Evolution of Plants | The Plant Life Cycle | Plant Adaptations to Life on Land
Bryophytes | Tracheophytes: The Vascular Plants | Vascular Plant Groups | The Psilophytes | The Lycophytes
The Sphenophyta | The Ferns | Learning Objectives | Terms | Review Questions | Links
The plant kingdom contains multicellular phototrophs that usually live on land. The earliest plant fossils are from terrestrial deposits, although some plants have since returned to the water. All plant cells have a cell wall containing the carbohydrate cellulose, and often have plastids in their cytoplasm. The plant life cycle has an alternation between haploid (gametophyte) and diploid (sporophyte) generations. There are more than 300,000 living species of plants known, as well as an extensive fossil record.
Plants divide into two groups: plants lacking lignin-impregnated conducting cells (the nonvascular plants) and those containing lignin-impregnated conducting cells (the vascular plants). Living groups of nonvascular plants include the bryophytes: liverworts, hornworts, and mosses. Vascular plants are the more common plants like pines, ferns, corn, and oaks. The phylogenetic relationships within the plant kingdom are shown in Figure 1.
Figure 1. Phylogenetic reconstruction of
the possible relationships between plant groups and their
green algal ancestor. Note this drawing proposes a green
algal group, the Charophytes, as possible ancestors for the
plants. Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates
(www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.
Fossil and biochemical evidence indicates plants are descended from multicellular green algae. Various green algal groups have been proposed for this ancestral type, with the Charophytes often being prominently mentioned. Cladistic studies support the inclusion of the Charophytes (including the taxonomic order Coleochaetales) as sister taxa to the land plants. Algae dominated the oceans of the precambrian time over 700 million years ago. Between 500 and 400 million years ago, some algae made the transition to land, becoming plants by developing a series of adaptations to help them survive out of the water.
Table 1. Photosynthetic pigments of algae and plants. Prokaryote groups are shown in red, protists in blue, and vascular plants in purple.
Cyanobacteria chlorophyll a, chlorphyll c, phycocyanin,
phycoerythrin Chloroxybacteria chlorophyll a, chlorphyll b Green Algae (Chlorophyta) chlorophyll a, chlorphyll b,
carotenoids Red Algae (Rhodophyta) chlorophyll a, phycocyanin, phycoerythrin,
phycobilins Brown Algae (Phaeophyta) chlorophyll a, chlorphyll c, fucoxanthin and other
carotenoids Golden-brown Algae (Chrysophyta) chlorophyll a, chlorphyll c, fucoxanthin and other
carotenoids Dinoflagellates (Pyrrhophyta) chlorophyll a, chlorphyll c, peridinin and
other carotenoids Vascular Plants chlorophyll a, chlorphyll b,
carotenoids
Vascular plants appeared by 350 million years ago, with forests soon following by 300 million years ago. Seed plants next evolved, with flowering plants appearing around 140 million years ago. This pattern is shown in Figure 2.
Figure 2. The fossil records of some
protist and plant groups. The width of the shaded space is
an indicator of the number of species. Image from Purves et
al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.
Plants have an alternation of generations: the diploid spore-producing plant (sporophyte) alternates with the haploid gamete-producing plant (gametophyte), as shown in Figure 3. Animal life cycles have meiosis followed immediately by gametogenesis. Gametes are produced directly by meiosis. Male gametes are sperm. Female gametes are eggs or ova.
Figure 3. Typical alternation of
generations life cycle, such as occur in some protistans and
plants. Image from Purves et al.,
Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.
The plant life cycle has mitosis occurring in spores, produced by meiosis, that germinate into the gametophyte phase. Gametophyte size ranges from three cells (in pollen) to several million (in a "lower plant" such as moss). Alternation of generations occurs in plants, where the sporophyte phase is succeeded by the gametophyte phase. The sporophyte phase produces spores by meiosis within a sporangium. The gametophyte phase produces gametes by mitosis within an antheridium (producing sperm) and/or archegonium (producing eggs). These different stages of the flowering plant life cycle are shown in Figure 4. Within the plant kingdom the dominance of phases varies. Nonvascular plants, the mosses and liverworts, have the gametophyte phase dominant. Vascular plants show a progression of increasing sporophyte dominance from the ferns and "fern allies" to angiosperms.
