Box Classification of the tracheophytes

Tracheophytes are the vascular land plants, and include modern ferns, horsetails, conifers and flowering plants, as well as numerous extinct groups. The basal groups are distinguished in terms of branching patterns and sporangial morphology.

Division Tracheophyta

Class RHYNIOPSIDA

• Simple vascular plants with dichotomously branching stems and terminal sporangia

• Mid Silurian to Early Devonian

Class Lycophyta

• Small to large plants with lateral sporangia and (usually) small leaves

• Late Silurian to Recent

Class EQUISETOPSIDA

• Horsetails; vertical stems with jointed structure and a whorl of fused leaves at the nodes; sporangia grouped in cones

• Late Devonian to Recent

Class FILICOPSIDA

• Ferns; dichotomously-branching flat leaves which uncurl as they develop; sporangia are grouped in clusters usually on the underside of leaves

• Mid Devonian to Recent

Class PROGYMNOSPERMOPSIDA

• Plants with gymnosperm-like wood but free sporing (fern-like) reproduction. Larger members include early trees such as Archaeopteris from the Late Devonian

Class SPERMATOPSIDA Subclass GYMNOSPERMAE Order MEDULLOSALES

• Primitive seed plants with large pollen grains and unusual stem anatomy

• Mississippian to Permian Order BENNETTITALES

• Bushy to tree-like plants with sterile scales between the seeds; frond-like leaves; flower-like cones with enclosing structures that surround ovules and pollen sacs

• Late Triassic to Late Cretaceous Order CYCADALES

• Bushy to tree-like plants with leaf traces that girdle the stem; frond-like leaves; seeds attach to a megasporophyll stalk below a leaf-like structure

• Mississippian-Recent Order GINKGO ALES

• Trees with seed-bearing shoots and with fan-shaped or more divided leaves

• Late Triassic to Recent Order CONIFERALES

• Conifers; trees with resin canals, and needle- or scale-like leaves

• Mississippian to Recent Order GNETALES

• Leaves opposite each other, and vessels in the wood; male and female cones are flower-like

• Late Triassic to Recent Subclass ANGIOSPERMAE

• Ovules are enclosed in carpels, within a flower, and fertilization is double (involving two sperm nuclei)

• Early Cretaceous to Recent relative abundance of each group at different points in plant history. The phylogeny highlights the three major bursts of land plant evolution, the first in the Devonian (rhyniop-sids, zosterophylls and other basal vascular plants), in the Carboniferous and Permian (lycopsids, ferns, horsetails, seed ferns) and in the Cretaceous (angiosperms).

Adapting to life on land_

The following are the key adaptations of vascular plants for life on land:

1 Spores or seeds with durable walls to resist desiccation.

2 Surface cuticle over leaves and stems to prevent desiccation.

3 Stomata (singular, stoma), or controllable openings, to allow gas exchange through the low-permeability cuticle.

4 A vascular conducting system to pass fluids through the plant.

5 The lignification of tracheids to resist collapse. The cellulose cell walls of the conducting tubes, or tracheids, of vascular plants are invested with lignin, the tough polymer that makes up all woody tissues, providing strength and waterproofing.

These key adaptations relate to the problems a water plant must overcome when moving onto land. In water, a plant may absorb nutrients and water all over its surface, but on land all such materials must be drawn from the ground, and passed round the tissues internally. Land plants typically have specialized roots that draw moisture and nutrient ions from the soil, which are passed through water-conducting systems that connect all cells. The system is driven by transpiration, a process powered by the evaporation of water from leaves and stems. As water passes out of aerial parts of the plant, fluids are drawn up into the water-conducting system hydrostatically.

Water loss is a second key problem for plants on land. Whereas in water fluids may pass freely in and out of a plant, a land plant must be covered with an impermeable covering - the waxy cuticle. Gaseous exchange and water transport are then facilitated in many land plants by specialized openings, the stomata (singular, stoma), often located on the underside of leaves. Typically, stomata open and close depending on carbon dioxide concentration, light intensity and water stress.

The third problem of life on land is support. Water plants simply float, and the water renders them neutrally buoyant. Most land plants, even small ones, stand erect in order to maximize their uptake of sunlight for photosynthesis, and this requires some form of skeletal supporting structure. All land plants rely on a hydrostatic skeleton, a stiff framework supported by water in tubes, and some groups have evolved additional structural support through lignification of certain tissues in the wood and cortex, the process whereby lignin encrusts cellulose fibers.

Plant reproductive cycles_

Plants may reproduce vegetatively and sexually. Vegetative reproduction, or budding, is an asexual reproductive process that involves no exchange of material from different individuals, no male and female cells. It is a property of many plants that they may multiply in this way, either naturally or by human intervention.

