DIERSITY OF LIFE

PHYLUM CONIFEROPHYTA (COULTER 1912)

EUKARYA>ARCHAEPLASTIDA>VIRIDIPLANTAE>STREPTOBIONTA>EMBRYOPHYTA>TRACHEOPHYTA>SPERMOPHYTA>CONIFEROPHYTA

Coniferophyta (ko-ni-fe-RA-fa-ta) is made from two Latin and one Greek root that mean cone (conus); bear (fero); plant (phyto -φυτο).  The reference is to plants that bear cones, a common attribute of most members of this phylum.

INTRODUCTION TO THE CONIFEROPHYTA

The conifers are remarkably successful as a group, and are among the most common plants in many temperate, alpine, and sub-arctic environments (Figures A-G).  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.  The wood of conifers is relatively soft and is the wood of choice for framing homes and the production of plywood.  Their leaves vary from needle-like to scale-like to strap-like.  And most, but not all, are evergreen.

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 Cordaites (Figure H).  Today, the Taxales have simplified the ovulate cone to single scales.

The life histories of the extant conifers are variations on that of Pinus (e.g. Figure A).  Early in the spring small simple strobili with abaxial sporangia (microsporangia) on microsporophylls.  The microspore mother cells undergo meiosis to form microspores within the microsporangium.  A microspore becomes a pollen grain when the microspore divides to form a generative cell and a prothallial cell.  The spore wall which is made of two layers, the entine and the exine, separates and the external wall becomes two balloon-like extensions (giving the pollen grain a Mickey Mouse appearance).  The prothallial cell divides again to form two cells.  This is what the gametophyte thallus has been reduced to in the conifers (and other seed plants).  The generative cell divides to form a tube cell and a generative cell (this will divide once more to form the two sperm).  The tube cell grows slowly through the megasporangial wall of the seed until 14 months after pollination the pollen tube enters the archegonial chamber.

Months earlier, the ovule begins to develop on the adaxial surface of the megastrobilus cone scale.  Before the formation of the megagametophyte, the ovule, which has its micropyle oriented toward the cone axis, exudes a droplet that catches pollen in the surface tension.  When the droplet is reabsorbed, the pollen grains caught by it are pulled through the micropyle and into the pollen chamber (this step is called pollination).  The megasporangium has a single megaspore mother cell which undergoes meiosis to form a linear tetrad of megaspores, three of which are aborted.  The megaspore begins to undergo free-nuclear enlargement after which it becomes cellular.  Some of the cells closest to the micropylar end differentiate into eggs (Pinus can have up to 11 eggs in a single ovule).  Fertilization occurs 14 months later when the pollination tube enters the archegonial chamber and one of the two sperm fuse with the egg and a zygote is formed.  The zygote begins to develop into an embryonic plant with multiple cotyledons.  The mature seed has a seed coat formed by the integuments (parental 2N), a megasporangium (parental 2N), a megagametophyte (1N), and an embryo (daughter 2N).  When released, the seed has an elongation of the integument which forms a propeller allowing for greater range of dispersal [See the Life History of Pinus]. 

Like the ferns, the conifers have been among the dominant plants almost from the time of their origin in the Carboniferous (Figures H&I). They increased in importance well into the Mesozoic when the flowering plants appeared.  Likely, the monopodial growth habit characteristic of many conifers served them in the evolutionary arms race with huge sauropods like Brachiosaurus.  The abundant conifers have left remains of many fossils of their wood and leaves.  In addition, the resin produced by most conifers has fossilized to form amber.

SYSTEMATICS OF THE CONIFEROPHYTA

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 gymnosperm phylogeny.  Crane (1996; Tree of Life Project) and Tudge (2000) show the conifers emerging with the ginkgophytes and cycads as sisters to the gnetophyte-flowering plant clade.  Chaw et al. (2000) 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. 

Doyle (2006) summarizes molecular and morphological cladistic analyses which point the conifers in two different directions.  The molecular work embeds the gnetophytes in a conifer clade that includes the Pinales.  The morphological analyses based on ovule position, sperm transfer support the molecular trees.  However, analyses centered on leaf organization suggest that the gnetophytes are separate from the conifers.

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, we will retain the classical taxonomic system.  Thus, we retain a tentative taxonomy of 4 classes, two of which are extant.

