Seedless Vascular Plants
Introduction:
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. Plants (kingdom Plantae) are autotrophs; they make their own organic nutrients. The term "organic" refers to compounds that contain carbon. Organic nutrients such as sugars are made by photosynthesis. Plants are adapted to living on land. For example, the above-ground parts of most plants are covered by a waxy layer called a cuticle to prevent water loss. Aquatic plants are secondarily adapted to living in water. Some evidence that suggests that plants evolved from the green algae is: They both use chlorophyll a, chlorophyll b, and carotenoid pigments during photosynthesis. The primary food reserve of both is starch.they both have cellulose cell walls. Genetic and morphological evidence indicates that plants evolved from a group of green algae called charophyceans. Many charophyceans inhabit shallow freshwater environments. Natural selection may have favored individuals capable of surviving occasional drying in these environments and this gave rise to land plants. These traits occur in plants but not charophyceans. Some evolved independently in other algae. Apical meristems Alternation of generations Spores with protective walls Spores produced in sporangia Gametes are produced in multicellular structures called gametangia; Antheridia produce sperm; Archegonia produce eggs
1
Seedless Vascular Plants Multicellular dependent embryos Many have a cuticle that waterproofs and offers some protection
2
Seedless Vascular Plants
EVOLUTION:
Evolution and Phylogenetic Relationship: On The evolutionary timetable, vascular seedless plants developed quite early. A number of distinct features set this group of plants apart from its predecessors, allowing the formation of a new taxonomic group known as the Vascular Seedless plants. Cook Sonia, the first vascular land plant, originated in the late Silurian period, some 420 million years ago, when little competition existed. The main evolutionary development in these plants was vascular tissue, xylem and phloem, which conducted water and nutrients, respectively. Vascular tissue, as shown on the clad gram to the left, is the single most important of the evolutionary developments that create this taxonomic category. A major developmental difference between ferns and the primitive mosses that came before is that ferns have a dominant sporophyte generation. The sporophyte is the 2N stage of the lifecycle, shown in the diagram to the lower right. Dominance of the diploid stage is attributed to exposure to UV radiation with the terrestrial expansion of plants; having two copies of the genetic material allows for greater success of the species. As well as a dominant sporophyte, ferns also demonstrate a trend toward smaller gametophytes, which are rarely observed to grow larger than 6mm in size. In addition to the sporophyte simply dominating the life cycle, it is also far more complex in vascular seedless plants than in bryophytes, indicating a connection with the development of vascular tissue and complex plant structures. In short, vascular seedless plants represent the evolutionary "jump" from a dominant gametophyte to a dominant sporophyte. Despite great differences, a large number of similarities with their predecessors, the nonvascular plants such as mosses, do exist. Bryophytes (mosses, liverworts, and hornworts) and vascular seedless plants both exhibit a diploid and haploid segment in their life cycle; the difference comes from which segment is dominant. In contrast, seeded plants are such that the diploid (sporophyte) stage nearly entirely dominates the life cycle, leaving only pollen as the male gametophyte and some egg cells as the female. This relationship provides the evolutionary order or development, suggesting that the order was in fact bryophytes, vascular seedless plants, and then vascular seeded plants. Structurally, there exists a great deal of evolutionary development between the bryophytes and vascular seedless plants. In moving from bryophytes to vascular plants, "multicellular gametangia, diplobiontic (alteration of generations) life cycle,
3
Seedless Vascular Plants stomates, and sporangium," have developed. In the image to the right, we see the underside of the fern frond showing ripe sporangia. These sporangia will release the spores that will form the gametophyte generation of the fern plant, the prothallum. The prothallum will then produce the sperm and egg that will fuse to form the sporophyte generation.
4
Seedless Vascular Plants
Figure (2) phylognetic relationship
Figur(3)Alternation of generation Figure (3) Alternation of generation
5
Seedless Vascular Plants
Energy/Mode of Nutrition:
Vascular seedless plants, like all plants, use sunlight to provide their energy. By carrying out photosynthesis, the plant is able to be self-sufficient, storing energy harnessed from the sun by carbon fixation. The energy captured in the photosynthetic process is used to produce a chemical called adenosine triphosphate, or ATP, as well as NADPH, which are used by the organism to assemble organic molecules such as sugars to use as food. The ATP molecule, as shown in figure (5), works on the principle of electromagnetic repulsion. To assemble an ATP molecule requires the positioning of two negatively charged particles within a close proximity, creating almost a "spring tension" from the repulsion force, thus storing the energy for a longer period in the form of chemical potential energy. Photosynthesis, the process by which plants obtain energy, occurs in two main steps; the dark and light reactions. In the light reactions, sunlight, water, ADP, and NADP+ go in, and ATP, NADPH, and oxygen are released. The NADPH and ATP are then used in the Calvin Cycle, along with carbon dioxide gas, to produce organic molecules (food molecules, i.e. sugars).
