Evolution of Plants

Modern classification systems, based largely on molecular evidence, divide living organisms into three domains: Bacteria (also called Eubacteria), Archaea, and Eukarya. Plants are classified as a kingdom (Plantae) within the Eukarya; organisms that possess a nucleus, mitochondria, an internal cytoskeleton, and, in photosynthetic species, chloroplasts. Most scientists rec­ognize three other eukaryotic kingdoms: Protista (most of which are single-celled organisms), Fungi, and Animalia (animals). The fungi, plants, and animals are thought to have evolved from different groups of protists.

Plants are multicellular organisms that have evolved the ability to live on land. The vast majority can carry out photosynthesis, but they are not the only organisms with this ability: many protists can photosynthesize too, as can several important groups of bacteria.

Algae in Plant Evolution

Photosynthetic protists (commonly called algae) are a diverse group of or­ganisms and are divided into several phyla. Many are unicellular, including most euglenoids (phylum Euglenophyta) and dinoflagellates (Dinophyta), and some diatoms (Bacillariophyta) and green algae (Chlorophyta). These, along with the cyanobacteria (often misleadingly called blue-green algae), form the phytoplankton of aquatic ecosystems. Others, including all brown algae (Phaeophyta), most red algae (Rhodophyta), and many green algae are multicellular. The large marine forms of these phyla are usually called sea­weeds.

Plants are thought to have evolved from a class of freshwater green al­gae called the charophytes. Two particular groups of charophyte, the Coleochaetales and the Charales, resemble the earliest land plants (bryophytes) in a variety of ways, including the structure of their chloro- plasts and sperm cells, and the way their cells divide during mitosis.

The Importance of Vascular Tissue

Plants are classified into two main groups: the bryophytes (nonvascular plants) and the tracheophytes (vascular plants). Both groups have multicellular embryos, which indicates that they are closely related to each another and distinguishes them from the green algae. Indeed, true plants are often referred to as embryophytes because of this feature. The bryophytes con­sist of the liverworts, hornworts, and mosses, and as their name implies none of these plants possess vascular tissues.

All other plants, including the ferns, gymnosperms, and angiosperms, are classified as tracheophytes. These possess specialized vascular tissues— phloem and xylem—to transport sugars, water, and minerals throughout their bodies. The oldest known vascular plants appeared in the middle Sil­urian period (439-409 million years ago); the oldest known bryophytes ap­peared later, in the Devonian (409-354 million years ago). Despite this, most scientists believe that bryophytes evolved before vascular plants, and that the earliest bryophytes have not been found because they fossilize poorly. This belief is supported by a variety of evidence, including morphological traits, ultrastructural features visible under the electron microscope, and molecular information obtained from gene sequencing.

Bryophytes

Since bryophytes are land plants, they need to support themselves in air. However, because they lack lignified vascular tissues, this support must be provided largely by the turgor pressure of their cells. Consequently, they cannot grow to be very tall, and most bryophytes are small and rather in­conspicuous. An additional important feature of their lifestyle is their re­productive system. The male gametes, produced by reproductive structures called antheridia, are free-swimming sperm cells that need water to trans­port them to the female gametes, which are enclosed within structures called archegonia. Because of the need for water, bryophytes are especially com­mon in wet habitats such as bogs, streambanks, and in moist forests. How­ever, they are not restricted to these habitats, and some mosses thrive in deserts, above the treeline, and in the Arctic tundra.

Among the living bryophytes, liverworts are probably most closely re­lated to the earliest land plants, since unlike hornworts, mosses, and all vas­cular plants they do not possess stomata. Indeed, the fact that stomata first appeared in hornworts and mosses is evidence that vascular plants evolved from one of these two groups. Vascular plants appear to be more closely related to mosses than to hornworts, because some mosses possess food- conducting cells (leptoids) and water-conducting cells (hydroids) that re­semble the phloem and xylem of vascular plants.

