Chapter 29 Plant Diversity I How Plants Colonized LandLecture Outline Overview The Greening of Earth For the first 3 billion years of Earth s history the land was lifeless Thin coatings of cyanobacteria existed on land about 1 2 billion years ago About 500 million years ago plants fungi and animals joined them More than 290 000 species of plants inhabit Earth today Most plants live in terrestrial environments including deserts grasslands and forests Some species such as sea grasses have returned to aquatic habitats The presence of plants has enabled other organisms to survive on land Plant roots have created habitats for other organisms by stabilizing landscapes Plants are the source of oxygen and the ultimate provider of food for land animals Concept 29 1 Land plants evolved from green algae Researchers have identified a lineage of green algae called charophyceans as the closest relatives of land plants Many key characteristics of land plants also appear in a variety of algal clades So do green algae dinoflagellates and brown algae Plants are multicellular eukaryotic photosynthetic autotrophs But red brown and some green algae also fit this description Plants have cell walls made of cellulose Plants have chloroplasts with chlorophyll a and b So do green algae euglenids and a few dinoflagellates Land plants share four key features only with the charophyceans 1 The plasma membranes of land plants and charophyceans possess rosette cellulose synthesizing complexes that synthesize the cellulose microfibrils of the cell wall These complexes contrast with the linear arrays of cellulose producing proteins in noncharophycean algae Also the cell walls of plants and charophyceans contain a higher percentage of cellulose than the cell walls of noncharophycean algae 2 A second feature that unites charophyceans and land plants is the presence of peroxisome enzymes to help minimize the loss of organic products as a result of photorespiration In those land plants that have flagellated sperm cells the structure of the sperm resembles the sperm of charophyceans Peroxisomes of other algae lack these enzymes 3 4 Finally certain details of cell division are common only to land plants and the most complex charophycean algae These include the formation of a phragmoplast an alignment of cytoskeletal elements and Golgi derived vesicles during the synthesis of new cross walls during cytokinesis Over the past decade researchers involved in an international initiative called Deep Green have conducted a large scale study of the major transitions in plant evolution Comparisons of nuclear and chloroplast genes support the hypothesis that the These researchers have analyzed genes from a wide range of plant and algal species charophyceans are the closest living relatives of land plants Many charophycean algae inhabit shallow waters at the edges of ponds and lakes where they experience occasional drying In such environments natural selection favors individuals that can survive periods when they are not submerged in water A layer of a durable polymer called sporopollenin prevents exposed charophycean zygotes from drying out until they are in water again This chemical adaptation may have been the precursor to the tough sporopollenin walls that encase plant spores The accumulation of such traits by at least one population of ancestral charophyceans enabled their descendents the first land plants to live permanently above the waterline The evolutionary novelties of the first land plants opened an expanse of terrestrial habitat previously occupied only by films of bacteria The new frontier was spacious The bright sunlight was unfiltered by water and plankton The atmosphere had an abundance of carbon dioxide The soil was rich in mineral nutrients At least at first there were relatively few herbivores or pathogens Concept 29 2 Land plants possess a set of derived terrestrial adaptations A number of adaptations evolved in plants that allowed them to survive and reproduce on land What exactly is the line that divides land plants from algae We will adopt the traditional scheme which equates the kingdom Plantae with embryophytes plants with embryos Some botanists now propose that the plant kingdom should be renamed the kingdom Streptophyta and expanded to include the charophyceans and a few related groups Others suggest the kingdom Viridiplantae which includes chlorophytes as well as plants Five key traits appear in nearly all land plants but are absent in the charophyceans We infer that these traits evolved as derived traits of land plants The five traits are Apical meristems Alternation of generations Multicellular embryo that is dependent on the parent plant Gametangia that produce gametes Sporangia that produce walled spores Apical meristems Light and carbon dioxide are mainly aboveground In terrestrial habitats the resources that a photosynthetic organism requires are found in two different places Water and mineral resources are found mainly in the soil Therefore plants show varying degrees of structural specialization for subterranean and aerial organs roots and shoots in most plants The elongation and branching of the shoots and roots maximize their exposure to environmental resources This growth is sustained by apical meristems localized regions of cell division at the tips of shoots and roots Cells produced by meristems differentiate into various tissues including surface epidermis and internal tissues Alternation of generations All land plants show alternation of generations in which two multicellular body forms alternate This life cycle also occurs in various algae However alternation of generations does not occur in the charophyceans the algae most closely related to land plants In alternation of generations one of the multicellular bodies is called the gametophyte and has haploid cells Gametophytes produce gametes egg and sperm by mitosis Fusion of egg and sperm during fertilization form a diploid zygote Mitotic division of the diploid zygote produces the other multicellular body the sporophyte Meiosis in a mature sporophyte produces haploid reproductive cells called spores A spore is a reproductive cell that can develop into a new organism without fusing with another cell Mitotic division of a plant spore produces a new multicellular gametophyte Unlike the life cycles of other sexually producing organisms alternation of generations in land plants and some algae results in both haploid and diploid stages