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Figure 4. The life cycle stages of a flowering plant. The above image is reduced from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Angiosperm/Angiosperm_life_cycle. Follow that link to view a larger image. |
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Plants have two further variations on their life cycles. Plants that produce bisexual gametophytes have those gametophytes germinate from isospores (iso=same) that are about all the same size. This state is referred to as homospory (sometimes referred to as isospory). A generalized homosporous plant life cycle is shown in Figure 5. Homosporous plants produce bisexual gametophytes. Ferns are a classic example of a homosporous plant.
Figure 5. A typical homosporous life
cycle. Note the production of a single type of bisexual
gametophyte that will eventually produce the antheridia
(sperm bearing structures) and archegonia (egg bearing
structures). Image from Purves et al., Life: The Science
of Biology, 4th Edition, by Sinauer Associates
(www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.
Plants that produce separate male and female gametophytes have those gametophytes germinate from (or within in the case of the more advanced plants) spores of different sizes (heterospores; hetero=different). The male gametophyte produces sperm, and is associated with smaller or microspores. The female gametophyte is associated with the larger or megaspores. Heterospory is considered by botanists as a significant step toward the development of the seed. A generalized heterosporous life cycle is shown in Figure 6.
Figure 6. Typical heterosporous life
cycle. Note the production of separate, unisexual male and
female gametophytes. Image from
Purves et al., Life: The Science of Biology, 4th
Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.
Organisms in water do not face many of the challenges that terrestrial creatures do. Water supports the organism, the moist surface of the creature is a superb surface for gas exchange, etc. For organisms to exist on land, a variety of challenges must be met.
Bryophytes are small, nonvascular plants that first evolved approximately 500 million years ago. The earliest land plants were most likely bryophytes. Bryophytes lack vascular tissue and have life cycles dominated by the gametophyte phase, as shown in Figure 7. The lack of conducting cells limits the size of the plants, generally keeping them under 5 inches high. Roots are absent in bryophytes, instead there are root-like structures known as rhizoids. Bryophytes include the hornworts, liverworts, and mosses.
Figure 7. The moss life cycle. The
haploid gametophyte phase is free-living and photosynthetic.
The diploid sporophyte grows from and is nourished by the
gametophyte. Images from Purves et
al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.
The vascular plants have specialized transporting cells xylem (for transporting water and mineral nutrients) and phloem (for transporting sugars from leaves to the rest of the plant). When we think of plants we invariably picture vascular plants. Vascular plants tend to be larger and more complex than bryophytes, and have a life cycle where the sporophyte is more prominent than the gametophyte. Vascular plants also demonstrate increased levels of organization by having organs and organ systems. The novel features oif the vascular plants are summarized in Table 2.
Table 2. Major evolutionary advances of the vascular plants.
|
Advance |
Green Algae |
Bryophytes |
Tracheophytes |
|
Development of the root-stem-leaf vascular system |
nonvascularized body (thallus) that may be variously shaped no levaes, shoots, or roots |
no vascular system leaflike structures are present, but lack any vascular tissue |
early vascular plants are naked, rootless vascularized stems later vascular plants develop vascularized leaves, then roots |
|
Reduction in the size of the gametophyte generation |
wide range of life cycles, some gametophyte dominant, others sporophyte dominant |
sporophyte generation dependant on gametophyte generation for food; gametophyte is free-living and photosynthetic |
progressive reduction in size and complexity of the gametoiphyte generation, leading to its complete dependance on the sporophyte for food in angiosperms, 3 celled male gametophyte and a (usually) 8 celled female gametophyte |
|
Development of seeds in some vascular plants |
no seeds |
no seeds |
seed plants retain the female gametophyte on the sporophyte |
|
Spores/Pollen |
spores for resisting environmental degradation |
Spores that germinate into the gametophyte generation |
Spores that germinate into the gametophyte generation or spores that have the gametophyte generation develop within themselves |
Vascular plants first developed during the Silurian Period, about 400 million years ago. The earliest vascular plants had no roots, leaves, fruits, or flowers, and reproduced by producing spores.
Cooksonia, shown in Figure 8, is a typical early vascular plant. It was less than 15 cm tall, with stems that dichotomously branched. Dichotomous branching (where the stem divides into two ewqual branches) appears a primitive or ancestral trait in vascular plants. Some branches terminated in sporangia that produced a single size of spore.
