Plant Timeline Evolution

1 Key excerpts on "Plant Timeline Evolution"

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  eBook - ePub

  Physiology and Behaviour of Plants

  • Peter Scott(Author)
  • 2013(Publication Date)
  • Wiley-Interscience

    (Publisher)

  1

  Introducing plants

  In this chapter, we look at how plants originated, what floral diversity there is today and the make-up of the plant and its ultrastructure.

  The beginning: the evolution of plants and the major divisions

  In the beginning, it is most probable that plants evolved from photosynthetic bacteria. From these bacteria the red and green algae evolved; and from freshwater-dwelling green algae the simple lower plants, such as mosses and ferns, evolved; and so on, up to the higher plants. A phylogenetic tree is shown in Figure 1.1 and Table 1.1 to demonstrate the relationship between the members of the plant kingdom and their relative abundance through the history of the planet.

  Figure 1.1 Schematic diagram of the phylogeny of plants. The diagram shows the evolutionary relationship between the different species and the relative abundance in terms of species numbers (shown by the width of the dark red lineage tree). On the y axis the different time periods of the evolutionary history of plants are shown. The diagram is based on that presented by Ridge 2002.

  Table 1.1 Estimates of numbers of species occurring in plant divisions.

  Plant division	Common name	Approximate number of species
  Hepatophyta	Liverworts	6000
  Anthocerophyta	Hornworts	100
  Bryophyta	Mosses	10 000
  Lycophyta	Club mosses and quillworts	1000
  Tree lycopods	Extinct
  Psilotum	Whisk fern	3
  Equisetum	Horsetail	15
  Pterophyta	Ferns	11 000
  Peltasperms	Seed ferns	Extinct
  Progymnosperms	Extinct
  Cycadophyta	Cycads	140
  Ginkophyta	Maidenhair tree	1
  Coniferophyta	Confers	550
  Gnetophyta	Vessel-bearing gymnosperms	70
  Angiosperms	Flowering plants	23 5000

  These estimates are based on there being around 260 000 different species of plant in the world. Species of plants based on data from Ridge 2002.

  Conquering the land

  The origin of plants was in water, where both photosynthetic bacteria and then algae originated. Light penetration of water reduces with depth, and on average only 1% of incident light reaches to a depth of over 15 m. As a consequence, there is a body of water at the surface known as the photic zone, where all of the photosynthetic activity in oceans occurs. A great deal of this photosynthetic activity still occurs at the shores of the oceans, where more complex algae have evolved; as a group these are commonly referred to as seaweeds. Algae are restricted to the oceans and freshwater bodies since, as part of the life cycle of algae, gametes (sex cells) that swim through water are required for sexual reproduction. This is thought to have been a major hindrance to plants attempting to colonize the land surface of the Earth and to overcome this, new reproductive systems needed to evolve.

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