Early Vascular Plants

The first detailed vascular plant fossils appear in rocks from middle Silurian, about 425 million years ago. The oldest of these, including a plant called Aglaophyton, appear to have possessed conducting cells similar to the hydroids of mosses. These ancient plants, which are sometimes called proto- tracheophytes, may have been an evolutionary link between the bryophytes and the true tracheophytes. Early vascular plants possessed two features that made them especially well adapted to life on land. First, their vascular tis­sues transported sugars, nutrients, and water far more efficiently than the conducting cells of mosses. Second, they evolved the ability to synthesize lignin, which made the cell walls of their vascular tissues rigid and sup­portive. Taken together, these features allowed them to grow much larger than their bryophyte ancestors and considerably reduced their dependence on moist habitats.

There are three major groups of tracheophytes: seedless vascular plants, gymnosperms, and angiosperms. Since the first appearance of tracheophytes in the Silurian, the fossil record shows three major evolutionary transitions, in each of which a group of plants that were predominant before the tran­sition is largely replaced by a different group that becomes predominant af­terward. The first such transition occurred in the late Devonian, approximately 375 million years ago. Prior to this time the most common plants were simple, seedless vascular plants in various phyla, several of which are now extinct. However, one phylum from this time, the Psilophyta, still has two living genera, including a greenhouse weed called Psilotum.

From the late Devonian until the end of the Carboniferous period (290 million years ago) larger, more complex seedless plants were predominant. The main phyla were the Lycophyta, the Sphenophyta, and the Pterophyta.

All three groups contain living relatives, including club mosses (Lycopodiaceae) in the Lycophyta, Equisetum (the only living genus of sphenophytes), and ferns, which are pterophytes. Only the ferns, which have about 11,000 living species, are common today, but in the Carboniferous these three phyla comprised a large fraction of the vegetation on the planet. Many grew to the size of trees and dominated the tropical and subtropical swamps that covered much of the globe at this time.

The second major transition was the decline of the lycophytes, spheno- phytes, and pterophytes at the end of the Carboniferous and their replace­ment by gymnosperms in the early Permian. Gymnosperms dominated the vegetation of the land for the next 200 million years until they themselves began to decline and were replaced by angiosperms in the middle of the Cretaceous. Although one group of gymnosperms (the conifers) is still abun­dant, the angiosperms have been the most diverse and widespread group of plants on Earth for the last 100 million years.

Gymnosperms

The gymnosperms probably evolved from an extinct phylum of seedless vas­cular plants, the progymnosperms, which appeared about 380 million years ago. The fossils of these plants, some of which were large trees, appear to form a link between the trimerophytes (another extinct phylum of seedless vascular plants) and true gymnosperms. Progymnosperms reproduced by means of spores like the former, but their vascular tissues were very similar to those of living conifers. The oldest true gymnosperms, which produce seeds rather than spores, first appeared about 365 million years ago. The evolution of seeds, with their hard, resilient coats, was almost certainly a key factor in the success of the group. A second factor was the evolution of pollen grains to protect and transport the male gametes. As a consequence of this, gymnosperms, unlike seedless vascular plants, were no longer dependent on water for successful fertilization and could broadcast their male gametes on the wind.

Several early gymnosperm groups are now extinct, but there are four phyla with living representatives: the cycads, the gnetophytes, the conifers, and one phylum (Ginkgophyta) that has only a single living species, the ginkgo tree (Ginkgo biloba). Of these, the conifers are by far the most abun­dant and diverse, and many species are of considerable ecological and eco­nomic importance. Most conifers are well adapted to dry environments, particularly in their leaf morphology, and some can withstand severe cold. These features may have enabled them to thrive in the Permian, when Earth became much drier and colder than it had been in the Carboniferous.

Angiosperms

The angiosperms, or flowering plants, are all members of the phylum An- thophyta. There are at least 250,000 species, making the group easily the most diverse of all plant phyla. They share a number of features that dis­tinguish them from other plant groups. The most obvious of these is the possession of flowers, highly modified shoots that carry the male and female reproductive structures. They also carry out a process called double fertil­ization, in which two male gametes (sperm nuclei) are released from the pollen tube into the ovule. One of these sperm nuclei fuses with an egg cell in a similar way to gymnosperms. The second nucleus (which degenerates in most gymnosperms) fertilizes other cells in the ovule called polar nuclei. Most commonly, two polar nuclei fuse with the sperm nucleus to form a triploid endosperm nucleus. The tissue that forms from this fusion is called endosperm, which in most angiosperms provides nutrients for the develop­ing embryo.