6
Seedless Vascular Plants
Figure (4) Energy/Mode of nutrition
Figure (5) molecules of ATP
7
Seedless Vascular Plants
World Distribution:
Vascular seedless plants are found throughout the globe. They generally tend to located in tropical areas, but this is not the case in all situations. Vascular seedless plants, having the primitive ability to transport water and nutrients within the plant, still require a generally moist environment to sustain a substantial population. There are currently about 12,000 living species, each specialized to a particular biological niche. Although vascular seedless plants can be found throughout the world, about 75% of the species are found in tropical regions. The remaining varieties are located in the temperate regions indicated on the map as shown in figure (6). With little or no vascular seedless plants surviving in the dry, cold, or Polar Regions.
8
Seedless Vascular Plants
Figure (6) World Distribution
9
Seedless Vascular Plants
Habitat Use:
Since vascular seedless plants possess flagellated sperm produced by their gametophytes, they are suited for life in many environments. Moisture and proximity, however, are a factor in determining whether or not reproduction can successfully occur. Some whisk ferns (Psilophyta) form parasitic associations with fungi, who provide the gametophytes with nutrients for development. Ferns (Pterophyta) employ primitive root structures known as rhizomes which spread sideways from the base of the plant into the soil, absorbing nutrients and water. Rhizomes are not true roots, however, because they lack vascular tissue, simply absorbing water and nutrients through diffusion. For this reasons, Ferns tend to locate themselves in shadier areas with moist soil.
Division of vascular seedless plants:
These three divisions along with the Pterophyta (ferns) are collectively know as the "seedless vascular plants". They have developed a vascular structure that permits the transport of water and nutrients but they do not reproduce by seeds Uptake and distribution of water became possible when plants developed roots and xylem. The movement of food and nutrients required the differentiation of phloem. In the more primitive plants the conducting tissues are arranged in a cylinder with phloem surrounding xylem. This is a protostele and is an arrangement that flowering plants have retained in their roots. It allows for vascular continuity to be maintained between the root and the shoot or the shoot and any structures arising from it.
Psilophyta:
If we could have wandered about on earth in the Devonian period the only conspicuous land plants would have been something like the whisk fern, Psilotum. It has virtually no leaves and no roots. It has underground stems from which the above ground parts branch off. Interestingly, the whisk ferns have developed mycorrhizal associations; perhaps they are necessary in the absence of true roots. Psilotum grows wild in Florida woodlands and all members of the division, Psilophyta, today are tropical plants. With the development of proper vascular systems it became possible to supply water to larger leaves: megaphylls were developed, perhaps by the coalescence of smaller microphylls. So, contrary to expectation, compound leaves may have developed before simple leaves.
10
Seedless Vascular Plants The other thing that becomes apparent as you study the life cycle is that the diploid sporophyte became dominant. Most of the seedless vascular plants are homosporous, the spores grow into a gametophyte that is usually bisexual with both antheridia and archegonia. The development of anatomy, physiology and reproductive mechanisms adapted to life on the land permitted an explosion of three divisions of plant life which survive to this day although they have been overshadowed by the success of the flowering plants. It probably took the animals time to catch up with the opportunity provided by this new development and perhaps that is why the Upper Carboniferous or Pennsylvanian period was the highpoint of the earth's photosynthetic productivity. We are using the remains of that productivity today. The coal measure forests were dominated by tree ferns, horsetails and clubmosses. There are no truly woody members of these divisions alive today. (The stems of living "tree ferns" are supported by a dense mass of roots). With the development of proper vascular systems it became possible to supply water to larger leaves: megaphylls were developed, perhaps by the coalescence of smaller microphylls. So, contrary to expectation, compound leaves may have developed before simple leaves. The other thing that becomes apparent as you study the life cycle is that the diploid sporophyte became dominant. Most of the seedless vascular plants are homosporous, the spores grow into a gametophyte that is usually bisexual with both antheridia and archegonia. The development of anatomy, physiology and reproductive mechanisms adapted to life on the land permitted an explosion of three divisions of plant life which survive to this day although they have been overshadowed by the success of the flowering plants. It probably took the animals time to catch up with the opportunity provided by this new development and perhaps that is why the Upper Carboniferous or Pennsylvanian period was the highpoint of the earth's photosynthetic productivity. We are using the remains of that productivity today. The coal measure forests were dominated by tree ferns, horsetails and clubmosses. There are no truly woody members of these divisions alive today. (The stems of living "tree ferns" are supported by a dense mass of roots).
