How Plants Colonized Land - Lecture Notes | BIOL 112, Study notes of Biology

Download How Plants Colonized Land - Lecture Notes | BIOL 112 and more Study notes Biology in PDF only on Docsity! Chapter 29 PLANT DIVERSITY I: HOW PLANTS COLONIZED LAND There are about 290,000 of land plants on Earth. LAND PLANTS EVOLVED FROM GREEN ALGAE Charophycean ancestor Charophyceans are the green algae most closely related to land plants. Land plants probably are probably derived from a group of green algae called charophytes. Land plants share with the charophyceans the following traits: 1. Rosette cellulose-synthesizing complexes: land plants and charophyceans posses a rosette-shape array of proteins that synthesize cellulose microfibrils in their cell wall. Other cellulose wall-containing algae (e. g. brown algae, dinoflagellates), have linear arrays of cellulose-producing proteins. This suggests a common ancestor between the charophytes and land plants. This rosette synthesizing system evolved independently of the cellulose making system of other green algae. 2. Peroxisomes enzymes: the charophyceans and land plants have enzymes in their peroxisomes that minimize the loss of carbohydrate due to photorespiration. Other alga groups do not have these enzymes in their peroxisomes. 3. Structure of the flagellate sperm: details of the sperm of charophyceans resemble those of land plants that have flagellated sperms. 4. Cell plate formation during cytokinesis: cell division features a complex network of microtubules and Golgi vesicles, the phragmoplast, again as found in all land plants. 5. DNA and RNA sequences support their close relation to the charophytes, especially Chara and Coleochaete. TERRESTRIAL ADAPTATION OF LAND PLANTS ADAPTATIONS ENABLING THE MOVE TO LAND A layer of sporopollenin protects charophytes from desiccation; sporopollenin is found in the spore wall of land plants. Danger of desiccation required new adaptations: transport tissue, cuticle, etc. Support against gravity. Plants are eukaryotic, multicellular, mostly autotrophic organisms, with haploid-diploid life cycles, which retain embryo within female sex organ on parent plant; the cell wall contains cellulose. Scientists are studying the ultrastructure of cells, analyzing macromolecules and comparing morphology with life cycles. There are several proposals to rearrange the boundaries of the kingdom Plantae:  Only the Embryophytes; the present and traditional system.  Expand it to include the charophyceans: Kingdom Streptophyta.  Expand it further to include all the green algae, Chlorophyta: Kingdom Viridiplantae. DERIVED TRAITS OF PLANTS The following characteristics are common to all four groups of land plants but are absent in the charophyceans. 1. Apical meristem: cluster of embryonic cells found at the tip of shoots and roots. 2. Alternation of generations: a characteristic life cycle. Alternation of generation does not occur in the charophyceans. This suggests that alternation of generation arose independently in land plants. A life cycle characterized by a multicellular haploid gametophyte stage followed by a multicellular diploid sporophyte stage. Multicellular, dependent embryos: The zygote is retained surrounded by tissues of the gametophyte. The parental tissue provides the embryo with nutrients. Placental transfer cells present in the embryo and sometimes in the gametophyte as well, enhance the transfer of nutrients. 3. Spores produced in sporangia: haploid reproductive cells that become a multicellular haploid gametophyte by mitosis. The multicellular sporangium contains sporocytes, the cells that undergo meiosis to form spores. Sporopollenin, the most durable organic material known, makes the walls of the spores. 4. Multicellular gametangia: the gametes of land plants are produced in multicellular organs called gametangia. Algae produce their gametes in unicellular gametangia, inside a single cell. Adaptations for water transport and conservation. 1. Waxy cuticle to protect against desiccation. The gametophores are anchored by fragile rhizoids. Rhizoids are either single elongated cells as those found in liverworts and hornworts, or filaments of cells as those of mosses. Rhizoids are not made of tissues and do not absorb any significant amount of water. In that way they differ from roots. Bryophytes have smallest and simplest sporophyte of any group. The sporophyte remains attached to the gametophyte throughout its lifetime, dependent of the gametophyte for food, water and minerals. The mature sporophyte of mosses consists of a foot embedded in the archegonium, a seta or stalk is present in the phylum Bryophyta, and a capsule or sporangium. The cap or calyptra closes the peristome or opening or the capsule. THE ORIGIN OF VASCULAR PLANTS. Ferns and other seedless vascular plants formed the first forests. 1. The next step in land plant evolution included the development of an independent sporophyte.  At first this sporophyte was of equal size as the gametophyte.  