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LAND PLANTS

THE MOVE TO LAND

Green algae had colonized both marine and fresh waters (as depicted with the sea lettuce and freshwater algae below) long before land plants evolved.

sea lettuce ALGAE
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LAND PLANTS

      The move to land presented a new challenge for algae in that water no longer bathed their cells and had to be transported throughout the individual.  As can be seen by the green algae on the statue in the photo below, algae do survive on land.

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The following photo shows not only algae growing on a rock, but also a land plant which descended from green algal ancestors.
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All land plants have a waxy cuticle to prevent water loss and support structures to compensate for the loss of buoyancy.  Many primitive plants are dependent on water for their reproduction because their sperm retain the primitive condition of possessing a flagella and needing to swim to the ova.  Unlike most algae, land plants have a multicellular jacket for protecting the reproductive cells.

 

BRYOPHYTES: liverworts, hornworts, mosses; 23,000 modern species

Spores of bryophytic plants represent the first known land plants from the Middle Ordovician. Bryophytic and vascular plants are found together in an Early Devonian hot spring environment (Park, 2007).

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      Bryophytes are the most primitive land plants alive today.  They that lack the vascular tissue that higher plants use to transport water from the roots to the leaves.  Since bryophytes cannot transport water well, they cannot grow to be very tall.  Genetic analysis indicates that they are the earliest land plants, a sister group to all other plants. Liverworts represent a more primitive group of plants than mosses. Analysis of the moss genome shows the acquisition of new genes which adapted them for land and the loss of algal genes for aquatic adaptations (Rensing, 2008).

A group of fossils called “nematophytes” are thought to represent the fossils of some of the most primitive land plants dating from the Cambrian to the Devonian.  At least some of these nematophyte fossils are comparable to structures in liverworts, the most primitive modern plants (Graham, 2004).A set of enigmatic spores are known from the Middle Cambrian through the Devonian which do not match modern or fossil algae. Although they may represent an extinct group of acritarchs, their structure is consistent with classification with liverworts. These fossils predate fossils of plants (trace fossils by 50 million years; whole plant fossils by 90 million years) ( Taylor, 2008).

A number of spores are known beginning in the Cambrian through the Devonian which are difficult to classify. The microscopic structure is more similar to that of primitive plants such as liverworts than to algae, acritarchs, or other groups ( Taylor, 2008).

 

VASCULAR PLANTS

     An important advance in the conquest of land by plants was the evolution of vascular tissue which allowed water to be transported from the ground to tissues high above the ground.  The first vascular plants lacked leaves and consisted of photosynthetic stems.

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Primitive Vascular Plants:

      Although there are only about 1000 modern species of primitive vascular plants,  their fossil relatives colonized terrestrial environments and were the ancestors of more advanced plants.  The first land plants are known from the Siluran. They lacked leaves and performed photosynthesis in their stems.  Leaves did not become common until about fifty million years after the origin of vascular plants.  In the early Devonian, the plant Eophyllophyton bellum possessed small megaphylls.  It is possible that the high concentrations of carbon dioxide in the Paleozoic inhibited leaf development (Osborne, 2004). The images below depict modern vascular plants which lack leaves and perform photosynthesis with their stems.

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Primitive modern groups of modern plants which possess leaves are represented by whisk ferns, horsetails, and clubmosses.

horsetail Although these plants today are fairly small and play a minor ecological role, some of their ancestors had reached heights of 9 meters by the end of the Devonian. Horsetail relatives reached 20-30 meters in height; one clubmoss relative reached 50 meters in height and 2 meters in diameter.  These plants composed the first forests and the remains of these plants and ferns (primarily ferns) formed coal deposits.  Most became extinct at the end of the Carboniferous, in part because of the development of drier climates. 
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 Below are photos of modern clubmosses.

