne in the same cone.
The pine is used as the representative example of this group. Pine may be viewed as an intermediate in the evolutionary sequence from fern to Angiosperm. The life cycle of the pine features certain evolutionary advances over the fern life cycle, which may be summarized as follows:
- In pine, there are two types of strobili, more complex
and well developed (than in lower tracheophytes), which contain microsporangia and megasporangia.
- These sporangia produce microspores and megaspores, which germinate into microgametophytes and megagametophytes.
- The gametophytes are much further reduced than those of
the ferns and are retained within the protective tissue
of the cone during a portion of their development or are
permanently retained.
- The gametophytes are dependent entirely upon the sporophyte.
- The gametes are not exposed to adverse environmental
conditions. The sperm moves to the egg through a pollen tube and the egg remains embedded in the gametophyte tissue.
- Fertilization is independent of outside conditions of moisture thus the sperm are typically non-flagellated.
- The integument of the megasporangium affords protection for the egg and developing embryo against injury and dessication
and enables the seed to retain its stored food.
The seed habit and dormancy are considered to represent major evolutionary advances in the plant world.
Cycadophyta
A remarkably diverse group which includes the cycads, palm
like plants of tropical and subtropical areas. Once so numerous
as to share equal billing during the Mesozoic with the "terrible lizards" - a time or Age of the Cycads and the Dinosaurs. This group is related to the seed ferns and transitional forms appear in the fossil record. All are dioecious with slow secondary growth from vascular cambium.
Ginkgophyta
Ginkgo biloba is the only modern representative of this group. Fossil records extend back to the Permian. A fairly large tree which may reach 100 feet, it has been cultivated in Asiatic countries and transported to numerous cities in the U.S. and Europe. Structurally it produces a vertical axis with horizontally held branches and the characteristic leaves are broadly fan-shaped and bilobed with dichotymous venation. Unlike other gymnosperms, it is deciduous and loses its leaves in the fall. The stem and root are anatomically much like the conifers.
Ginkgo is strictly dioecious. It is especially resistant to air pollution and the seeds from the female plant produce a rancid odor as a result of butyric acid in the seed coat.
Gnetophyta
This group contains an unusual array of three genera with varying appearances. Gnetum,Ephedra,
and Welwitschia all show a distinct relationship to the angiosperms.
THE ANGIOSPERMS
Comparative Morphology and Fossil records seem to indicate that the earliest Angiosperms were woody plants with active cambium. MONOCOTS originated as early herbaceous (probably aquatic) offshoots from primitive DICOTS. Monocots filled niches that had been previously occupied mainly by ferns and their allies.