Algae show all kinds of reproduction, vegetative, asexual and sexual. Sexual reproduction (Fig. 18.6a) involves the combination of cellular material from two organisms of the same species. The reproductive cells, or gametes (sperms from the "male" and eggs from the "female"), contain a single set of n chromosomes, the haploid condition. When the gametes combine, forming a zygote, the chromosome number doubles to the diploid condition, 2n. The diploid plant stage, the sporophyte, produces haploid (n) spores, each of which develops into a haploid gametophyte plant stage. It is the gametophyte that produces the haploid sperms and eggs.

In typical vascular plants, the green plant that we see is the sporophyte, while the game-tophyte is very small (Fig. 18.6b). The opposite is the case in bryophytes, where the visible mosses and liverworts are haploid gameto-phytes, and the sporophyte is a small plant that depends for nourishment on the larger gametophyte (Fig. 18.6c). Hence, in Sporogo-nites (see Fig. 18.3), numerous sporophytes appear to be growing from a portion of the larger flattened gametophyte phase. Translat-

spermatozoid n n egg

Figure 18.6 The origin of vascular land plant (tracheophyte) life cycles: (a) simplified plant life cycle showing alternation of phases; (b) life cycle of a hypothetical tracheophyte, with a dominant sporophyte phase and reduced gametophyte, in comparison with the life cycle of a hypothetical bryophyte (c), where the dominant phase is the gametophyte, and the sporophyte is a reduced dependent structure. (Based on various sources.)

ing to the human case, this would be like having the haploid sperm or egg dominant, and the diploid body (sporophyte) repressed!

Vascular plants in the Silurian and Devonian

As we have seen, non-vascular land plants had arisen at least by the Mid Ordovician, and vascular plants by the Mid to Late Silurian, some 425 Ma. Vascular plants are characterized by the possession of tracheids, true vascular conducting systems. Lignin and stomata are typical of vascular plants, but may not have been present in the earliest forms.

The oldest vascular plant is Cooksonia from the Mid Silurian of southern Ireland, a genus that survived until the end of the Early

Devonian. Cooksonia (Fig. 18.7a-d) is composed of cylindrical stems that branch in two at various points and are terminated by cap-shaped sporangia, or spore-bearing structures, at the tip of each branch. The specimens of Cooksonia range from tiny Silurian examples, only a few millimeters long, to larger Devonian forms up to 65 mm long. Extraordinary anatomic detail has been revealed by studies of specimens of these tiny plants that have been freed from the rock by acid digestion, and then mounted in resin. The sporangia have been dissected to reveal that they were packed with spores, the vascular conducting tissues of Early Devonian examples have thickened walls, and there are stomata on the outer surfaces of the stems (Edwards et al. 1992).

Cooksonia Caledonica

Figure 18.7 Early vascular plants. (a-d) The oldest land plant, Cooksonia from the Silurian to Early Devonian. Early Devonian examples from Wales, showing a complete sporangium at the end of a short stalk (a), a stoma (b) and spores (c). The sporangium is 1.6 mm wide, the stoma is 40 |im wide and the spores are 35 |im in diameter. (d) Reconstruction of Cooksonia caledonica, a Late Silurian form, about 60 mm tall. (e) Zosterophyllum, a zosterophyllopsid from the Early Devonian of Germany, 150 mm tall. (a-d, courtesy of Dianne Edwards; e, based on Thomas & Spicer 1987.)

Figure 18.7 Early vascular plants. (a-d) The oldest land plant, Cooksonia from the Silurian to Early Devonian. Early Devonian examples from Wales, showing a complete sporangium at the end of a short stalk (a), a stoma (b) and spores (c). The sporangium is 1.6 mm wide, the stoma is 40 |im wide and the spores are 35 |im in diameter. (d) Reconstruction of Cooksonia caledonica, a Late Silurian form, about 60 mm tall. (e) Zosterophyllum, a zosterophyllopsid from the Early Devonian of Germany, 150 mm tall. (a-d, courtesy of Dianne Edwards; e, based on Thomas & Spicer 1987.)

Cooksonia is a member of the Rhyniop-sida, the basal group of vascular plants, the Tracheophyta. Rhyniopsids are known most fully from the Early Devonian Rhynie Chert of northeast Scotland, a deposit that has preserved numerous plants and arthropods exquisitely in silica (Box 18.3). Some of the Rhynie rhyniopsids reached heights of 180 mm. They consisted of groups of vertical stems supported on horizontal branching structures that probably grew in the mud around small lakes.

Several other groups of vascular land plants arose in the Early Devonian. Zosterophyllum (Fig. 18.7e), a zosterophyllopsid, shares many features with the rhyniopsids, but has numerous lateral sporangia, instead of a single terminal one, on each vertical stem. Later in the Devonian, some basal tracheophytes became taller, as much as 3 m, the size of a shrub, and these indicate the future evolution of some vascular plants towards large size.

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  • mackenzie mckay
    What are the system of classification of tracheophytes?
    8 years ago
  • fulvus
    What is classification of tracheophytes short answer?
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  • negassi
    What are the superclass of the tracheophytes?
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  • italo
    What two groups formed the super group called tracheophyta?
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