Chapter 25 Notes: Intro to Plants
Section 1: Evolution of Plants
The fossil record indicates great change in the diversity of plants throughout geologic time. In the movie Jurassic Park, little is noted (2 short observations, as I recall) of the change in plants over time. In fact, a true Jurassic Park would not work if they placed the dinosaurs with common plants from current-day earth (we are in the Cenozoic area, while dinosaurs lived in the Mesozoic).
As a broad generalization, plants come in four flavors:
Bryophytes – nonvascular plants that don’t produce seeds. This group includes mosses, liverworts and hornworts. These are likely the earliest plants, colonizing areas near water where humidity was high and soil was soggy. The plants lack vascular tissue, specifically xylem (moves water and nutrients) and phloem (moves dissolved sugars).
Seedless Vascular Plants – Ferns and similar plants do have vascular tissue but they don’t produce seeds. They produce sperm that require standing water to complete fertilization.
Gymnosperms – These plants include vascular plants that reproduce without flowers. They do produce seeds, but these do not develop from the ovaries of the flower. The most common examples are conifers (such as pines and spruce) and ginko trees.
Angiosperms- This is the group of plants that has been enjoying an evolutionary explosion over the past 130 million years. These are vascular plants that produce flowers and seeds.
To begin our unit on plants, you should know a few basic botany (study of plants) terms and concepts. As we study plants, we will examine them in light of their role in evolution. As early earth developed, the earliest colonizers would likely have been the bacteria, fungi (likely the lichens) and, then, the bryophytes. Why move onto land?
Open niche – Early oceans would have presented many problems to plant growth. Many plants would compete for available sunlight, just as trees in modern forests compete for available sunlight. Also, early land colonizers would have faced few predators (herbivores).
Carbon Dioxide – The air contains more carbon dioxide than the air. It is believed that the atmosphere in ancient times, such as the Devonian era, was characterized by a denser atmosphere, containing more carbon dioxide than currently exists within our atmosphere.
What challenges would be faced by plants making the move to terrestrial (land) environments?
Lack of water – Because they are no longer bathed in water, terrestrial plants are presented with a challenge of conserving water. Three adaptations are especially notable in terms of water conservation: vascular tissue for moving water from soil to upper reaches of a plant, cuticle (waxy covering on leaves, which serves to conserve water) and stomata (opening on leaves that allow them to take in CO2 without losing much water).
Strength – Water has considerable buoyancy, therefore aquatic plants get a lot of support from water. Terrestrial plants need stronger tissue, and some produce lignin in their cell walls to provide extra support.
Alternation of Generations – In the fungi and the early plants, the gametophyte generation is the dominant life stage. This stage produces gametes (haploid cells, 1n). In fungi, the zygote is the only diploid life stage, and it quickly proceeds through meiosis, resulting in a haploid gametophyte generation. In many plants, the sporophyte generation becomes the dominant life stage.
In modern gymnosperms and angiosperms, the sporophyte generation produces (through meiosis) a gametophyte generation capable of producing specialize cells known as pollen and eggs.
Section 2: Bryophytes
These organisms earn the moniker ‘early plants’ because they were the first plants to colonize terrestrial environments. They lack vascular tissue, and are therefore short due to their inability to transport water upwards. Think of these plants as being like sponges: they can absorb water and dissolved nutrients and transport them a short distance, but their ability to transport is limited. The do have rhizoids, structures that serve the purpose of anchoring the plant and absorbing moisture (not true roots as they lack vascular tissue). These plants do display cuticles (waxy covering on leaves) but, unlike the other plants, the gametophyte is the dominant life stage and it is not dependent upon the sporophyte. Examples include mosses, liverwort and hornworts. The gametes require water in order to unite (no wind or insect pollination).
Section 3: Seedless Vascular Plants
Plants such as ferns are generally taller than bryophytes, but still are usually found in moist areas. These plants have true vascular tissue and the sporophyte is the dominant life stage, but you will not find true seed or flowers. Ferns produce spores on the undersides of their leaves, which ultimately will grow into a gametophyte that release motile sperm, meaning they need some standing water in order to reproduce.
Section 5: Seed-bearing Plants
The plants best adapted to life on land are the seed-bearing plants. With the evolution of gymnosperms and angiosperms, plants were able to adapt to fertilization in dry areas and the new offspring gained dispersal advantages. With the male parts of a sporophyte, microspores give rise to pollen grains (which contain many haploid pollen cells). The female parts of the sporophyte contain ovules, which become seeds if they are fertilized. Seed are fertilized eggs with a store of food and a coating to protect them from the elements.
Section 6: Gymnosperms
The gymnosperms are recognizable as pine trees (conifers – ‘cone bearing’), cycads and ginkgos. The gymnosperms have well-developed cell walls contain lignin (allowing them to reach great heights, such as the giant redwoods), vascular tissue and seed (produced by cones or other means, but NO flowers). Like the angiosperms, the spruce trees you see around town are the sporophyte generation. Only in the spring will you be able to observe the gametophyte generation (clouds of yellow pollen coming from the male cones). The tradition pinecone consists of fertilized female eggs, giving rise to seeds atop each scale of the cone. These seeds are not surrounded by the ripened ovary (as seen in angiosperms) but they often have a store of food to allow the new sporophyte generation to get started after seed dispersal.
Section 8: Angiosperms
When you think of plants, 90% of the species you think of are probably angiosperms. Anything with flowers, or fruits, is an angiosperm. The flower is used to attract pollinators (in most cases), thus serving to increase sexual reproduction. When eggs are fertilized, they will often send out a hormonal signal for the surrounding tissue (ovary) to develop into a fruit. This fruit (not produced in gymnosperm, thus the name meaning ‘naked fruit’) can either nourish the developing seed or entice other organisms to help disperse the seeds.