The Colonization of Land The Evolution of Land Plants

From the time of Linnaeus ( ) until fairly recently (1969)*, the diversity of life was organized into two main groups: plants and animals. The plant kingdom was thought to include plants, algae, fungi, and later, bacteria (i.e. organisms we could see, but that were clearly not animals). More recent classifications have recognized the differences among these groups, and treat the plant kingdom as limited to the Embryophyta, photosynthetic eukaryotes that have multicellular, dependent embryos. * The 5-kingdom classification scheme was proposed by Robert H. Whittaker, of Cornell University. Retracing biological thought… What are the problems associated with classifying Euglena and fungi, according to this scheme?

What challenges exist in the terrestrial environment? Consider the 4 main groups of land plants: Bryophytes (mosses) Pteridophytes (ferns) Gymnosperms (conifers) Angiosperms (flowers) What are the benefits of living in a watery environment?

What challenges exist in the terrestrial environment? Consider the abiotic needs: water, nutrients, space, exposure to UV & wind, etc.

What challenges exist in the terrestrial environment? Consider the major themes in the study of life: metabolism, growth, homeostasis, reproduction, etc.

In other words, how do we define “land plants”? Kingdom Plantae –Eukaryotic –Multicellular –Photosynthetic autotrophs

General Characteristics of Land Plants Chloroplasts, containing chlorophyll a and b Cell walls contain cellulose Starch is the energy storage molecule What is the primary function of the central vacuole?

Adaptations to life “on the border” Related to characteristics of growth and reproduction: Apical meristems Dependent embryos Alternation of generations –Sporophyte produces durable spores –Gametophyte produces gametes How compatible are dry land and swimming sperm? How big can a plant body get? How do sessile organism disperse?

Meristems …persistent populations of small unspecialized cells with embryonic characteristics Fig Fig Are there cells in humans that serve a similar function?

Reproduction parental tissues provide nutrition haploid dominant  diploid dominant …genetic advantage? motile gametes  nonmotile gametes seedless  seeds Water disbursement  wind and animals –gametes, spores and seeds Conifers Bryophytes How are male gametes transported to female gametes in these two groups of land plants?

When does the haploid stage occur in humans? Is it multicellular (as in plants)? Do humans have alternation of generations? Alternation of Generations HumansPlants

Haploid Dominant  Diploid Dominant Fig The gametophyte is haploid. What type of cell division will produce gametes? What is the advantage of a larger sporophyte for seedless plants?

Additional terrestrial adaptations Water conservation –Cuticle –Stomata Transport –Xylem –Phloem Secondary compounds stomata cuticle xylem phloem Where do you think there would be more stomata, on the upper or lower leaf surface? Why?

Fixed Carbon secondary metabolites

Modified Leaves Tendrils Spines BractsSucculent Speculate as to why natural selection favored the leaf morphology of each plant shown.

Underground Stem Storage Rhizomes (lateral stem) Ginger, many grasses, some ferns... Tubers Rhizomes: potatoes, yams... Bulbs Onion, Daffodil, Tulip, Lilies... Iris Speculate as to the adaptive advantage of these structures for these plants.

What is the most likely ancestor to the group of land plants? How are land plants different than the photosynthetic protists (algae)? How are they similar? Euglena Brown algae DinoflagellateSpirogyra

The search for structural and molecular homology* Plant cell walls contain cellulose –but also present in some dinoflagellates and phaeophyta (browns) Plant cells contain chlorophylls a and b in chloroplasts –but also present in Euglena, some dinoflagellates and green algae Fig *see p. 438

The closest relatives to the land plants are the Charophyceans Rosette cellulose- synthesizing complexes –as opposed to synthesizing proteins arranged in a line Molecular comparisons Which molecules are compared? Similarities in cytokinesis Similarities in sperm cell structure

1. What major innovations allowed the initial invasion of land? Leaves --> advantage? Wood --> advantage? 2. What other innovation occurred along with seeds? Proposed phylogeny of land plants

I think we should rename the plant kingdom “Terrestrial Algae”. But algae don’t flower or produce seeds! I think the most important difference is that plants protect their embryos. Isn’t the big difference based on whether they have swimming sperm?