jimtrue.com : school : BSC2011 : CH 29: Plant Diversity I: How Plants Colonized Land

Posted by Jim True on October 5, 2004 6:01 PM. Last Updated October 22, 2006 9:23 PM

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CH 29: Plant Diversity I: How Plants Colonized Land

Plant Evolution

• The Kingdom Plantae are commonly referred to as the "green plants".
• Comparatively few of the 275,000+ species in the kingdom live in aquatic environments.
• The Plantae evolved as a terrestrial group.
• Adaptations to living on land (not necessarily present in all groups today) include:
• Offpsring develop from multicellular embryos that remain attached to the parent plant for nourishment and protection. Involves placental transfer cells - cells in embryo and parent plant for transfer of nutrients.
• Development of vascular tissues (xylem and phloem). Critical development which allowed plants to invade habitats away from standing water. Also allowed for greatly increased size of plants.
• Presence of apical meristems for growth.
• Development of the seed - complete embryonic plant plus a food supply encased within a protective coating.
• Development of the flower and the fruit as means of enhancing reproductive success and dispersal of offspring.
• Alternation of generations present in all species.
• Production of both spores and gametes.
• Cuticle on leaves and some stems prevents dehydration.
• Variety of chemical compounds produced for: protection from terrestrial herbivores, e.g. alkaloids, terpenes and tannins (bitter tastes, possibly toxic); absorption of UV (flavonoids); hardened tissues to provide structural support (lignin).

Plant Characteristics

• Highly diverse group of multicellular eukaryotes
• Thought to have evolved from the protistan phylum chlorophyta (Order Charaophyceae).
• All are photosynthetic autotrophs, typically contain chlorophylls a and b, plus carotenoids and xanthophylls as main photosynthetic pigments.
• Store excess carboyhydrates (CHO's) as amylose (starch).
• Possess cell walls of cellulose produced by structures that differ from most algae and other cellulose containing protists.
• Cells contain peroxisomes with enzymes that differ from all phtosynthetic protists except charaophyceans. (Detoxifies hydrogen peroxide from cells).
• Flagellated sperm (in those groups with motile gametes) structurally are most similar to charophyceans.
• Phragmoplasts - construction formed by cytoskeletal proteins and vesicles from the Golgi complex that form cell plates during cytokinesis.

Plant Reproduction

• The typical reproductive pattern of plants is called alternation of generations (AoG).
• In AoG, plants alternate between a haploid gametophyte generation (asexual) and a diploid sporophyte generation (sexual).
• Gametophytes produce gametes within multicellular sex organs called gametangia:
  1. Antheridium - Male gametangium. Produces sperm.
  2. Archegonium - Female gametangium. Produces ovum (ova - eggs).
• Sperm and egg cells (gametes) are haploid. Fusion produces a diploid zygote that develops in the archegonium.
• If we start with the haploid gametophyte:
1. Antheridia produce haploid sperm via mitosis, archegonia produce haploid ova via mitosis.
2. In primitive forms, sperm are motile and reach the archegonium in various ways but all involving water. They swim to ovum.
3. Gamete nuclei fuse, produce diploid (2n) zygote.
4. The 2n zygote forms a multicellular plant by mitosis, which matures into the sporophyte plant.
5. When mature, the sporophyte develops sporangia, in which haploid cells develop by meiosis from spore mother cells (sporogenic cells).
6. Plant spores are encased in an extremely tough polymer which resists water loss and environmental conditions.
7. Spores are dispersed mainly by wind.
8. When spores contact a favorable environment, they germinate, developing into the new haploid gametophyte generation.

Plant Taxonomy

• Plant taxonomists are evaluating three possible kingdoms for plants, but the currently accepted kingdom is the Plantae, consisting of the terrestrial green plants (plus their few aquatic relatives).
• All of these are embryophytes, producing a multicellular embryo.
• Regardless of whether one of the alternate suggestiions for the kingdom is adopted, the plant kingdom is monophyletic, derived from a single ancestral type.
• Four major groups of plants are currently recognized, which can be divided into two main categories:
1. Non-Vascular (Avascular) Plants - These plants lack vascular tissues (xylem and phloem), and have no true organs. The gametophyte is the dominant form. Dominance means longer duration, or most visible lifeform (more likely to be observed in nature), and perhaps larger size; it does not necessarily mean 'stronger'.
2. Vascular Plants - Possess vascular tissues and organs. The sporophyte generation dominates.
• Three of the four major groups fall within Category II: seedless plants, plants with exposed seeds, and plants with enclosed seeds.