Many scientists now consider "Cooksonia" an evolutionary grade rather than a true monophyletic taxon. Their main argument is that not all stems of Cooksonia-type plants have vascular tissue. The evolutionary situation of a grade would have some members of the group having the trait, others not. The shapes of sporangia on various specimens of Cooksonia also vary considerably.
Figure 8. Cooksonia fossil
specimen (L) and reconstruction (R).
Both Images from http://www.ucmp.berkeley.edu.
Rhynia, shown in Figure 9, is another early vascular plant. Like Cooksonia, it lacked leaves and roots. One of the species formerly assigned to this genus, R. major, has since been reclassified as Aglaophyton major. Some paleobotanists consider A. major (Figure 10) a bryophyte, however, it does have a separate free-living sporophyte that is more prominent than the sporophyte, but appears to lack lignified conducting cells. The remaining species, R. gwynne-vaughanii is an undoubted vascular plant.
Figure 9. Rhynia gwynne-vaughanii
(L) stem cross section from the Rhynie Chert in
Scotland. Image cropped and reduced
from http://www.uni-muenster.de/GeoPalaeontologie/Palaeo/Palbot/rhynie.html.
(R) Reconstruction of the plant, from http://www.ucmp.berkeley.edu/IB181/VPL/Elp/Elp2.html.
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Figure 10. Reconstruction of Aglaophyton major (A-C) and Lyonophyton rhyniensis, another Rhynie Chert plant thought to be the gametophyte of Aglaophyton. Image from the UCMP Berkeley website. |
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Devonian plant lines included the trimerophytes and zosterophyllophytes, which have been interpreted as related to ferns and lycophytes.
The Division Psilophyta consists of Psilotum nudum (the whisk fern, shown in Figure 11), a living plant that resembling what paleobotanists believe Cooksonia to have been: a naked, photosynthetic stem bearing sporangia. Also in the group is Tmesipteris, which resembles Psilotum except for its possession of smallo vascularized leaves arising on opposite sides of the stem. However, most paleobotanists doubt that Psilotum is a direct descendant of Cooksonia. Molecular studies suggest an affiliation with ferns for Psilotum. Psilotum also has three fused sporangia, termed a synangium, located on the sides of the stems (instead of the tips of stems as in Cooksonia).
Figure 11. Psilotum nudum from
Hawaii. Note the synangia, the
roundish structures on the side of the green stems. Image
from http://www.botany.hawaii.edu/faculty/carr/images/psi_nud_mid.jpg.
The next group, the Division Lycophyta, have their sporangia organized into strobili (singular: strobilus). A strobilus is a series of sporangia and modified leaves closely grouped on a stem tip. The leaves in strobili are soft and fleshy as opposed to the hard, modified leaves in cones.
Leaves that contained vascular tissue are another major advance for this group. The presumed evolutionary pathway for the leaf is shown in Figure 12. The leaves in lycophytes, both living and fossil forms, are known as microphylls. This term does not imply any size constraint, but rather refers to the absence of a leaf gap in the vascular supply of the stem at the point where the leaf vascular trace departs. Ferns and other plants have megaphylls, leaves that produce this leaf gap.
Figure 12. Proposed steps in the
evolution of the microphyll leaf. Note that microphylls do
not leave a leaf gap in the stem's vascular cylinder. If we
wanted to place Psilotum-like plants on the left top,
we would have Lycopodium-like plants on the right
top. Image from Purves et al.,
Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.
Today there are fewer genera of lycophytes than during the group's heyday, the Paleozoic Era. Major living lycophytes include Lycopodium (commonly called the club moss [shown in Figure 13], although it is NOT a moss), Isoetes, and Selaginella (the so-called resurrection plant). Lycopodium produces isospores that germinate in the soil and produce a bisexual gametophyte. These spores are all approximately the same size. Selaginella and Isoetes are heterosporous, and thus produce two sizes of spores: small spores (termed microspores) that germinate to produce the male gametophyte; and larger spores (megaspores) that germinate to produce the female gametophyte. The production of two sizes of spores, and also making separate unisexual gametophytes, is thought an important step toward the seed. Modern lycophytes are small, herbaceous plants. Many of the prominent fossil members of this group produced large amounts of wood and were significant trees in the Carboniferous-aged coal swamps.
