Chapter 22 SEEDLESS VASCULAR PLANTSEVOLUTION OF VASCULAR PLANTS Notes from Raven et al., 7th. Edition. Plants had an aquatic ancestor probably a Coleochaete-like alga, of the Chlorophyta.Coleochaete algae are monobiontic – one multicellular generation.Plant evolution shows a tendency toward greater independence from water as they progressively occupiedthe land.Air is drier than water and less buoyant. Land plants had to develop adaptations to conserve and transport water with its solutes, absorb water from the environment, support itself facing the direction of sunlight, solve reproductive problems like fertilization and nourishment of embryo, and dispersal of offspring.1. Dominant sporophyte and reduced gametophyte.- In bryophytes, the gametophyte is dominant generation.- Water is required for fertilization. - Pollen and embryo sac are much reduced gametophytes.- The occupation of the land by the bryophytes was undertaken with emphasis on the gamete-producing generation, which requires water for fertilization.- Dibiontic: two multicellular generations.2. Development of fluid-transport system, the xylem and phloem.- Aquatic plants take water throughout their entire body.- On land, soil is the water reservoir; the air is dry in comparison to cells.3. The ability to synthesize lignin.- Early land plants were small and probably stayed upright by means of turgor pressure.- Lignin adds rigidity to the cell wall and allows the plant to reach greater heights.4. Development of apical meristems.- Bryophyte sporophyte growth is subapical and unbranched. - It allows the sporophyte to branch many times.5. Ability to produce many sporangia.- Only one sporangium is produced the bryophyte sporophyte.- The many branches of vascular plants became capable of bearing many sporangia.6. More diverse plant body through the development of roots stems and leaves.- Roots for absorption, storage and anchorage.- Stems for support above ground, transport and growth toward the light.- Leaves for photosynthesis.7. Evolution of seeds.- Provides the embryo with food and protection.- Dispersal of the species to new locations.HYPOTHESES ABOUT THE ORIGIN OF THE ALTERNATION OF GENERATIONSNotes from Mauseth.INTERPOLATION THEORYThis theory presumes that the earliest land plants did not have a sporophyte generation; the zygote germinated by meiosis that produced haploid spores that grew into new haploid gametophytes.- A small sporophyte came into existence when a zygote germinated mitotically instead of meiotically.- The sporophyte generations would have gradually evolved in complexity while the gametophyte generation remained small. - A sporophyte generation was inserted (interpolated) into the monobiontic life cycle.- Some bryophytes may represent an intermediate stage in the progression from green algae to vascular plants.- Riccia and Ricciocarpus have simple, almost alga-like gametophyte, and their sporophytes consist of just a small globose sporangium with not foot or seta. The zygote undergoes several mitotic divisions and then some cells undergo meiosis.- Speculation: the sporophyte then evolved more complex with a foot and seta, and later developed an apical meristem to branch and live free of the gametophyte.TRANSFORMATION THEORY- After the dibiontic life cycle originated, both gametophyte and sporophyte became larger, more complex, and vascularized, in a life cycle with an alternation of isomorphic generations. - No living plants have generations that are alike but many algae do and so do some fossil plants- The transformation theory postulates that plants evolved into two clades:1. Nonvascular plants in which the sporophyte became much simpler and dependent on the gametophytes2. Vascular plants in which sporophytes became increasingly elaborate whereas gametophytes became reduced.EARLY VASCULAR PLANTSNotes from Raven et al.There are three divisions of extinct seedless vascular plants: Rhyniophyta, Zosterophyllophyta and Trimerophytophyta.The genera Rhynia, Zosterophyllum and Trimerophyton are members of these phyla.The earliest known go back about 425 million years ago and most went extinct by the end of the Devonian about 370 million years ago.These three groups were the dominant vegetation from the mid-Silurian to the mid-Devonian, 425 to 370 million years ago.For the most part they were relatively simple plants 18 in to 36 inches tall. They had the following characteristics: 1. Naked photosynthetic stems 2. Terminal sporangia (some lateral) 3. No roots or leaves 4. They were all homosporous 5. They had protostelesPteridophytes, lycophytes and progymnosperms are more complex groups that were dominant from the Late Devonian through the Carboniferous, from about 370 to 290 million years ago. Seed plants arose starting in the Late Devonian period, about 380 million years ago, and evolved many new lines by the Permian (290-248 million years ago).Gymnosperms dominated the land floras throughout the Mesozoic until about 100 million years ago. Angiosperms appeared in the fossil record about 125 million years ago. It became the dominant group about 30 – 40 million years ago, and has remained so until the present.XYLEM STRUCTURE OF EARLY VASCULAR PLANTS.Early vascular plants had two types of xylem organization: endarch protostele and exarch protostele.- The protostele consists of a solid, central mass of vascular tissue with no pith.- In endarch protostele the protoxylem is located in the center of the protostele, and the metaxylem surrounds it.- In the exarch protostele, the metaxylem is located in the center and the protoxylem grouped into several clusters around the metaxylem.Siphonostele evolved after the protostele. Siphonostele has a central pith surrounded by xylem.Xylem consisted of tracheids.Around the xylem there was a layer of phloem-like cells and then a parenchymatous cortex and epidermis.Division RhyniophytaThe Rhyniophytes are a group of early land plants originally described from the Rhynie Chert, Scotland.The earliest vascular plant fossil belongs to the genus Cooksonia.It became extinct in the mid-Devonian about 380 million years ago.Plants with these characteristics are called rhyniophytes.- Seedless; produced spores.- Dichotomous branching.- Terminal sporangia.- Homosporous.- Plant body was not differentiated into roots, stems and leaves.- Epidermis with a cuticle, cortex of parenchyma cells, and protostele.- Underground rhizome with rhizoids.- Protostele consisting of a