The conifers are remarkably successful as a group.  Like the ferns, they have been among the dominant plants almost from the time of their origin  (in the Carboniferous).  And still dominate in some temperate, alpine, and sub-arctic environments.  The conifers generally have a strong monopodial vegetative growth.  The stems and roots have an active vascular cambium, and produce large amounts of wood.  This attribute, and their tendency to grow in almost pure stands of uniform age has made them economically important wood producers.  Their leaves vary from needle-like to scale-like to strap-like.  Their reproductive structures almost always occur in strobili.  Most modern conifers have compound ovulate cones and simple staminate cones.  This is a simplification of the compound ovulate and staminate cones characteristic of the Paleozoic conifers like the Cordaites. Today, the Taxales have simplified the ovulate cone to single scales.

The origin of conifers has been a puzzle.  Their similarities to the progymnosperms have led authors like Pearson (1995) to place them near the root of the gympnosperm phylogeny.  Tudge (2000) shows the conifers emerging with the ginkgophytes and cycads as sisters to the gnetophyte-flowering plant clade.  Chaw et al. (1999) and Bowe (2000) present similar analyses based on multiple genes (3 genomes: nuclear, mitochondrial, and chloroplast) in which Ginkgo is a sister to the gnetophyte-conifer clade.  Their analyses show the conifers separating into two groups.  Conifer group 1 is made of the Pinaceae and the Gnetophytes.  Conifer group 2 is all of the other extant conifers.  Surprisingly, Taxus is firmly embedded in the Conifer 2 group.

Bold et al. (1987) present a classical taxonomic treatment of the conifers. They show two major groups based on ovulate cones: Coniferopsida (compound ovulate cones) and Taxopsida (simple, reduced isolated ovules on sporophylls).  Until a new taxonomic system evolves from the current work, which itself is controversial, I will retain the classical taxonomic system.  I offer it as a tentative taxonomy of 4 classes, two of which are extant.

I. PHYLUM CHARACTERISTICS

A. Structure

Habit: The conifers have needle-like or scale-like leaves and usually have a strong monopodial habit. Ovules are borne in compound cones (except the Taxopsida).

Pollen: Pollen walls with 1 suture and a large saccus or 2 sacci. The microgametophyte has 0-2 prothallial cells, a stalk cell, 2 nonflagellated sperms and a tube cell.

Microstrobilus: Simple or compound; the microsporangia are adaxial.

Seeds: Ovules with 1 integument of 3 tissue layers; no pollen chamber or nucellar beak. Archaegonia (each with 2 tiers of 4 neck canal cells) develop at the micropylar end of the megagametophyte. The embryo has many (2-18) cotyledons.

Megastrobilus: Compound with ovules associated with cone scales; each scale is subtended by a bract.

Stems: Monopodial growth with extensive wood. Often, leafy shoots of 2 types: long shoots and spur shoots. Stems with many resin canals.

Leaves: Needle-like or scale-like. Usually there is a difference between the long shoot leaves and spur shoot leaves.

Roots: Protostelic, usually diarch. They can undergo extensive secondary growth.

B. Ecology: These plants are found world-wide. Dominants in the northern temperate and sub arctic forests. These plants have a fossil history which dates from the Carboniferous. Mesozoic.

II. TAXONOMY: As I have defined this phylum, the CONIFEROPHYTA has 4 classes, each with a single order.

CLASS CORDAITOPSIDA

These plants are extinct and are dominants in the Carboniferous period. Very large trees (up to 20m), their upper branches have long, strap-like leaves. The stems have chambered pith and endarch xylem. Although they have a well developed cambium, they have no growth rings. The leaves are thin, strap-like and long (more than a meter), with sparsly dichotomous venation making them appear to have parallel veins. Both staminate and ovulate cones are compound. The single order is CORDAITALES.

example: Cordaites

CLASS VOLTZIOPSIDA

These plants are extinct with a fossil history which ranges from the Carboniferous to the Triassic. They are trees with whorls of branches. The leaves are dimorphic; flattened and broad on the large branches and needle-like on the ultimate leafy branches. They have extensive secondary xylem with cones on the ends of branches. The ovulate cones are compound while the staminate cones are simple. The single order is VOLTZIALES.

example: Lebachia

CLASS CONIFEROPSIDA

These plants are extant and have a fossil history which dates from the Triassic. They are trees, rarely shrubs. Usually, the leaves are dimorphic. They have extensive secondary xylem. The ovulate cones are compound and the staminate cones are simple. The single order is CONIFERALES. This is a dominant group of living plants and has 6 families.

FAMILY PINACEAE

examples: Pinus, Pseudotsuga, Tsuga, Picea, Abies, Larix, Cedrus.

FAMILY TAXODIACEAE

examples: Taxodium, Sequoia, Sequoiadendron.

FAMILY CUPRESSACEAE

examples: Juniperus, Cupressus, Chamaecyparis, Thuja.

FAMILY ARAUCARIACEAE

examples: Agathis, Araucaria.

FAMILY PODOCARPACEAE

example: Podocarpus.

FAMILY CEPHALOTAXACEAE

example: Cephalotaxus.

CLASS TAXOPSIDA

These plants are dioecious and have no megastrobili; the megasporophylls become fleshy (arils). The ovules produce only a single archaegonium. The plants have simple microstrobili, the pollen of which produce no prothallial cells. This class has a single order, TAXALES, and a single family, TAXACEAE.

example: Taxus.

revised: 04/15/2003