Intro to Plants and Plant Diversity

B. The important challenges of life on land

1) Dehydration and getting enough water for photosynthesis 2) Support of plant structure above ground (overcoming gravity) 3) Transport of water within the plant from buried or submerged cells to air-exposed cells 4) Transport of photosynthate sugars to non-photosynthesizing cells, such as "root" cells 5) Sexual reproduction in dry environments where flagellated sperm can't swim. 6) Exposure to harmful ultraviolet radiation (blue end of the wavelength spectrum); fortunately some pigments evolved to absorb blue wavelengths and offer protection.

A. Resources out of the water

1) Sunlight unfiltered and unabsorbed by water, and hence energy is more available 2) CO2 is more available in air than in water, and photosynthetic organisms were being driven to the most surface waters and even momentarily out of the water and onto land to get enough CO2 [diffusion of CO2 occurs about 10,000 times faster in air than water] 3) Freedom from heterotrophs, at least early in the evolution of land plants.

Some major characteristics of Kingdom Plantae

1. Chloroplasts with chlorophyll a, b and β-carotene for wider spectral sensitivity - in green algae and all land plants 2. Multiple membrane layers in chloroplasts [thylacoid condition = stacked and flattened vesicles without connection to the inner of two membranes enclosing the chloroplast), pp.116 and 178-179] for more efficient extraction of radiant energy 3. Cellulose cell wall outside of cell membrane (for greater protection - and structural support) 4. Starch as an energy storage product

Conditions leading to the evolution of Plantae: 'Evidence of early competition for energy sources in the aquatic environment:'

A. Elongated cells in multicellular algae (to reach above neighbors to access sunlight and CO2) in the very surface water. Elongated cells appear in multicellular algae to reach areas with more light (e.g. kelp), sometimes 30 m above the sea floor. It is easier to diffuse substances within parts of the same cell than transferring substances across cell membranes/cell walls, even if intercellular pores are present. B. Increased contractile fibers and cytoplasmic streaming to distribute energy products and gases within different parts of larger and elongated cells, and between cells, in multicellular organisms. Larger cells evolved to reduce predation risk and to help expose at least some plant parts to increased radiant energy, such as at or above water surfaces. c. Cellulose, silicon and calcium carbonate were used by the photosynthetic protists (e.g. diatoms) to strengthen their cell membranes against predation, with cellulose becoming the common constituent of the cell wall of all land plants. This physical protection could also protect those protists pushed to the edge of the sea in the splash zone along ancient shores.

Conditions leading to the evolution of Plantae: 'CO2 was also becoming less available'

CO2 was also becoming less available, and oxygen more available, in the shallow marine waters congested with marine autotrophs. Here again, the air above the water had much more readily available CO2 than was dissolved in sea water. Clearly, the early aquatic environment had become saturated with early autotrophs competing for a shrinking resource base of energy and CO2. Some early life forms at this time evolved the ability to attack other life forms for food/energy instead of competing for solar radiation. And so, an evolutionary progression was evident in the early oceans from autotrophy with different pigments, to heterotrophy (organisms eating organisms as an energy source), and eventually to parasitism (nibbling the surface of larger forms that couldn't be engulfed). There were few available niches left in the early sea, at least for single cell or simple multicellular organisms.

Major Plantae Groups: 'Non-vascular plants with cuticle'

Parts of plants out of water have a cuticle that gives dehydration resistance, but lack of support structure (and vascular tissue) results in low, sprawling growth (e.g. true mosses). Extending out of the water just a little greatly increases wavelength and CO2 availability. 1. Mosses have specialized conducting tubular cells, 2. First to have stoma, 3. Water/nutrient intake and intercellular transport mainly by diffusion. 4. The gametophyte is the dominant life cycle stage 5. Diversity: 3 Divisions (Phyla): mosses, liverworts and hornworts A. Largest Phylum: Bryophyta (Mosses) B. First land plants (mosses); but not the ancestors of higher plants. C. Sphagnum moss during drought becomes dormant until water returns

Conditions leading to the evolution of Plantae: 'Wavelength resource limitation'

Photosynthesis evolved in early unicellular bacteria with the appearance of a pigment molecule able to absorb radiant energy and transform it into high-energy chemical bonds that could be used as an energy source by the cell for many metabolic purposes (Prokaryote: Cyanobacteria). Different pigments eventually evolved as the competition for different wavelengths of light penetrating the sea surface intensified with the increased diversity of aquatic autotrophs. Unfortunately, water also absorbs radiant energy, especially in the red end of the visual spectrum (which makes ocean water appear blue). Thus, the radiant energy available to marine organisms is much less just below the sea surface than a few centimeters above it, and natural selection would favor early autotrophs able to reach above the sea surface even if just for short periods.