11
Seedless Vascular Plants
Figure (7) life cycle of pterophytes
12
Seedless Vascular Plants
Lycophyta:
The Lycophyta include Lycopodium and Selaginella. Lycopodium species can be found in Ohio, and throughout the world - in a wide range of habitats but usually growing beneath other plants. They have rhizomes from which arise adventitious roots; the stems are clothed in microphylls. Sporangia may be spread all over the plant or they may be clustered in a cone-like strobilus. Lycopodium is used by florists to some extent as a foliage plant. Running pine is sometimes classified as Diphasiastrum rather than Lycopodium and can be used as a ground cover in the landscape. Selaginella species are mostly tropical; they are similar in structure to Lycopodium but rather more delicate and they usually grow in damp places. A big difference from Lycopodium is that they are heterosporous. The strobilus contains micro- and mega-sporophylls. They find some use as an indoor "ground-cover" plant in interiorscapes.
13
Seedless Vascular Plants
Figure (8) Running pine or Lycopodium complanatum, an Ohio native
. Figure (9) An ornamental Lycopodium
14
Seedless Vascular Plants
Sphenophyta:
The Sphenophyta are represented today by one genus, Equisetum the horsetails or scouring rushes. These are widely distributed, usually growing in marshes and waterlogged soil. The plant is essentially stem, it has a rhizome which puts out adventitious roots. The leaves are a whorl of non-photosynthetic scales at each node. Some species produce lots of feathery branches. Their cell walls contain silica which makes the stems coarse textured, and led to their use as a natural scouring pad for cook ware. Spores are produced in strobili and although the plant is homosporous the gametophytes are unisexual. Horsetails can be aggressive and troublesome weeds, although they are sold for water gardens and have some decorative value if they can be kept within bounds.
15
Seedless Vascular Plants
Figure (10) Equisetum palustre vegetative fronds
Figure (11) same with stobili
16
Seedless Vascular Plants
Ecological Roles:
Vascular seedless plants, contrary to common belief, play an important ecological role in the biosphere. Firstly, by consuming carbon dioxide gas, through carbon fixation and photosynthesis, plants allow the planet to sustain aerobic organisms such as humans. The carbon cycle is shown in the image below. In this cycle, carbon dioxide released from human factories, as well as animal respiration and decomposition in the soil is consumed by plants (represented by trees), and converted back to oxygen for future consumption by another organism. In a larger respect, vascular seedless plants are autotrophic primary producers. They obtain energy from the sun as described in the "Energy" section of the site, converting this energy into food for the plant as well as any "first-order consumers." Since plants (for the most part) are not predators, they contain no means by which to fight back against an enemy organism. Although some plants have evolved defense mechanisms such as toxins and foul tastes, plants do not directly challenge any predators, and serve in the overall biosphere as primary producers.
Impacts on Human Society:
Vascular seedless plants are often overlooked in their importance to human society. Often times, these plants serve as merely decorative landscaping or erosion preventing flora in moist regions, taking advantage of the fact that vascular seedless plants thrive in moist shady areas. The most important of uses for pterophytes include aesthetics, food, and medicinal value. In India, the species Azolla pinnata is used as rice fertilizer and feed for chickens. Countless other species serve localized purposes such as cooking seasonings, water repellants, dye-production, as well as the production of mats, baskets, and packing materials. In one special case, the species Equisetum arvense Linn. demonstrates an affinity for gold, reffered to by the locals as a "gold indicator." Medicinally, there are a number of species that serve to cure nasal polyps, kidney infections, and even the common headache. Numerous third world illnesses of the lungs and kidney are also cured by a species Lycopodium clavatum. In third world nations, where modern medicine is largely out of reach, these tried and true practices of the past are proving that vascular seedless plants have many medicinal benefits that are just beginning to be discovered. The ones mentioned are merely those documented to work at the present time, it is expected that hundreds more are yet to be confirmed.
17
Seedless Vascular Plants
18