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FERNS
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    Ferns were a dominant type of plant in the past.  Long before there were grasslands (grass did not evolve until fairly recently) there existed vast fern prairies.  Some ferns grew to be tree-sized, a few of which still survive today.  Although most ferns reproduce using spores, one group evolved seeds although they were not ancestral to other seed plants.  Fern leaves possess a highly branched vascular system.  Some ferns in New Zealand have leaves that are 6-9 m long and probably the most complex leaves in the plant kingdom. 

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 Many ancient ferns grew to be tree sized. Some tree ferns still survive today such as those South American tree ferns depicted below.

tree ferntree fern

SEED PLANTS

      "Seeds" are another adaptation in which plant embryos are surrounded by a desiccation-resistant coat to protect them from water loss.  The earliest seeds are known from the Late Devonian and these early seeds are the most primitive known (Gillespie, 1981).  There are two modern groups of seed plants: gymnosperms and angiosperms.  Gymnosperms are known from the end of the Paleozoic in the Permian Period. 

 

GYMNOSPERMS

gymnosperms

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     Gymnosperms were the dominant trees on the planet during most of the Mesozoic Era.  They declined sharply after the rapid spread of flowering plants.  There are several groups of conifers today that are rare and compose only about 100 modern species, including cycads and ginkgos.

       Cycads are now approaching extinction but they were abundant in the Late Paleozoic and Mesozoic.  Their sperm still retain the primitive condition of needing to swim within the pollen tube to reach the ova. Cycads originated in the Late Paleozoic and by the Triassic they had spread to Europe, North America, and Antarctica (Hermsen, 2009).

cycad

    

The fossil relatives of the modern tree Ginkgo, the Ginkgoales, are divided into 5 to 6 families. They first appeared on southern continents in the Permian (or even the Late Carboniferous) but spread throughout the world to reach their peak distribution in the Triassic. Only one species survives today (Zhou, 2009). They were thought to be extinct until wild trees were found in China.  There are separate male and female trees.  Experienced landscapers know that they should only plant one gender or the other in an area because of the strong odor produced by the seeds if these trees are able to reproduce.

       Other Gymnosperms include the group Gnetophyta. One of the gnetophytes, Welwitschia, produces only 2 enormous leaves and obtains almost all of its water from fog.

CONIFERS

cedar pine
      The best known gymnosperms are the conifers, a group which consists of about 550 modern species of pines, spruces, cedars, firs, hemlocks, junipers, and larches.  Some Permian gymnosperms resemble conifers in their needles but their reproductive structures were too simple to be classified as conifers.  The conifers (which first appear in the Early Permian) are either descended directly from a Late Pennsylvanian to Early Permian group called the cordaites or share a common ancestor with them.   The first modern families of conifers are known by the Triassic and all modern families of conifers are known by the Cretaceous.  The most primitive conifers today (groups such as Taxodiaceae and Cupressaceae which include cypress tress) are more closely related to fossil forms than any modern group (Miller, 1999b).

Today, conifers are successful in cold environments.  Their needlelike leaves have less surface area which protects them from water loss in areas of little precipitation and extreme cold (water is hard to obtain when the ground is frozen).  Conifers also include one of the tallest plants, the sequoia, which can reach 4,000 years of age and grow to 340 ft. tall and 25 feet wide.   Conifers also include the oldest organisms on earth, given that living bristlecone pines have reached 4600 years of age.

 

 

ANGIOSPERMS

     The plants which are alive to day are unlike the plants of the fossil past.  Angiosperms, the flowering plants, are the most dominant group of plants alive today and compose the vast majority of both modern species and individuals of plants.  The first flowering plants evolved in the mid-Mesozoic and the group did not become dominant until the end of the Mesozoic.  The majority of the groups of modern flowering plants did not evolve until the Age of Mammals in the Cenozoic.  Grasses, for example, dominate in a large percentage of terrestrial ecosystems today, but they did not exist at all during almost 400 million years of the history of land plants.  Angiosperms will be discussed in more detail with the material at the end of the Mesozoic.