Trends in Plant Evolution (from Mike Farabee, Maricopa Community College)




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Trends in Plant Evolution (from Mike Farabee, Maricopa Community College)


Several evolutionary trends within the plant kingdom have been noted. The monophyletic nature of this kingdom is not in dispute, with the first major division being between vascular ( those with xylem and phloem) and nonvascular plants. Within the vascular plants we see increasing changes in the relationship between sporophyte and gametophyte, culminating in flowering plants.

Developing from green algal ancestors, plants show a trend for reduction of the complexity, size, and dominance of the gametophyte generation. In nonvascular plants the gametophyte is the conspicuous, photosynthetic, free-living phase of the life cycle. Conversely, the angiosperm gametophyte is reduced to between three and eight cells (hence it is very inconspicuous) and is dependent on the free-living, photosynthetic sporophyte for its nutrition.

Plants also developed and refined the root-shoot-leaf axis with its specialized conducting cells of the xylem and phloem. The earliest vascular plants, such as Cooksonia and Rhynia, were little more than naked (unleafed) photosynthetic stems.



Cooksonia

Some plants later developed secondary growth that produced wood. Numerous leaf modifications are known, including "carnivorous" plants such as the Venus flytrap, as well as plants that have reduced or lost leaves, such as Psilotum and the cacti.

A third trend is the development of the seed to promote the dormancy of the embryo. The seed allows the plant to wait out harsh environmental conditions. With the development of the seed during the Paleozoic era, plants became less prone to mass extinctions.

The fourth trend in plant evolution is the encasing of a seed within a fruit. The only plant group that produces true fruit is the flowering plants, the angiosperms. Fruits serve to protect the seed, as well as aid in seed dispersal.





Table 1. Plant classification.





Characteristics

Examples and Uses

Bryophyta

nonvascular plants, gametophyte dominanted life cycle

mosses, liverworts, hornworts

Psilophyta

naked photosynthetic stems, no leaves, free-sporing

Psilotum nudum, Tmesipteris;no commercial uses

Lycophyta (the club mosses)

microphyll leaves, sporangia in strobili; more significant in the Paleozoic

Lycopodium, a homosporous plant, Selaginella, a heterosporous plant; no commercial uses except as ground cover in some areas

Sphenophyta (= Equisetophyta; the horsetails)

megaphyll leaves, sporangia in tight cones; plants with jointed stems; more significant in the Paleozoic

Equisetum, the scouring rush or horsetail; no commercial applications

Filicophyta (ferns)

megaphyll pinnately compound leaves, sporangia on underside of leaves, clumped in sori; leaves arise by circinnate vernation (uncoiling like a sheperd's crook)

Ornamental plants such as the Boston fern; tree ferns; commercial applications as ornamentals; ecologically important plants in some areas of the world

Cycadophyta (the cycads)

long, pinnately compound megaphyll leaves with a leathery feel arising from soft woode4d stems by circinnate vernation; new leaves arise as a crown or whorl; reproduction by seeds produced in female cones, pollen produced in male cones

Cycas revoluta, a common ornamental known as the Sago palm;Zamia floridana, only cycad native to the 48 contiguous United States; commercial uses as ornamental plants

Ginkgophyta (the ginkoes)

fan shaped, usually bilobed leaves on a woody tree with long anmd short shoot anatomy; seeds produce a foul odor when mature; pollen elliptical, monoaperturate

Ginkgo biloba, the maidenhair tree; commercial uses as ornamental plants and allegedly as a memory aid in herbal form

Coniferophyta

needle-like or scale-like leaves with thick cuticles and sunken stomata; soft wooded plants; seeds lacking fruit, dispersede from hardened cones; pollen can be bisaccate (Pinus) or lacking sacs (Taxodium)

Pinus(wood, resin, pine straw, pine nuts, paper); Taxus (yew) a natural source for the anticancer drug taxol; Douglas fir (wood); ornamental plants

Gnetophyta

living plant group most closely related to flowering plants; vessels in some members of group; reproductive structures close to flowers; pollen elliptical eith usually one aperture

Ephedra (Mormon tea) source of ephedrine; Gnetum; Welwitschia have no commercial uses

Magnoliophyta

flowering plants (angiosperms) that encase their seeds within a ripened ovary wall known as a fruit; fruits may be dry or fleshy, single or multiple; two major classes the monocots and dicots; xylem may include vessels as well as tracheids; range from herbaceous annuals to periennel trees

Monocots: cereal grains form staple of economy and diet; palms are used in many places as building material; ecologicalimpact as pioneer species in disturbed habitats;

Dicots: food crops such as beans, peas; fibers from cotton used to make clothing and paper; illegal drugs such as marijuana, cocaine, heroin; legal products such as chocolate, tea, and coffee; Nicotiana leaves make tobacco a dangerous legal "drug"; ornamental plants




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