Figure 13.
Lycopodium venustulum, from from
Hawaii. Note the strobili on the tips
of the stems, as well as the stems covered with numerous
spirally arranged microphylls. Image from http://www.botany.hawaii.edu/faculty/carr/images/lyc_ven.jpg.
Selaginella is a heterosporous member of the lycophytes. Some species of this genus are able to withstand drying out by going dormant until they are rehydrated. For this reason these forms of the genus are commonly called resurrection plants. An example of this is shown in Figure 14.
Figure 14. Selaginella
lepidophylla. The top part of the image shows the dried
plant, the middle part shows the resurrected or rehydrated
plant. Image from http://www.orchideeitalia.it/selaginella.jpg.
Baragwanathia, shown in Figure 15, is an undoubted lycophyte from the middle Silurian deposits of Australia. It has microphyllous leaves spirally attached to the stem, and sporangia clustered in some areas of the plant, although not in terminal strobili as in modern lycophytes.
Figure 15. Left: Reconstruction of
Baragwanathia longifolia, from the middle Silurian of
Australia. Image from
http://www.ucmp.berkeley.edu/IB181/VPL/Lyco/Lyco1.html;
Right: Reconstruction of
Drepanophycus , a middle Devonian lycophyte. Note the
numerous microphyll leaves, placement of sporangia on the
upper surface of the sporophylls, and stem anatomy that are
all consistent with modern lycophytes.
Image from http://www.ucmp.berkeley.edu/IB181/VPL/Lyco/Lyco2.html.
Drepanophycus is a middle Devonian lycophyte from the Northern Hemisphere, also shown in Figure 15. Its features are very similar to modern lycophytes.
The Lycophytes became significant elements of the world's flora during the Carboniferous time (the Mississippian and Pennsylvanian are terms used for this time span in the United States). These non-seed plants evolved into trees placed in the fossil genera Lepidodendron and Sigillaria, with heights reaching up to 40 meters and 20-30 meters respectively. Lepidodendron stems are composed of less wood (secondary xylem) that usually is found in gymnosperm and angiosperm trees.
We know much about the anatomy of these coal-age lycopods because of an odd type of preservation known as a coal ball. Coal balls can be peeled and the plants that are anatomically preserved within them laboriously studied to learn the details of cell structure of these coal age plants. Additionally, we have some exceptional petrifactions and compressions that reveal different layers of the plants' structure. Estimates place the bulk, up to 70%, of coal material as being derived from lycophytes.
Lepidodendron, pictured in Figures 16 and 17, was a heterosporous lycophyte tree common in coal swamps of the Carboniferous time. As with many large plant fossils, one rarely if ever finds the entire tree preserved intact. Consequently there are a number of fossil plant genera that are "organ taxa" and represent only the leaves (such as Lepidophylloides), reproductive structures (Lepidostrobus), stem (Lepidodendron), spores (Lycospora), and roots (Stigmaria). Lepidodendron had leaves borne spirally on branches that dichotomously forked, with roots also arising spirally from the stigmarian axes, and both small (microspores) and large (megaspores) formed in strobili (a loose type of soft cone). Lepidodendron may have attained heigths of nearly 40 meters, with trunks nearly 2 meters in diameter. The trees branched extensively and produced a large number of leaves. When these leaves fell from the branches, they left behind them the leaf scars characteristic of the genus.
Figure 16. External stem features typical
of arborescent lycopods, collectively called lepidodendrids,
based on the diamond-shaped "snakeskin" type pattern
produced by the helically arranged leaf cushions. On the
left is a lower magnification view of this type of pattern,
showing the general features of many of these trees. Each
leaf abscissed, so that if you are looking, as you are here,
at the outside of the stem, you can see a characteristic
appearance. On the right is a higher magnification photo
showing details of leaf cushions. Each diamond shaped
cushion has a smaller central area called the leaf base
where the leaf attached. In the center of the leaf base you
can see the leaf trace, or vein to the leaf. The vertical
stripe running down each cushion is probably the result of
increased girth from secondary cortical growth inside the
stem. Images and text from
http://lsvl.la.asu.edu/plb407/kpigg/lepidodendrid.htm,
used with permission of K.B. Pigg, Arizona State
University. Figure 17. Top: Cross section through a
branch (approximately an inch in diameter) of a large
lepidodendrid tree. In the very center is a pith, surrounded
by primary xylem and a small fringe of secondary xylem
[wood, MJF]. Then there is black gunk and an open
white area. Phloem and innermost cortical tissues are
typically not well preserved, and this black gunk and white
areas probably represent their positions in the branch. The
outermost part of the stem is gone. Images and text from
http://lsvl.la.asu.edu/plb407/kpigg/lepidostemxs.htm,
used with permission of K.B. Pigg, Arizona State University.
