The four phyla of living gymnosperms are of separate clades or lineages, unlike the angiosperms, which are a monophyletic, single lineage. “Gymnosperm” means “naked seed” and the name draws attention to the ovules and resulting seeds that are exposed openly on the megasporophylls.
Phylum Coniferophyta
The conifers are woody, mostly evergreen trees, with needle‐shaped or flattened leaves, which occupy the drier and cooler sites in the world today just as their ancestors probably did in the Permian. They are the familiar pines, firs, spruces, yews, hemlocks, and junipers of the Northern Hemisphere forests and the Araucaria species of the Southern. The Sequoias are among the tallest living trees (the Australians credit the Eucalyptus, an angiosperm, as being the tallest), and the bristlecone pines are among the oldest living plants. ( The oldest plant, purportedly, is a clone of creosote bush in the Mojave Desert that is well over 4,000 years of age.)
The pines (pinus species). The leaves of the 90+ species of pines are needles that, in the seedlings, are borne singly along the stem. As the seedling matures, however, the needles appear in fascicles (bundles) of several (the number varying by species) on short shoots covered with scale‐like leaves. The fascicle is a branch of determinate growth, a feature of evolutionary significance.
The pine needles are adapted for a xeric environment, one in which water is unavailable either because it is frozen most of the time or else because it is climatically scarce. Needles have a thick cuticle, an epidermis, and an underlying hypodermis of thick‐walled cells, which further protect the mesophyll from drying out. The vascular bundles are surrounded by transfer tissue of parenchyma cells. Resin canals are present in regular patterns within the needle. Needles are shed at intervals of two to four or more years, but not all of the needles on the tree are dropped at the same time. Thus, a tree remains evergreen.
The wood of pines and conifers (called softwood by lumbermen) in general lacks vessels and is composed of tracheids with circular bordered pits. Parenchyma is almost entirely restricted to ribbons of narrow rays. The vascular cambium is bifacial; that is, it produces secondary xylem (wood) toward the center and secondary phloem toward the outside of the stem.
The life cycle of a pine is a slow, two‐year process (Figure ). Pollination occurs in the spring of one year, and the pollen tube begins its growth towards the megagametophyte about this time although its goal, the egg, is not as yet differentiated. In fact, the megaspore mother cell has not yet divided, the megagametophyte does not exist, and there are no archegonia, let alone eggs within the ovulate cones. This seeming lack of syncronization is of little concern because it takes the pollen tube over a year to digest its way through nucellular tissues to the archegonia—which gives ample time for megagametophyte preparations—and for the immature male gametophyte (the four‐celled germinated pollen grain) to produce two sperm cells by division of the generative cell.
In the spring of the year following pollination, events come together: The eggs in the two to three archegonia are fertilized (polyembryony), and development of the new sporophytic generation begins. Normally only one embryo survives to maturity in the seed. The pine seed consists of tissues from two sporophyte and one gametophyte generation. That is, the parent 2 n sporophyte tissue remains in the seed as the seed coat (mature integuments); the embryo is the new 2 n sporophyte, which is surrounded by the 1 n megagametophyte. Sometime during the summer of the year following pollination the seed matures and is shed. If it germinates in a suitable habitat, a new tree grows to initiate still another cycle.
Other conifers. Not all of the conifers resemble the needle‐leaved pines in appearance or length of time to complete the sexual reproductive cycle—most take only a year. Some conifers are deciduous, such as larch ( Larix), bald cypress ( Taxodium), and the dawn redwood ( Metasequoia). The yews ( Taxus) have flattened leaves and instead of a cone have a fleshy red cup, an aril.
As a group, the conifers occur throughout both the Northern and Southern Hemispheres, some in large numbers and widespread across sites largely unsuitable for angiosperm tree growth. Many other genera are restricted in species numbers and have a relict distribution. Some, such as the dawn redwood and the Wollemia pine, have only a few living individuals left in isolated sites although botanists knew of neither species until fairly recently.
Seventeen species of conifers are growing in small numbers along the California coast—and nowhere else. The same story is repeated in other genera, which leads to the conclusion that the conifers reached their heyday in the Mesozoic and then started a decline that continues to the present.
Figure 1
In the spring of the year following pollination, events come together: The eggs in the two to three archegonia are fertilized (polyembryony), and development of the new sporophytic generation begins. Normally only one embryo survives to maturity in the seed. The pine seed consists of tissues from two sporophyte and one gametophyte generation. That is, the parent 2 n sporophyte tissue remains in the seed as the seed coat (mature integuments); the embryo is the new 2 n sporophyte, which is surrounded by the 1 n megagametophyte. Sometime during the summer of the year following pollination the seed matures and is shed. If it germinates in a suitable habitat, a new tree grows to initiate still another cycle.
Other conifers. Not all of the conifers resemble the needle-leaved pines in appearance or length of time to complete the sexual reproductive cycle—most take only a year. Some conifers are deciduous, such as larch ( Larix), bald cypress ( Taxodium), and the dawn redwood ( Metasequoia). The yews ( Taxus) have flattened leaves and instead of a cone have a fleshy red cup, an aril.
As a group, the conifers occur throughout both the Northern and Southern Hemispheres, some in large numbers and widespread across sites largely unsuitable for angiosperm tree growth. Many other genera are restricted in species numbers and have a relict distribution. Some, such as the dawn redwood and the Wollemia pine, have only a few living individuals left in isolated sites although botanists knew of neither species until fairly recently.
Seventeen species of conifers are growing in small numbers along the California coast—and nowhere else. The same story is repeated in other genera, which leads to the conclusion that the conifers reached their heyday in the Mesozoic and then started a decline that continues to the present.