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Lecture 10

BIO153 Lecture 10.pdf

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Department
Biology
Course
BIO153H5
Professor
Christoph Richter
Semester
Fall

Description
2009 BIO153: Lecture 10 The Vascular Plants February 11, 2009 Origin of the vascular plants: ▯ current thinking (2006) suggests that the non-vascular plants (bryophytes) are not a monophyletic group ▯ sister taxon of the vascular plants appears to be the hornworts ▯ about 50 million years after the appearance of land plants, fossil evidence of the first vascular plants Early vascular plants: a diverse and poorly understood group. Much of our knowledge of early vascular plants comes from a large fossil deposit called the Rhynie Chert (see below). e.g. Rhyniophyta: ▯ very simple structure: bifurcating (2- way branching) stems; no leaves, no seeds, no flowers ▯ no roots (horizontal stems connected to soil by root hairs took the function of roots – anchoring the plant; likely providing nutrients: note the difference bewteen these and rhizoids in bryophytes) ▯ sporophyte dominant: sporangia borne in structures at the top of the plant – aid in spore dispersal The Rhynie Chert: a treasure-trove for plant paleontologists (in this way, analogous to the 1 Burgess Shale for Cambrian animals, which we will see in an upcoming lecture). The Rhynie Chert is a silicified moor located about 40 km from Aberdeen, Scotland. About 400 million years ago, geysers spouted boiling hot, siliceous (silica-rich) water, preserving the plant life so perfectly that the fossils can be examined at the cellular level. Beautiful! Fossils from the Rhynie Chert: ▯ vascular tissue with lignin (strong material that permits greater growth) ▯ stomata & cuticles ▯ sporangia that split (dehisce) – spores land away from plant (dependent on moist conditions to complete the life cycle) ▯ start to see the first leaves (likely evolved independently in several groups) ▯ 2 types of leaf structure: microphylls and megaphylls Recall that we discussed how plant structure reflects adaptations to living on land. There were still unsolved problems: There are distinct competitive advantages to being taller than your neighbours if you are a plant. However, it’s hard to grow tall when you grow only via apical meristem (new cell growth only at the tips of the plant). Palm “trees” grow this way: stems thicken but do not grow outward, making the plant very unstable! Solution: grow wide as well as tall! This is accomplished by secondary growth (outward growth) via vascular cambium. Today, there are a few vascular seedless plants that retain many of the features seen in these early vascular plants. These are: 1. Lycophyta (club mosses) 2. Sphenophyta (horsetails) 3. Psilophyta (whisk ferns) 4. Pteridophyta (ferns) 1. Lycopods - club-mosses ~ 1100 spp. (**I do not expect you to memorize the 2 numbers of species; only to have a general idea of which groups are large and diverse, and which groups are small and relictual). ▯ Lycopods are present today as small, moss-like plants (you can find some on the UTM campus!), but in the Carboniferous, they formed huge forests of large “trees”. 2. Sphenophytes - horsetails - 15 spp. ▯ all within a single genus (Equisetum); they are called scouring rushes due to the high silica content in their tissues 3. Psilophytes - whisk ferns ~3 spp. ▯ These resemble the Rhyniophyta of the Rhynie Chert! 4. Pteridophyta - ferns ~10,500 spp. ▯ The largest and most diverse group of seedless vascular plants. They typically have large megaphylls, and many are epiphytic (live on other plants). Seedless vascular plants dominated the terrestrial landscape for much of the Palaeozoic, where conditions were mostly warm and moist, but in the Permian, conditions especially in the centers of the continents began to get cold and dry. This was very problematic for plants that depended on moist conditions to complete the life cycle, and many of these forms began to die out. The next major evolutionary step was the seed. Advantages of the seed: ▯ protects young plant (sporophyte) from desiccation ▯ the embryo develops within a seed coat; supplied by nutritive tissues ▯ seeds can lie dormant and germinate only when conditions are favourable ▯ the seed is analogous to the cleidoic egg in animals, which we will examine in an upcoming lecture. 3 The seed plants are: 1. (Seed ferns) 2. Gymnosperms ▯ Gnetophytes ▯ Cycads ▯ G
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