Bottom: Reconstructed diorama of Carboniferous forest
scene. Note the ferns and sphenopsids growing around
the fallen Lepidodendron trunk, and a large calamite
tree in the right foreground. Image and text from
http://seaborg.nmu.edu/earth/carbonif/car01b.html.
Sigillaria was another arborescent lycopod, and is also common in coal-age deposits. In contrast to the spirally borne leaves of Lepidodendron, Sigillaria had leaved arranged in vertical rows along the stem.
The Division Sphenophyta contains once dominant plants (both arborescent as well as herbaceous) in Paleozoic forests, equisetophytes are today relegated to minor roles as herbaceous plants. Today only a single genus, Equisetum, survives. The group is defined by their jointed stems, with many leaves being produced at a node, production of isospores in cones borne at the tips of stems, and spores bearing elaters (devices to aid in spore dispersal). Sporophyte features are seen in Figure 18. The gametophyte is small, bisexual, photosynthetic, and free-living. Silica concentrated in the stems give this group one of their common names: scouring rushes. These plants were reportedly used by American pioneers to scour the pots and pans. The fossil members of this group are often encountered in coal deposits of Carboniferous age in North America and Europe.
Figure 18. Top: A branched species of
Equisetum from Hawaii. Note that the branches arise
from the same node/area along the stem. Image from
http://www.botany.hawaii.edu/faculty/carr/images/equ_sp.jpg.
Bottom: Closeup view of the cone of Equisetum hyemale
from Hawaii. Note the small leaves at
the joints near the lettering in the picture.
Image from http://www.botany.hawaii.edu/faculty/carr/images/equ_sp_cu.jpg.
Ferns reproduce by spores from which the free-living bisexual gametophyte generation develops. There are 12,000 species of ferns today, placed in the Division Pteridophyta. The fossil history of ferns shows them to have been a dominant plant group during the Paleozoic Era. Most ferns have pinnate leaves, exhibiting small leaflets on a frond, as shown in Figure 19. Ferns have megaphyllous leaves, which cause a leaf gap in the vascular cylinder of the stem/rhizome, as shown in Figure 20. The first ferns also appear by the end of the Devonian. Some anatomical similarities suggest that ferns and sphenophytes may have shared a common ancestor within the trimerophytes.
Figure 19. Three genera of the fern
family Gleicheniaceae from Hawaii: Upper left: Sticheris
owhyhensis, lower left: Diploterygium pinnatum
(uluhe lau nui), right: Dicranopteris linearis
(uluhe). Image from http://www.botany.hawaii.edu/faculty/carr/gleicheni.htm.
Figure 20. Formation of leaf gaps by a
megaphyllous leaf. Most plants above the ferns have
megaphyllous leaves. Image from Purves et al., Life: The
Science of Biology, 4th Edition, by Sinauer Associates
(www.sinauer.com)
and WH Freeman (www.whfreeman.com),
used with permission.
The fern gametophyte has both sexes present and is referred to as a prothallium. Prothallia develop from spores shed from the underside of the sporophyte leaves, shown in Figure 21. Once fertilization occurs, the next generation sporophyte develops from the egg located in the prothallium.
Figure 21. Sporangia on the underside of
Polypodium pellucidum from Hawaii.
Image from http://www.botany.hawaii.edu/faculty/carr/images/pol_pel.jpg.
Composite of 4 segmented diagrams of the fern life cycle. Note: to view this in its proper sequence you will need to open your browsert window as wide as possible. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
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Charophytes |
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homospory and heterospory |
isospores |
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strobilus |
All text contents ©1995, 1999, 2000, 2001, 2003, 2004, by
M.J. Farabee. Use of the text for educational purposes is
encouraged.
Email: mj.farabee@emcmail.maricopa.edu
Last modified:
Tuesday May 18 2010
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