Plant Diversity! How the Plants Move to Land? Part 1

Türkçe İçin : http://gfreethought.blogspot.com.tr/2016/02/bitki-cesitliligi-bitkiler-karalara.html

Plant Diversity : How the Plants Colonized Land?

   Geochemical analysis and fossil evidence suggest that thin coatings of cyanobacteria and protists existed on land by 1.2 billion years ago. But it was only within the last 500 million years ago, tall plants appeared, leading to the first forests.

   

   Today, there are more than 290,000 known plant species.

Land plants evolved from green algae

   Green algae called charophytes are the closest relatives of land plants.

Morphological and Molecular Evidence

   Many key traits of land plants also appear in some algae. For example, plants are multicellular, eukaryotic, photosynthetic autotrophs, as are brown, red and certain green algae.

  

   Plants have cell walls made of cellulose, and so do green algae, dinoflagellates, and brown algae. And chloroplasts with chlorophylls a and b are present in green algae, euglenids, and a few dinoflagellates, as well as in plants.

   

Figure 1 Rings of cellulose-synthesizing
proteins.

   However, the charophytes are the only present-day algae that share the following distinctive traits with land plants, suggesting that they are the closest living relatives of plants :

• Rings of cellulose-synthesizing proteins.
• Structure of flagellated sperm.
• Formation of a phragmoplast.
• Peroxisome enzymes.

Figure 2 Coleochaete orbicularis

Adaptations Enabling the Move to Land

   Many species of Charophyta algae inhabit shallow waters around the edges of ponds and lakes, where they are subject to occasional drying. In such environments natural selection favors individual algae that can survive periods when they are not submerged. In charophytes, a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out. A similar chemical adaptation is found in the tough sporopollenin walls that encase plant spores.

Derived Traits of Plants

   A series of adaptations that facilitate survival and reproduction on dry land emerged after land plants diverged from their algal relatives.

                                                         Figure 3 Three possible "plant" kingdoms.

   Charophyte algae lack the key traits of land plants in this part : alternation of generations; multicellular, dependent embryos; walled spores produced in sporangia; multicellular gametangia and apical meristems. This suggests that these traits were absent in the ancestor common to land plants and charophytes but instead evolved as derived traits of land plants. Not every land plant exhibits all of these traits, certain lineages of plants have lost some traits over time.

Alternation of Generations :

                                                                            Figure 4 Alternation of Generations

1. The gametophyte produces haploid gametes by mitosis.

2. Two gametes unite (fertilization) and form a diploid zygote.

3. The zygote develops into a multicellular diploid sporophyte.

4. The sporophyte produces unicellar haploid spores by meiosis.

5. The spores develop into multicellular haploid gametophytes.

Multicellular, Dependent Embryos :

   As part of a life cycle with alternation of generations, multicellular plant embryos develop from zygotes that are retained within the tissues of the female parent (a gametophyte). The parental tissues protect the developing embryo from harsh environmental conditions and provide nutrients such as sugars and amino acids. The embryo has specialized placental transfer cells that enhance the transfer of nutrients to the embryo through elaborate ingrowths of the wall surface (plasma membrane and cell wall). The multicellular, dependent embryo of land plants is such a significant derived trait that land plants are also known as embryophytes.

                                 Figure 5 Embryo (LM) and placental transfer cell (TEM) of Marchantia (a liverwort)

Walled Spores Produced in Sporangia :

   Plant spores are haploid reproductive cells that can grow into multicellular haploid gametophytes by mitosis. The polymer sporopollenin makes the walls of plant spores tough and resistant to harsh environments. This chemical adaptation enables spores to be dispersed through dry air without harm.

   

   The sporophyte has multicellular organs called sporangia (singular, sporangium) that produce the spores. Within a sporangium, diploid cells called sporocytes, or spore mother cells, undergo meiosis and generate the haploid spores.

   

   Although charophytes also produce spores, these algae lack multicellular sporangia, and their flagellated, water-dispersed spores lack sporopollenin.

                                          Figure 6 Sporophytes and sporangia of Sphagnum (a moss)

Multicellular Gametangia :

   Another feature distinguishing early land plants from their algal ancestors was the production of gametes within multicellular organs called gametangia. The female gametangia are called archegonia (singular, archegonium). The male gametangia are called antheridia (singular, antheridium).

                                       Figure 7

Apical Meristems :

   Though plants cannot move from place to place, their roots and shoots can elongate, increasing exposure to environmental resources. This growth in length is sustained throughout the plant’s life by the activity of apical meristems, localized regions of cell division at the tips of roots and shoots.

                                       Figure 8

   Additional derived traits that relate to terrestrial life have evolved in many plant species. For example, the epidermis in many species has a covering, the cuticle that consists of wax and other polymers.

                                                                Figure 9 Cuticle of Agapanthus from my microscope

Thanks for reading! But that's not enough of course. This is only part I, to be continued...

Reference : Biology: A Global Approach, Global (Tenth) Edition - PEARSON