Pinus-bristlecone-siu.jpg (67787 bytes)

A. A wizened bristlecone pine (Pinus), one of the longest-lived species on earth.

Giant-sequoia-ucsd.jpg (1356373 bytes)

B. Sequoia, a genus of giant and long-lived trees.  This plant shows the typical monopodial growth habit of many conifers.

juniperus_virginiana-cones-ncsu.jpg (21076 bytes)

C. The berry-like cones of the red cedar (Juniperus).

araucaria-cunninghamii-anbg.jpg (27442 bytes)

D. Araucaria with a typical monopodial growth habit.

Podocarpus-stalked-fruit-csu.jpg (30526 bytes)

E. The stalked "fruit" of  Podocarpus.  The fleshy fruit-like structure beneath the seed is a modified fleshy megasporophyll called an aril.

cephalotaxus-fruit-richmond.jpg (46113 bytes)

F. Cephalotaxus also has seeds borne on a lateral branch that I interpret as a very loose ovulate cone.

taxus-baccata-funet.jpg (112516 bytes)

G. The seeds of Taxus are borne in arils that occur individually on the stem.

cordaites-ucmp.gif (7377 bytes)

H. A reconstruction of Cordaites, a very successful Paleozoic conifer that had long strap-like leaves.

lebachia-berkeley.jpg (31222 bytes)

I. A compression of a Lebachia leafy branch from the lower Mesozoic.

Images taken from:
A: http://www.science.siu.edu/landplants/Coniferophyta/coniferophyta.html
B: http://sysnet.ucsd.edu/~iramani/sequoia/Standing%20tall%20-%20Giant%20sequoia.htm
C: http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/trees-new/
D: http://www.anbg.gov.au/anbg/conifers/araucaria-cunning.html
E: http://farrer.riv.csu.edu.au/ASGAP/APOL25/mar02-5l.html
F: http://www.richmond.edu/~jhayden/conifers/cephalotaxus.html
G: http://www.funet.fi/pub/sci/bio/life/plants/magnoliophyta/pinophytina/taxaceae/taxus/
H: http://www.ucmp.berkeley.edu/seedplants/cordaitales/cordaites.gif
I: http://www.ucmp.berkeley.edu/IB181/VPL/CorCon/CorConVGIII.html

SYNOPTIC DESCRIPTION OF THE CONIFEROPHYTA

The following description comes from Bold et al. (1987), Chamberlain (1966), and Pearson (1995).

I. SYNONYMS: conifers

II. NUMBER: ~ 560 extant species

III. 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 and some of the Coniferopsida).

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 sperm 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.

Life History of:
Pinus

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 and Mesozoic.

HIERARCHICAL CLASSIFICATION OF THE CONIFEROPHYTA

This system is a modification of Bold et al (1987).

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 sparse dichotomous venation making them appear to have parallel veins. Both staminate and ovulate cones are compound. 

ORDER CORDAITALES.

Amyelon, Cardiocarpus, Cordaianthus, Cordaites, Premnoxylon

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. 

ORDER VOLTZIALES.

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. 

ORDER CONIFERALES.

Family Pinaceae: Abies, Cedrus, Larix, Pinus, Pseudolarix, Pseudotsuga, Tsuga, Picea

Family Taxodiaceae: Cryptomeria, Cunninghamia, Metasequoia, Sciadopitys, Sequoia, Sequoiadendron, Taxodium

Family Cupressaceae: Callitris, Chamaecyparis, Cupressus, Juniperus, Thuja

Family Araucariaceae: Agathis, Araucaria, Aucarioxylon+

Family Podocarpaceae: Podocarpus

Family Cephalotaxaceae: 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. 

ORDER TAXALES

Family Taxaceae: Taxus

LITERATURE CITED

Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi. 5th Edition. HarperCollins Publishers, Inc. New York.

Bowe, L. M., G. Coat, and C. W. dePamphilis. 2000. Phylogeny of seed plants based on all three genomic compartments: Extant gymnosperms are monophyletic and Gnetales' closest relatives are conifers. Proceedings of the National Academy of Sciences (USA) 97:4092-4097. 

Chamberlain, C.J. 1966 (originally published 1935). Gymnosperms, Structure and Function. Dover. New York (facsimile of 1935 edition published by University of Chicago Press. Chicago.)

Chaw S.-M., C. L. Parkinson, Y. Cheng, T. M. Vincent, and J. D. Palmer. 2000. Seed plant phylogeny inferred from all three plant genomes: Monophyly of extant gymnosperms and origin of Gnetales from Conifers. Proceedings of the National Academy of Sciences (USA) 97:4086-4086. 

Crane, P.. 1996. Spermatopsida. Seed Plants. Version 01 January 1996 (temporary). http://tolweb.org/Spermatopsida/20622/1996.01.01 in The Tree of Life Web Project, http://tolweb.org/

Doyle, J. A. 2006. Seed ferns and the origin of angiosperms. Journal of the Torrey Botanical Society 133(1): 169-209.

Pearson, L. C. 1995. The Diversity and Evolution of Plants. CRC Press. New York. 

Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York.

By Jack R. Holt.  Last revised: 03/25/2012