Major Plantae Groups: 'Non-vascular plants without cuticle'

Purely aquatic, simple plants with no major adaptations for existence on land (e.g. green algae). 1. Mainly immersed in nearshore and freshwater habitats 2. Division (Phylum) Chlorophyta (Green algae) Ex. Charales. An order of green alga believed to be the closest relatives of green land plants. They are branched, multicellular, chlorophyll-using plants that grow in fresh water. They are often called stoneworts, because the plants can become encrusted in lime (calcium carbonate) in hard water. There is no specialized transport tissue.

Major Plantae Groups

Showing an ever increasing ability to live out of water as aquatic and then terrestrial niches gradually became more congested with different plant types: the competition observed in the early seas among the bacteria and protists just continued out onto land, showing the structural adaptations listed in Fig 31.9, and concurrently the reproductive adaptations acquired to achieve gamete union in a dehydrating environment in Fig. 31.23. The plant groups below reflect this trend of increasing structural complexity to deal with more harsh/competitive environments from the simplest to the most highly evolved plants.

Introduction

The Kingdom Plantae includes all the land and aquatic plants and the green algae, but recently the red algae and an additional small, primitive algal group have been included in the Plantae. For our purposes though, we will just include the green algae and land/aquatic plants as constituting the Kingdom Plantae, and exclude other algae, as presented in Chapter 31 (Fig. 31.6).

Major Plantae Groups: 'Vascular, seedless plants:'

The first plants with true vascular tissue enabling them to grow to greater heights, reaching above the non-vascular plants to compete for sunlight, but still limited sexually to moist environments because of depending on flagellated sperm for reproduction (e.g. ferns). Again, notice the evolutionary consequences of congested living. 1. Vascular system 2. The sporophyte is the dominant life cycle stage, as in all higher (land) plants (Fig. 31.14). 3. Diversity: 4 major Divisions: Club mosses, Horsetails, Whisk ferns, Ferns (see lab exercise for Phylum names). Largest Phylum: Pteridophyta (Ferns): largest leaves, many epiphytes, oldest vascular plants, ancient 20 m high tree ferns.

Major Plantae Groups: 'Vascular plants with fruit-covered seeds (=angiosperms, the flowering plants)'

The fruit increases the protection and dispersal of seeds; the largest number of species of all plants. 1. Flower: a major innovation over gymnosperms: they facilitate pollination by attracting animal Pollinators. 2. Fruit = a mature, ripened ovary enclosing seed(s); 3. Diversity: One large Division: Anthophyta: the most widely distributed group of all land plants. a. Traditionally separated into monocotyledonous and dicotyledonous plants (Fig. 31.24) [monocots have one leaf at germination, scattered vascular bundles, parallel leaf veins, and flower petals in multiples of 3; dicots have 2 leaves at germination, a circular arrangement of vascular bundles, branching leaf veins, and petals in multiples of 4 or 5]. b. Most angiosperm plants are annuals (die off at the end of the year leaving only seeds for the next growing season) and herbaceous (are short-lived and non-woody [little or no lignin]).

Major Plantae Groups: Vascular, naked-seed plants (=gymnosperms):'

The innovation of pollen (~ air-borne sperm) and dehydration-resistant seeds enable greater freedom (and dispersal) of sexual reproduction on land (e.g., cycads and conifer trees), but absence (= naked) of fruit (= developed ovary) compromises even greater seed dispersal. 1. Seed is 1st major innovation: (1) protects embryo against water loss, (2) protects the embryo against herbivores, (3) permits dormancy, and (4) promotes dispersal. 2. Pollen is 2nd major innovation (see Fig. 31.19). 3. Cones of males (small) and females (large) usually in separate trees; scales partially protect female megaspore (eggs). 4. Needles of conifers reduce the photosynthetic surface, but this feature in combination with pollen allow conifers to thrive in more arid habitats. 5. Diversity: 4 major Divisions: Cycads, Ginkgos, Gnetophytes, Conifers (pines, spruces, firs) a. Largest Division: Coniferophyta (Pines/spruces/firs) b. Includes the largest (redwoods, sequoias) and oldest (Bristle cone pine ~ 4,900 yr) plants

Conditions leading to the evolution of Plantae: 'New niche'

Untapped possibilities (resources) were just centimeters away in the air above very shallow water or on land: Land invasion and the evolution of land plants.