1. Category I Plants - Non-Vascular Plants - Bryophytes
• Until recently, all nonvascular plants were included in the Phylum Bryophyta ("bryo" - moss; "phyt" - plant) and were commonly known as the true mosses, hornworts and liverworts.
• However, the hornworts and liverworts are now each placed in their own phyla, the Hepatophyta ("hepato" - liver) and Anthocerophyta ("antho" - flower; "cero" - horn).
• Because their biology is essentially the same, we will focus on the Bryophyta (lecture and lab).
• A few bryophytes have tubular cells for water conduction, but they lack hardened walls. The lack of the rigid vascular system imposes size and habitat limits.
• Bryophytes are typically small and live in wet areas. this is because the water can't be transported any distance within the plant.
• About 15,000 species (9K mosses, 6K liverworts, about 100 hornworts).
• Lack true roots, stems and leaves although they may have structures that appear similar.
• Typically are anchored to firm substrates by filamentous structures called rhizoids ("root" + "similar")
• Gametophyte generation is dominant (largest, most visible generation). Diagnostic characteristic for the non-vascular plants.
• Gametangia are usually located on stalks growing out of the top of the gametophyte
• In some species, the gametophyte can form both M and F gametangia, in other species, separate plants form separate gametangia.
• Water is essential for sperm transport, another reason these are in moist environments.
• The 2n embryo becomes a sporophyte which grows as a small, stalked structure on top of the gametophyte.
• The sporophyte body consists of a foot, the connection to the gametophyte body, a seta ("hair"), a long thin stalk, and a capsule, which is the sporangium.
• Sporogenous cells in the capsule produce haploid spores by meiosis, which are spread by wind and rain.
• Tens of millions of spores can be released PER CAPSULE!
• Spore germinates into a haploid threadlike protonema ("proto" - first + "nema" - thread), which develops into the new gametophyte.
• Liverworts can asexually produce small, cuplike structures called gemmae in which a ball of haploid tissue develops into a new liverwort.
• Although structurally different from mosses, the reproductive cycles of hornworts and liverworts are similar.
2. Category II A - Seedless Vascular Plants (Pteridophytes)
• We can refer to category II A as the "seedless" vascular plants.
• From this point onward, Sporophyte generation is dominant (largest, most visible generation). Diagnostic characteristic for the vascular plants.
• Two phyla are currently recognized:
1. Phylum Lycophyta - the "club mosses".
• Actually not mosses at all.
• Very simple body with very small leaves (microphylls) into which the vascular tissues does not branch unlike all other vascular plant leaves (megaphylls).
• Specialized leaves (sporophylls) bear the sporangia. Spores produced by meiosis are released and produce tiny inconspicuous gametophytes.
• In the lab, Sphenophytas were considered separate Phyla, but the Text combines them into this Phyla.
2. Phylum Pterophyta - includes the whisk ferns, horsetails and the most familiar of the members, the ferns.
   • Whisk ferns are very simple, having lost both root and leaves.
   • Sporangia form as knobby protrusions on the stems.
   • Horsetails possess greatly reduced leaves. the stems are the photosynthetic organs (same in whisk ferns).
   • Sporangia appear as cone shaped structures off the stems.
   • The ferns are the most diverse members of the phylum, ca. 12,000 identified modern species.
   • The fern sporophyte is composed of:
   • Underground stem, called the rhizome, which bears roots and which can asexually produce new plants.
   • Above ground leaves called fronds.
   • All of the ferns structures contain vascular tissue.
   • Because of need for water for fertilization (sperm are motile), ferns, whisk ferns and horsetails are typically found in moist environments.
   • Growth cycle:
   1. Ferns form haploid spores by meiosis in sporangia. The sporangia are usually contained in a button-like or strand-like cluster called a sorus (sori).
   2. Spores fall to the ground, germinate and develop into a tiny gametophyte called a prothallus. This is a tiny, heart shaped nonvascular disk which is anchored to the soil by rhizoids to keep moisture on the underside.
   3. On the underside of the prothallus, antheridia, containing numerous sperm, and archegonia, each with one ovum, codevelop.
   • In some species, archegonia and antheridia occur on same prothallus, in others they are separate.
   4. Sperm swim from antheridia to archegonium and fertilize ovum.
   5. The new 2n zygote develops within the archegonium and emerges as a new sporophyte.
   6. Once the sporophyte is rooted, the prothallus dies.

Spore Production

• Homospory - Among all of the plants we have examined thus far, spore production from the sporophyte plant involved a single type of spore.
• From this, a bisexual gametophyte, capable of producing both sperm and egg, developed.
• In a few aquatic ferns, and in all of the more advanced terrestrial plants (category 2B and C), the sporophytes are heterosporous. (Two separate gamete producing bodies).
• Heterospory - some ferns and all seed plants produce two different types of spores in two different sporangia (sporgenous cells).
• Microsporangia -> Microsporocytes -> Meiosis -> Microspores -> Male gametophyte -> Sperm.
• Megasporangia -> Megasporocytes -> Meiosis -> Megaspores -> Female gametophyte -> Ovum.
• In heterosporous plants, the gametophytes are not free living, but remain enclosed within their respective sporangia.
• Heterospory is evolutionarily important -- led to the formation of the seed, and ultimately to the development of the flower.

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