Cooksonia caledonica, from the Silurian (~420 million years ago) rocks of Europe and North America, is the oldest known land plant.  Small, leafless, rootless, dichotomous axes with terminal sporangia. 2. Transport in xylem and phloem.  Phloem for the transport of dissolved carbohydrates.  Xylem for water and mineral transport.  Lignin strengthens the vascular tissue cells. 3. Evolution of roots.  Roots anchor plants and allow the absorption of water and nutrients from the soil.  Root tissues of living plants closely resemble stem tissues of early vascular plants preserved in fossils.  Roots may have evolved from the lowest subterranean parts of the stem.  The oldest lycophyte fossil had simple roots 400 million years ago. 4. From an evolutionary perspective, there are two kinds of leaves:  Microphylls are single veined leaves associated and evolved as superficial outgrowth of the stem.  Microphylls first appear in the fossil recorda about 410 million years ago.  Megaphylls have a complex venation pattern, and evolved from a branch system.  Megaphylls appeared about 370 million years ago, at the end of the Devonian. 5. The sporophyte became the dominant generation. Sporophylls are modified leaves that bear spores. Sporophylls may be grouped into cone-like structures called strobili (sing. strobilus).  Homospory: production of one kind of spores. - Spores produce a bisexual gametophyte that produces eggs and sperms.  Heterospory: production of two kinds of spores. - Haploid megaspores develop into a female gametophyte. - Haploid microspores develop into a male gametophyte. CLASSIFICATION OF SEEDLESS VASCULAR PLANTS There are two phyla of pteridophytes found in the modern flora: Licophyta and Pterophyta. 1. Phylum Lycophyta. There are about 15 genera of lycophytes and approximately 1000 living species. This phylum includes the Lycopods (club mosses), Selaginella (spike moss) and Isoetes (quillwort). This evolutionary line extends back into the Devonian (409-363 mya) but were most prevalent in the wet swamps of the Carboniferous period (363-290 mya). They eventually split up into two evolutionary lines.  The first were very large woody trees that did not survive in the drier climate at the end of and after the Carboniferous age. In the Carboniferous some lycophytes were forest- forming trees more than 35 meters tall.  The second and the surviving group of Lycopods are the small and herbaceous trees. Lycophyta remains became the largest coal deposits of all geologic time. The sporophytes of lycophytes consist of true roots, stems and leaves (microphylls). Some Selaginella are heterosporous; Lycopodium is homosporous. Sporophylls are specialized leaves that bear sporangia and are organized into a structure called the strobilus (pl. strobili). 2. Phylum Pterophyta Psilophytes (whisk ferns) It includes two living genera, Psilotum and Tmesipteris, from tropical and subtropical regions of the world...  Sporophyte with a dichotomously branching aerial and subterranean stem system.  True roots lacking.  Underground stems with rhizoids and with a fungal association.  Aerial stems lacking leaves but with scale-like or larger leaf-like structures (enations) Until recently they were placed in a phylum of their own, but DNA sequences analysis and sperm ultrastructure study has shown that they are related to present day fern. The lack of roots and leaves may be due to simplification, a derived or secondary characteristic, rather than a maintained characteristic from ancient ancestors, a primitive characteristic. Sphenophytes (Horsetails) Sphenopsids extend back to the Devonian (409-363 mya) and reached their maximum development in the Carboniferous (363-290 mya.) A family of one extant genus, Equisetum (ca. 15 species), of nearly worldwide distribution in damp habitats such as riverbanks, lakeshores, and marshes. Michigan is a center of diversity for the genus with nine native species.  The sporophyte of Equisetum is differentiated into an underground rhizome that bears adventitious roots and an upright, photosynthetic stem with whorls of microphylls...  Tough perennial herbs with jointed, ridged aerial stems with distinct nodes.  Stems rough, accumulating silica and metals, and complex anatomically.  The aerial stems contain a large central pith region, which in mature plants is hollow.  Surrounding the pith cavity are discrete bundles of vascular tissue; this arrangement of conducting tissue is known as a eustele. Recent molecular data suggest that they are closely related to ferns and should be classified with them. Ferns The fossil record of ferns extends back into the Carboniferous (363-290 mya) but their origins is in the Devonian (409-363 mya). There are about 12,000 species of ferns in the world. Most species are tropical.  There are about 380 species of ferns in U.S.A. and Canada.  Costa Rica has over 1,000 species.  Philippines has over 950 species.  Sporophyte is differentiated into true roots, stem (rhizome) and leaves (megaphylls).