The ginkgo tree (Ginkgo biloba) is the only living species of the early gymnosperm phylum Ginkgophyta.

A third feature that separates angiosperms from gymnosperms is that angiosperm embryos are protected by an ovary wall, which develops into a fruit after fertilization has taken place. In contrast, gymnosperm embryos are held relatively unprotected on the surfaces of ovule-bearing scales in the female cones.

Angiosperm Evolution

Angiosperms first appear in the fossil record about 130 million years ago, and by 90 million years ago they had become the predominant group of plants on the planet. English naturalist Charles Darwin considered the sud­den appearance of angiosperms to be an “abominable mystery,” and scien tists have debated about the origin of the group for many years. Compara­tive studies of living species suggest that angiosperms evolved from the gnetophytes, a group of gymnosperms with three living genera of rather strange plants: Ephedra, Gnetum, and Welwitschia. Double fertilization has been shown to occur in both Ephedra and Gnetum, and the reproductive struc­tures (strobili) of all three genera are similar to the flowering stalks of some angiosperms. Some gene sequencing studies also indicate that gnetophytes and angiosperms are closely related to each other and to an extinct group of gymnosperms called the Bennettitales. However, more recent molecular studies suggest that gnetophytes are more closely related to conifers than they are to angiosperms.

In 1998, the discovery of an angiosperm-like fossil called Archaefructus, which apparently existed 145 million years ago, also cast some doubt on the idea that angiosperms descended from gnetophytes or Bennettitales. Al­though a great deal of information has been obtained since the time of Dar­win, the origin of angiosperms is still something of a mystery.

Early Angiosperms, Monocots, and Eudicots

The oldest known angiosperms were a diverse group of plants called magnoliids. Some of these were herbs with simple flowers; others were woody plants with more complex flowers that were very similar to living magno­lias. Magnoliids, probably those with small, inconspicuous flowers, gave rise to the two main groups of angiosperms, monocots and eudicots, al­though a few angiosperm families, including the water lilies, may have evolved earlier.

These plants possessed a number of adaptations that were probably cru­cial to their eventual success. Their vascular tissues were particularly effi­cient, their embryos were enclosed in a protective seed coat, their leaves were resistant to desiccation, and they were pollinated by insects, rather than by the wind. This last feature made pollen transfer much more effi­cient and was almost certainly a key innovation in the diversification of the group, as coevolution of plants and their pollinators, particularly bees, gave rise to increasing specialization of both flowers and insects.

The orchid family contains some of the most specialized insect-pollinated flowers of all and has more species (at least 24,000) than any other plant fam­ily. Other groups of angiosperms re-evolved the ability to be pollinated by wind. One of these groups—the grasses—appeared about 50 million years ago, diversified rapidly, and became the dominant plants over many regions of the planet. They still thrive and are crucial to human well-being. Approximately 54 percent of the food eaten by people is provided by grain (seed) from cul­tivated varieties of just three grasses: rice, wheat, and corn.

References

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Iwatsuki, K., and P. H. Raven, eds. Evolution and Diversification of Land Plants. Berlin, Germany: Springer-Verlag, 1997.

Kenrick. P., and P. R. Crane. “The Origin and Early Evolution of Plants on Land.” Nature 389 (1997): 33-39.

Pryer, K. M., et al. “Horsetails and Ferns Are a Monophyletic Group and the Clos­est Living Relatives to Seed Plants.” Nature 409 (2001): 618-622.

Qiu, Y. L., et al. “The Earliest Angiosperms: Evidence from Mitochondrial, Plastid, and Nuclear Genomes.” Nature 402 (1999): 404-407.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

Stewart, W. N., and G. W. Rothwell. Paleobotany and the Evolution of Plants, 2nd ed. Cambridge: Cambridge University Press, 1993.

Sun, G., et al. “In Search of the First Flower: A Jurassic Angiosperm, Archaefructus, from Northeast China.” Science 282 (1998): 1692-1695.